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Volume 65 
Part 3 
Pages 269-290  
June 2009  

Received 9 July 2008
Accepted 27 March 2009

Complex ceramic structures. I. Weberites

aDepartment of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400, USA
Correspondence e-mail: jnino@mse.ufl.edu

The weberite structure (A2B2X7) is an anion-deficient fluorite-related superstructure. Compared with fluorites, the reduction in the number of anions leads to a decrease in the coordination number of the B cations (VI coordination) with respect to the A cations (VIII coordination), thus allowing the accommodation of diverse cations. As a result, weberite compounds have a broad range of chemical and physical properties and great technological potential. This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB3 and A3B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interesting properties is discussed.

1. Introduction

The weberite crystal structure (space group: Imma, No. 74), with typical stoichiometry A2B2X7 (A and B are cations, X is an anion, O or F), is a type of anion-deficient fluorite superstructure (AX2). While several other compounds possess the same stoichiometry (pyrochlores, layered perovskites etc.), weberites are isomorphic with the mineral Na2MgAlF7. This mineral was originally found in Ivigtut in southwestern Greenland and was named after Theobald Weber (Bogvad, 1938[Bogvad, R. (1938). Meddelelser om Grønland, 119, 1-11.]). In 1944, Byström (1944[Byström, A. (1944). Ark. Kemi Miner. Och Geol. B, 18, 1-7.]) determined the crystal structure, basing his studies on the pyrochlore structure, which is another fluorite-related superstructure.

While the structure has a cationic sublattice arrangement similar to that found in the fluorite structure (face-centered cube), owing to distortions in the anion sublattice, the crystal structure has a high potential to accommodate diverse metals. The cations in (011) planes have nearly the same symmetry as in the hexagonal tungsten bronze (HTB) structure. In addition, the triangular network in Na2B2+B3+F7 weberites, which is formed by B2+ and B3+ cations in the HTB-like planes, potentially supports various magnetically ordered systems. To date, fluorine-based weberites such as Na2B2+B3+F7 and Ag2B2+B3+F7 have attracted most of the attention owing to their interesting magnetic properties (Cosier et al., 1970[Cosier, R., Wise, A., Tressaud, A., Grannec, J., Olazcuag, R. & Portier, J. (1970). C. R. Hebd. Seances Acad. Sci. C, 271, 142-145.]; Dance et al., 1974[Dance, J. M., Grannec, J., Jacoboni, C. & Tressaud, A. (1974). C. R. Hebd. Seances Acad. Sci. C, 279, 601-604.]; Frenzen et al., 1992[Frenzen, G., Massa, W., Babel, D., Ruchaud, N., Grannec, J., Tressaud, A. & Hagenmuller, P. (1992). J. Solid State Chem. 98, 121-127.]; Laligant et al., 1989[Laligant, Y., Calage, Y., Heger, G., Pannetier, J. & Ferey, G. (1989). J. Solid State Chem. 78, 66-77.]; Laligant, Ferey et al., 1987[Laligant, Y., Ferey, G., Heger, G. & Pannetier, J. (1987). Z. Anorg. Allg. Chem. 553, 163-171.]; Pankhurst et al., 1991[Pankhurst, Q. A., Johnson, C. E. & Wanklyn, B. M. (1991). J. Magn. Magn. Mater. 97, 126-130.]; Ruchaud et al., 1992[Ruchaud, N., Grannec, J., Gravereau, P., Nunez, P., Tressaud, A., Massa, W., Frenzen, G. & Babel, D. (1992). Z. Anorg. Allg. Chem. 610, 67-74.]; Thompson et al., 1992[Thompson, G. R., Pankhurst, Q. A. & Johnson, C. E. (1992). J. Magn. Magn. Mater. 104, 893-894.]; Tressaud et al., 1974[Tressaud, A., Dance, J. M., Portier, J. & Hagenmul, P. (1974). Mater. Res. Bull. 9, 1219-1226.]; Heger, 1973[Heger, G. (1973). Int. J. Magn. 5, 119-124.]). Investigations on A2B2O7 weberites have mainly focused on crystallography because of the close relationship between the weberite and the pyrochlore structures (Cordfunke & Ijdo, 1988[Cordfunke, E. H. P. & Ijdo, D. J. W. (1988). J. Phys. Chem. Solids, 49, 551-554.]; Groen & Ijdo, 1988[Groen, W. A. & IJdo, D. J. W. (1988). Acta Cryst. C44, 782-784.]; Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]; Reading et al., 2002[Reading, J., Knee, C. S. & Weller, M. T. (2002). J. Mater. Chem. 12, 2376-2382.]; Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]; Bonazzi & Bindi, 2007[Bonazzi, P. & Bindi, L. (2007). Am. Mineral. 92, 947-953.]; Desgardin et al., 1976[Desgardin, G., Robert, C. & Raveau, B. (1976). Can. J. Chem. 54, 1665-1671.]; Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.], 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]; Ivanov & Zavodnik, 1990[Ivanov, S. A. & Zavodnik, V. E. (1990). Kristallografiya, 35, 842-846.]). Both of the structures form three-dimensional BX6 networks and HTB-like layers. Some compounds, for example Ca2Sb2O7, can form a metastable pyrochlore phase, which can be converted into the weberite structure by heating above 973 K (Brisse et al., 1972[Brisse, F., Stewart, D. J., Seidl, V. & Knop, O. (1972). Can. J. Chem. 50, 3648.]). Recently, weberite oxides and weberite-related oxides have been reported to possess interesting physical properties (ferroelectric, dielectric and magnetic) as well as photocatalytic activity (Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]; Abe et al., 2004[Abe, R., Higashi, M., Zou, Z. G., Sayama, K., Abe, Y. & Arakawa, H. (2004). J. Phys. Chem. B, 108, 811-814.], 2006[Abe, R., Higashi, M., Sayama, K., Abe, Y. & Sugihara, H. (2006). J. Phys. Chem. B, 110, 2219-2226.]; Cai & Nino, 2007[Cai, L. & Nino, J. C. (2007). J. Eur. Ceram. Soc. 27, 3971-3976.]; Cai et al., 2007[Cai, L., Guzman, J., Perez, L. & Nino, J. C. (2007). Solid-State Chemistry of Inorganic Materials VI, Materials Research Society Symposium Proceeding, 998E, 0988-qq0901-0904.]; Cava et al., 1998[Cava, R. J., Krajewski, J. J. & Roth, R. S. (1998). Mater. Res. Bull. 33, 527-532.]; Ebbinghaus et al., 2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]; Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]; Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]; Lin et al., 2006[Lin, X. P., Huang, F. Q., Wang, W. D., Wang, Y. M., Xia, Y. J. & Shi, J. L. (2006). Appl. Catal. Gen. 313, 218-223.]; Khalifah et al., 1999[Khalifah, P., Erwin, R. W., Lynn, J. W., Huang, Q., Batlogg, B. & Cava, R. J. (1999). Phys. Rev. B, 60, 9573-9578.]; Nishimine et al., 2004[Nishimine, H., Wakeshima, M. & Hinatsu, Y. (2004). J. Solid State Chem. 177, 739-744.], 2005[Nishimine, H., Wakeshima, M. & Hinatsu, Y. (2005). J. Solid State Chem. 178, 1221-1229.]; Plaisier et al., 2002[Plaisier, J. R., Drost, R. J. & IJdo, D. J. W. (2002). J. Solid State Chem. 169, 189-198.]; Wakeshima & Hinatsu, 2006[Wakeshima, M. & Hinatsu, Y. (2006). J. Solid State Chem. 179, 3575-3581.]; Hinatsu et al., 2004[Hinatsu, Y., Wakeshima, M., Kawabuchi, N. & Taira, N. (2004). J. Alloys Compd, 374, 79-83.]; Harada & Hinatsu, 2002[Harada, D. & Hinatsu, Y. (2002). J. Solid State Chem. 164, 163-168.], 2001[Harada, D. & Hinatsu, Y. (2001). J. Solid State Chem. 158, 245-253.]; Lam et al., 2003[Lam, R., Langet, T. & Greedan, J. E. (2003). J. Solid State Chem. 171, 317-323.]; Wiss et al., 2000[Wiss, F., Raju, N. P., Wills, A. S. & Greedan, J. E. (2000). Int. J. Inorg. Mater. 2, 53-59.]; Wltschek et al., 1996[Wltschek, G., Paulus, H., Svoboda, I., Ehrenberg, H. & Fuess, H. (1996). J. Solid State Chem. 125, 1-4.]; Gemmill et al., 2005[Gemmill, W. R., Smith, M. D., Mozharivsky, Y. A., Miller, G. J. & zur Loye, H. C. (2005). Inorg. Chem. 44, 7047-7055.]).

There are a considerable number of publications on weberite ceramics. However, information on weberites is scattered. While presenting their work on the structure determination of Na2Fe2F7, Yakubovich et al. (1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]) devoted more than half of the paper to a comparison of different types of weberite structures in Na2B2+B3+F7 compounds and their relationship to the fluorite and the pyrochlore structures, but their discussion was limited to the crystallographic aspects. Lopatin et al. (1985[Lopatin, S. S., Averyanova, L. N. & Belyaev, I. N. (1985). Zh. Neorg. Khim. 30, 867-872.]) and Sych et al. (Sych, Kabanova, Garbuz et al., 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) focused on the stability-field region of these compounds. However, there is no article that correlates the structure and properties of weberites. Therefore, this paper is intended to provide a discussion of weberite ceramics, covering crystallographic aspects including the relationship between weberite, fluorite and pyrochlore, their stability field with respect to pyrochlore, and their interesting properties.

In §2[link] the structural features of weberite and the characteristics of weberite-like structures are discussed. Several different descriptions of the structure are given and a stacking vector and stacking angle are used for the first time to represent the weberite structure. The crystallographic relationship between the fluorite and the pyrochlore structures is also discussed. It is also shown that the cation sublattices of pyrochlores can be transformed to the weberite-like lattice. The different stacking of neighboring AB3 and A3B layers lead to the different coordination environments of anions in weberite and pyrochlore. A stability field is developed to predict the formation of pyrochlore and weberite oxides. In §3[link] ferroelectric and dielectric properties of some weberite compounds are discussed.

2. Crystal structure

2.1. Classic orthorhombic weberite

The space group of the orthorhombic weberite structure is Imma (No.74) with four formula units per unit cell (Z = 4). However, the correct space group of weberites was a subject of controversy for a long time, as it was described as both Imma and Imm2 (Sych, Kabanova & Andreeva, 1988[Sych, A. M., Kabanova, M. I. & Andreeva, S. G. (1988). Zh. Neorg. Khim. 33, 2756-2760.]; Haegele et al., 1978[Haegele, R., Verscharen, W., Babel, D., Dance, J. M. & Tressaud, A. (1978). J. Solid State Chem. 24, 77-84.]; Giuseppetti & Tadini, 1978[Giuseppetti, G. & Tadini, C. (1978). Tschermaks Min. Petr. Mitt. 25, 57-62.]; Knop et al., 1982[Knop, O., Cameron, T. S. & Jochem, K. (1982). J. Solid State Chem. 43, 213-221.]; Byström, 1944[Byström, A. (1944). Ark. Kemi Miner. Och Geol. B, 18, 1-7.]). The detailed history of the determination of the space group has been reported by Yakubovich et al. (1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]). The two space groups are closely related as Imm2 is a subgroup of Imma. The only evidence ruling out Imma was the observation of very weak (hk0) with h = 2 n + 1 reflections. It was later proven that the existence of (hk0) with h = 2n + 1 reflections from Na2NiFeF7 single crystals and Na2NiAlF7 originated from the Renninger effect (`double reflection' process; Schmidt et al., 1992[Schmidt, R. E., Massa, W. & Babel, D. (1992). Z. Anorg. Allg. Chem. 615, 11-15.]; Laligant et al., 1989[Laligant, Y., Calage, Y., Heger, G., Pannetier, J. & Ferey, G. (1989). J. Solid State Chem. 78, 66-77.]). Thus, there is no doubt that the true space group of the orthorhombic weberite is Imma. The atomic positions and site symmetry are given in Table 1[link].

Table 1
Weberite structure data (origin at A cations) in space group Imma

      Atomic position
Atoms Wyckoff position Site symmetry x y z
A1 4d 2/m 0.25 0.25 0.75
A2 4a 2/m 0 0 0
B1 4c 2/m 0.25 0.25 0.25
B2 4b 2/m 0 0 0.5
X1 8h m 0 y1 z1
X2 16j 1 x2 y2 z2
X3 4e mm2 0 0.25 z3
X1 is at the center of A3B, X2 is in the A2B2 tetrahedron and X3 is inside A4B2.

In weberites the A ions sit in the 4a and 4d atomic positions with site symmetry 2/m and establish a coordination number of 8 with the anions. The A ions have two different coordination environments. The A1 cations (in atomic position 4d) lie in a highly distorted cube (or square prism) where there are two different A1-X bond lengths. The cubes are edge-shared to form a series of chains in the [100] direction. The A2 cations (in atomic position 4a) are located within bi-hexagonal pyramids in which anions are spaced at three different distances from the central cations. Each pyramid is corner-shared with two other pyramids and edge-shared with four A1X8 cubes. As presented in Table 1[link] there are three Wyckoff positions for anions (X1 at 8h, X2 at 16j and X3 at 4e). A1 ions only connect to X1 and X2, while A2 link to all three types of anions (two X1, four X2 and two X3).

The B ions are located in the 4b and 4c Wyckoff positions (site symmetry 2/m) and have a coordination number of 6, i.e. A2VIIIB2VIIX7. The weberite structure can be described as a network of corner-shared BX6 octahedra with the penetration of A cations (see Fig. 1[link]). There are two types of BX6 octahedra: B-1 (B2+ in the case of A2B2+B3+F7, A = Na+ or Ag+) in 4c Wyckoff positions, and B-2 (B3+ in the case of A2B2+B3+F7) in 4b Wyckoff positions. Each of the six vertices of B-1 octahedra connects to another B octahedron, while only four vertices of a B-2 octahedron link to other B octahedra. The two unpaired vertices are in a trans configuration. As will be discussed later, trigonal or monoclinic weberite variants cause a cis configuration (see Fig. 2[link]). The B-1 octahedra are corner-linked to each other and form B-1 octahedral chains parallel to the A1 chains (in the [100] direction). The B-2 octahedra are isolated from each other and link the B-1 octahedral chains to form a three-dimensional octahedral network.

[Figure 1]
Figure 1
B-octahedral network and A cations (a) in the [1, 0.04, 0.07] direction and (b) in the [0.1, 0, 1] direction; the black lines are the unit cell.
[Figure 2]
Figure 2
(a) A B-1 octahedron in the center and its connection to another six octahedra; (b) trans configuration of B-2 octahedra; (c) cis configuration of B-2 octahedra.

The arrangements of A and B ions lead to three different cation tetrahedra. Six anions occupy the two A3B (A3BX, X1) and four A2B2 tetrahedral interstices (A2B2X, X2) and none are located inside the two AB3 sites (AB3[], where [] represents a vacant site) in a formula unit. The remaining anion (X3) maintains four coordination and lies outside the two edge-shared AB3 tetrahedra, very close to the shared B-B edge (see Fig. 3[link]). X3 can also be considered to sit inside the octahedron (A4B2), which shares faces with two adjacent AB3 tetrahedra, and distort towards the B-B edge (Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]).

[Figure 3]
Figure 3
Anion coordination: (a) X-1 in A3B; (b) X-2 in A2B2; (c) X-3 in A4B2.

The weberite structure can also be considered as a stacking of repeated layers or slabs. The most common way to examine the structure is to view it as stacked, alternating close-packed metal layers A3B and AB3 on (011) parallel planes. In A3B layers, four A-1 and two A-2 ions form a hexagonal ring with B-2 occupying the center. In other words the A cations form Kagomé-type networks. [Kagomé in Japanese means a bamboo-basket woven pattern. It is formed by interlaced triangles and each lattice point has four equivalent bonds. `Kagomé' was introduced by Husimi after he and his co-worker Syôzi found a new antiferromagnetic lattice by star-to-triangle transformation from a honeycomb lattice (Mekata, 2003[Mekata, M. (2003). Phys. Today, 56, 12-13.]). Syôzi published the first Kagomé paper in 1951 (Syozi, 1951[Syozi, I. (1951). Prog. Theor. Phys. 6, 306-308.]); see Fig. 4[link].] In AB3 layers the BX6 octahedron arrangement is nearly identical with the basal plane of the hexagonal tungsten bronze (HTB) structures and A-2 cations are in the center of the hexagonal rings. The HTB-like layers can also be simplified by a Kagomé net representation (Fig. 5[link]). The HTB-like layers are displaced with respect to each other by an interlayer stacking vector (SV) which is defined as the projected distance, viewed down the (pseudo-) sixfold axis, between crystallographically similar ions in adjacent layers. White (1984[White, T. J. (1984). Am. Mineral. 69, 1156-1172.]) and Cohelho et al. (1997[Coelho, A. A., Cheary, R. W. & Smith, K. L. (1997). J. Solid State Chem. 129, 346-359.]) used SV as an alternative description for zirconolite, zirkelite, pyrochlore and polymignyte. Here, SV is used to describe weberite. The stacking vectors in the weberite structure are nearly in the [[0\bar11]], [[\bar 31\bar 1]] and [[31\bar 1]] directions. They are typically of the order 4 Å. The angle between successive stacking vectors (SA) is approximately 120°. The distance between two neighboring HTB-like layers along the (pseudo-) sixfold axis is approximately 5.8 Å. Fig. 6[link] shows the stacking vectors between three sequences of HTB layers.

[Figure 4]
Figure 4
(a) Kagomé net presentation of A cations on A3B layers; (b) AX8 polyhedral representation of A3B layers, which are parallel to the (011) plane.
[Figure 5]
Figure 5
(a) Kagomé net presentation of B cations on AB3 layers; (b) BX6 polyhedral representation of AB3 layers, which are parallel to the (011) plane.
[Figure 6]
Figure 6
Left: Stacking vectors (black arrows) between three sequences of HTB layers; right: Kagomé nets of three successive HTB layers (purple arrows are stacking vectors). This figure is in color in the online version of this paper.

There is yet another way to consider the weberite repeated layers. The first layer is formed by the alternating B-1 octahedral chains and A-1 distorted cube (or square prism) chains, which are in the [100] direction for classic orthorhombic weberites. In this layer the B-1 octahedra are edge-shared with A-1 cubes. The second layer is alternating B-2 octahedra and A-2 bi-hexagonal pyramids in the [100] direction as in Fig. 7[link] (Rossell, 1979[Rossell, H. J. (1979). J. Solid State Chem. 27, 115-122.]; Renaudin et al., 1988[Renaudin, J., Leblanc, M., Ferey, G., Dekozak, A. & Samouel, M. (1988). J. Solid State Chem. 73, 603-609.]).

[Figure 7]
Figure 7
(a) The layer consisting of A1 and B1 lines; (b) A-2 and B-2 layers, viewed in the [010] direction.

2.2. Relationship to fluorite and pyrochlore

Weberite and pyrochlore (A2B2X7) are both fluorite-related (AX2 or A4X8) superstructures. The coordination number of A and B is the same in both structures. These two structures have a similar cationic sublattice, which is comprised of stacked cubic close-packed cation layers, the same as (111) planes in fluorite. These layers alternate between the compositions A3B and AB3 and are parallel to (111) planes in pyrochlore and (011) planes in weberite. AB3 layers in pyrochlore can also be described as HTB-like layers. The length of SV and the value of SA of the pyrochlore structure are almost the same as weberite. However, the difference between the weberite and the pyrochlore structures is the different stacking of two successive AB3 and A3B layers, which will be discussed later in this section. The crystallographic relationship between the weberite and the pyrochlore structures is further confirmed by the fact that the space group of weberite (Imma) is a subgroup of [Fd\bar 3m], the space group of pyrochlore. If the lattice parameter of pyrochlores is 2a with respect to fluorite a (a [asymptotically equal to] 5 Å), then the lattice parameters of the classic orthorhombic weberites are approximately 21/2a, 2a and 21/2a. The rotation of 45° about the b axis of the pyrochlore cation sublattice leads to the weberite-like cation sublattice (Fig. 8[link]). The (111) planes of pyrochlore are transformed to the (011) of the new lattice. The transformation relationship can be written as

[W = P\left(\matrix{0.5 & 0 & 0.5\cr 0 & 1 & 0\cr -0.5 & 0 & 0.5}\right). \eqno(1)]

The transformation leads to the space group Imcm, which is a different setting of Imma. The Imma lattice can be achieved by the 90° rotation of the coordinate system of Imcm. The transformation matrix is

[\left(\matrix{0 & 1 & 0\cr 0 & 0 & 1\cr 1 & 0 & 0}\right). \eqno(2)]

The resulting lattice parameters in Imma are 2a, 21/2a and 21/2a. In order to match the weberite lattice parameters, the space group Imcm is preferred when presenting the atomic positions of pyrochlore in the weberite-like orthorhombic lattice (Table 2[link]).

Table 2
Pyrochlore (A2B2X6X') structure data in space group Imcm

      Atomic position
Atoms Wyckoff position Site symmetry x y z
A 4b 2/m 0 0.5 0
  4c 2/m 0.25 0.25 0.25
B 4a 2/m 0 0 0
  4d 2/m 0.25 0.75 0.25
X 4e mm2 0.5 x + 0.25 0.25
  4e mm2 0 x 0.25
  16j 1 x - 0.125 0.125 x + 0.125
X' 4e mm2 0.25 0.375 0
The x is the oxygen parameter inside the A2B2 tetrahedral site. The value is between 0.3125 and 0.375.
[Figure 8]
Figure 8
(a) Cationic network on (111) of fluorite; (b) AB3 layer; (c) A3B layer on (011) of weberite; dashed lines are the unit cell; (d) axial transformation of pyrochlore to weberite [black dashed lines: a fluorite cell; blue lines: double pyrochlore cells; red dotted lines: weberite lattice; green shadow (011)w and (111)p]. This figure is in color in the online version of this paper.

It is easy to recognize weberite and distinguish the three structures from powder diffraction. As is well known, in Cu K[alpha] radiation to 2[theta] [asymptotically equal to] 70°, the five fluorite characteristic peaks are (111), (200), (220), (311) and (222). The (111) reflection is at 2[theta] [asymptotically equal to] 30° with the highest intensity. In pyrochlore, owing to the doubling of the lattice parameter with respect to fluorite, the five fluorite peaks become (222), (400), (440), (622) and (444). The appearance of several weak reflections, especially the (111) peak at 2[theta] [asymptotically equal to] 15°, is a major difference between the X-ray diffraction (XRD) patterns of fluorite and pyrochlore. In orthorhombic weberite, the five fluorite peaks are split, for example, the most intense (111)f or (222)p are split into (022)w and (220)w. There are several more reflections in weberite, which are systematic absences in pyrochlore, for example, (101)w and (020)w [corresponding to (200)p]. Details on the XRD reflection for fluorite, pyrochlore and weberite are listed in Table 3[link]. For space reasons, only reflections up to (222)f are presented.

Table 3
XRD reflections for fluorite, pyrochlore and weberite

XRD reflections
hkl (fluorite) hkl (pyrochlore)   hkl (weberite) Corresponding pyrochlore plane
  111   011 111
    [\big\{] 101 200
    020 020
  220 [\bigg\{] 002 202
  121 220
  200 [20\bar 2]
  311 [\bigg\{] 112 311
  211 [3 1 \bar 1]
  031 131
111 222 [\big\{] 022 222
220 [2 2 \bar 2]
200 400 [\big\{] 202 400
040 040
  331 [\bigg\{] 013 313
  132 331
  231 [3 3 \bar 1]
    [\cases{ & \cr &\cr &\cr &}] 103 402
    222 420
    301 [4 0 \bar2]
    141 240
    [\bigg\{] 123 422
    042 242
    321 [4 2 \bar 2]
      240 [2 4 \bar 2]
  333   033 333
  511 [\bigg\{] 213 511
  312 [5 1 \bar 1 ]
  051 151
220 440 [\bigg\{] 004 404
242 440
400 [4 0 \bar 4 ]
  531 [\cases{ & \cr &\cr &\cr &\cr&\cr&}] 114 513
  233 531
  152 351
  332 [5 3 \bar 1 ]
  251 [3 5 \bar 1 ]
  411 [5 1 \bar 3 ]
  442 [\cases{ & \cr &\cr &\cr &}] 024 424
  143 442
  341 [4 4 \bar 2 ]
  420 [4 2 \bar 4 ]
    [\big\{] 303 600
    060 060
  620 [\cases{ & \cr &\cr &\cr &}] 204 602
  323 620
  402 [6 0 \bar 2 ]
  161 260
  533 [\bigg\{] 134 533
  053 353
  431 [5 3 \bar 3]
311 622 [\cases{ & \cr &\cr &\cr &}] 224 622
062 262
422 [6 2 \bar 2]
260 [2 6 \bar 2 ]
222 444 [\big\{] 044 444
440 [4 4 \bar 4]

It is important to recall that in fluorites, each anion is at the center of the cationic tetrahedra (A4X). The arrangement of A and B leads to different cation tetrahedra: AB3, A3B and A2B2 in weberites, and A4, B4 and A2B2 in pyrochlores. The reason for the formation of different cation tetrahedra is that weberites and pyrochlores are different in stacking two neighboring AB3 and A3B layers, although generally they follow the pattern of cubic close-packed cation layers. The three nearest-neighbor metal ions in these layers form pseudo-equilateral triangles. The distribution of A and B cations in AB3 layers will lead to two types of triangles: AB2 and B3. The cations in the following A3B layer lie above the centers of these triangles. If an AB3 layer is a reference, there are 21/2a/2 along [100]w or [[10\bar 1]]P displacement in the above A3B layer between weberite and pyrochlore. As a result, in the A3B layer, A cations are above the center of AB2 triangles and B cations are above B3 triangles in pyrochlore, while in weberite, 2/3 A (A-1) and all B cations are above the AB2 triangles and the remaining A cations (A-2) are above B3 triangles (see Fig. 9[link]). Therefore, these arrangements lead to two AB3, two A3B and four A2B2 in a formula unit of weberite, and in the case of pyrochlore, one A4, six A2B2 and one B4. In addition, different stacking of two neighboring AB3 and A3B layers can explain why the transformation of the pyrochlore cation sublattice into a weberite-like lattice results in a different setting of the space group.

[Figure 9]
Figure 9
A3B and AB3 slabs of (a) weberite and (b) pyrochlore. Cations connected by dotted lines are on the AB3 layer and by grey lines are on the A3B layer. The red lines (or dotted line) show cation tetrahedra. This figure is in color in the online version of this paper.

As stated in §2.1[link], in a formula unit the X3 anion of weberite is located outside the cation tetrahedra and leaves two AB3 tetrahedra with a vacant center. In contrast, all anions in the pyrochlore structure are inside the cation tetrahedra. Therefore, it can be argued that pyrochlore is more closely related to fluorite than weberite since the former preserves all the anions in cation tetrahedral interstices (Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]; Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]). In weberite it is understandable that the X-deficient site is more favorable in B-rich tetrahedra (AB3 than A2B2 and A3B), because B ions have a smaller coordination number (CN). However, it raises the question: why there are two AB3 tetrahedra with a vacant center and the X3 is not inside the cation tetrahedra? Grey et al. (2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]) argued that in Ca2Ta2O7 weberite, the sum of valence ([\textstyle\sum v/CN]) in CaTa3 tetrahedra is so highly over-saturated that CaTa3 cannot accommodate X3. Actually, the highly over-saturated AB3 tetrahedra occur in all weberite compounds: A21+B2+B3+F7, A22+B25+O7 and A21+B26+O7. The nominal sum of valence in the center of AB3 is 1.46 for A21+B2+B3+F7, 2.75 for A22+B25+O7, 3.13 for A21+B26+O7. Thus, anions should distort largely towards A cations to meet the required valence, which would then result in a shorter A-X distance than a B-X distance. However, A ions are larger and have a larger CN than B ions and so the A-X bond length should be larger than the B-X bond length. The end result is that anions cannot move towards A cations and the required valence cannot be achieved. By contrast, the sum of valence in the center of AB3 is under-saturated, being 0.875 for A21+B2+B3+F7, 1.58 for A22+B25+O7 and 1.375 for A21+B26+O7. Anions are required to move towards B cations, which is favored by the bond length argument above. As for A2B2 tetrahedra, the sum of valence is 1.08 for A21+B2+B3+F7, 2.17 for A22+B25+O7 and 2.25 for A21+B26+O7. In this case the sum of valence is close to the anion oxidation state.

In order to check the stability of the X3, the empirical equation by Brese & O'Keeffe (1991[Brese, N. E. & O'Keeffe, M. (1991). Acta Cryst. B47, 192-197.]) is used to calculate the valence of the X3

[V_i = \sum v_{ij} = \sum \exp((R_{ij} - r_{ij})/b)\eqno(3),]

where Rij is the bond-valence parameter, rij is the bond distance and b is a constant. Three representative compounds were chosen for detailed analysis: Na1+2Mg2+Al3+F7, Sr2+2Sb5+2O7 and Ag1+2Te6+2O7. There are few, if any, reported bond-length data for Ag2B2F7. Na2MgAlF7 was chosen since it is the aristotype of the weberite compounds. The rij of Na1+2Mg2+Al3+F7 is from Knop et al. (1982[Knop, O., Cameron, T. S. & Jochem, K. (1982). J. Solid State Chem. 43, 213-221.]) based on single-crystal XRD. Sr2+2Sb5+2O7 was chosen because it is a stable weberite even under high pressure and neutron diffraction data are available (Knop et al., 1980[Knop, O., Demazeau, G. & Hagenmuller, P. (1980). Can. J. Chem. Rev. 58, 2221-2224.]; Groen & Ijdo, 1988[Groen, W. A. & IJdo, D. J. W. (1988). Acta Cryst. C44, 782-784.]). As for A21+B26+O7 compounds, only the crystal structure Ag2Te2O7 has been reported (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]). Table 4[link] lists the detailed valence information including all three anion types as well as cations by the empirical equation above using the bond-valence parameters (Brese & O'Keeffe, 1991[Brese, N. E. & O'Keeffe, M. (1991). Acta Cryst. B47, 192-197.]; Brown, 2002[Brown, D. I. (2002). The Chemical Bond in Inorganic Chemistry: The Bond Valence Model. Oxford University Press.]) and the bond lengths (rij) from the literature. The valence of X3 is close to its oxidation state in these compounds. It is worth noting that the discrepancy is small in Ag21+Te26+O7 for all three oxygen anions. Therefore, A21+B26+O7 weberites are possible.

Table 4
Examples of bond-valence sum for anions and cations by Brese and O'Keeffe

X1 in A3B tetrahedra X2 in A2B2 tetrahedra X3 outside the AB3 tetrahedra
Bonds rij (Å) vij [\sum]vij Bonds rij (Å) vij [\sum]vij Bonds rij (Å) vij [\sum]vij
Na2MgAlF7
Al-F 1.825 0.469 0.972 Mg-F 1.960 0.360 1.022 Mg-F 1.951 0.368 0.866
Na2-F 2.211 0.236 Al-F 1.793 0.512 Mg-F 1.951 0.368
Na1-F 2.423 0.133 Na2-F 2.549 0.095 Na2-F 2.689 0.065
Na1-F 2.423 0.133 Na1-F 2.749 0.055 Na2-F 2.689 0.065
Na11+: [\sum]vij = 0.779; Na21+: [\sum]vij = 0.981; Mg2+: [\sum]vij = 2.17; Al3+: [\sum]vij = 2.89
Sr2Sb2O7
Sb2-O 1.922 1.056 2.280 Sb1-O 1.971 0.926 2.151 Sb1-F 2.005 0.843 1.963
Sr2-O 2.363 0.515 Sb2-O 2.202 0.512 Sb1-F 2.005 0.843
Sr1-O 2.502 0.354 Sr1-O 2.833 0.145 Sr2-F 2.850 0.138
Sr1-O 2.502 0.354 Sr2-O 2.599 0.273 Sr2-F 2.850 0.138
Sr12+: [\sum]vij = 1.996; Sr22+: [\sum]vij = 2.40; Sb15+:[\sum]vij = 5.39; Sb25+: [\sum]vij = 5.34
Ag2Te2O7
Te2-O 1.825 1.281 1.864 Ag1-O 2.806 0.067 2.041 Ag2-O 2.806 0.067 1.983
Ag2-O 2.388 0.207 Ag2-O 2.549 0.134 Ag2-O 2.806 0.067
Ag1-O 2.465 0.168 Te1-O 1.997 0.806 Te1-O 1.946 0.925
Ag1-O 2.465 0.168 Te2-O 1.905 1.034 Te1-O 1.946 0.925
Ag11+: [\sum]vij = 0.939; Ag21+: [\sum]vij = 1.084; Te16+: [\sum]vij = 5.785; Te26+: [\sum]vij = 5.983
rij is the bond distance, vij is the bond valence.

Another significant difference is the formation of BX6 networks. All of the anions in weberites participate in the formation of BX6 octahedra, but only 6/7 of the anions in pyrochlores do. The BX6 octahedral network in both structures is fairly rigid. Therefore, in order to maintain the octahedral network it is difficult for the weberite to form vacancies at anion sites. By contrast, the pyrochlore structure tolerates X deficiency or paired A and X deficiencies relatively easily. Examples of such pyrochlore oxides are Bi1.5Zn0.92Nb1.5O6.92 and Tl2B2O6 (B = Nb, Ta and U), Tl2Os2O7-x and Pb2Os2O7-x (Nino, 2002[Nino, J. C. (2002). PhD dissertation. The Pennsylvania State University.]; Subramanian et al., 1983[Subramanian, M. A., Aravamudan, G. & Rao, G. V. S. (1983). Prog. Solid State Chem. 15, 55-143.]; Reading et al., 2002[Reading, J., Knee, C. S. & Weller, M. T. (2002). J. Mater. Chem. 12, 2376-2382.]). In addition, the substitution of small amounts of oxygen by F- may prevent the formation of weberites. For example, Ca2Sb2O7 pyrochlore transforms into weberite irreversibly above 973 K, but CaNaSb2O6F and Ca1.56Sb2O6.37F0.44 pyrochlores are stable (Aleshin & Roy, 1962[Aleshin, E. & Roy, R. (1962). J. Am. Ceram. Soc. 45, 18-25.]; Aia et al., 1963[Aia, M. A., Mooney, R. W. & Hoffman, C. W. W. (1963). J. Electrochem. Soc. 110, 1048-1054.]).

2.3. Weberite-like structures

The weberite structures show a wide variety of different modifications including monoclinic and trigonal variants. Grey et al. (2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]) proposed the use of the nomenclature of the International Mineralogical Association Commission New Minerals and Mineral Names (IMA-CNMMN), which was initially approved for zirconolite CaZrTi2O7 (Bayliss et al., 1989[Bayliss, P., Mazzi, F., Munno, R. & White, T. J. (1989). Mineral. Mag. 53, 565-569.]). As discussed above, the basic building unit is a slab formed by one A3B and one AB3 layer. The differences between weberites are the crystal system and the number of slabs (N) in a unit cell. A notation which combines N and the first letter of the crystal system is used to indicate different weberites. For example, the notation of the classic orthorhombic weberite is 2O because it has two slabs in a unit cell. The reported weberites include 2O, 2M, 3T, 4M, 5M, 6M, 6T, 7M and 8O. Tables 5[link] and 6[link] list different types of A2B2F7 and A2B2O7 weberites, respectively. (NaCu)Cu2F7 (or NaCu3F7, space group C2/c) and (Ca0.5Ln1.5)(Ca0.5Sb1.5)O7 (or CaLn1.5Sb1.5O7, space group I2/m11, Ln = La, Pr, Nd and Y) are special 2M weberites and more like pseudo-2O weberites for they maintain the structural feature of 2O rather than 2M, as will be discussed later. Ca2Ta2O7-based compounds are important in the weberite family since for N > 4, only Ca2Ta2O7-based compounds have been reported. Grey and co-workers (Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.], 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.], 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]) have shown that Ca2Ta2O7 compounds can crystallize into 3T, 4M, 5M, 6T, 6M and 7M by different doping or synthesis methods and later Ebbinghaus et al. (2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]) also synthesized an 8O Ca2Ta2O7 single crystal using the optical floating zone method.

Table 5
List of A2B2F7 weberites

    Space   Lattice parameters       Properties
Type   group Z a (Å) b (Å) c (Å) [beta] (°) RA (Å) RB (Å) RA/RB investigated
2O Na2MgCrF7 (Chassain, 1969[Chassain, J. (1969). C. R. Hebd. Seances Acad. Sci. C, 268, 2188.]) Imma 4 7.39 7.15 10.20   1.18 0.667 1.768  
2O Na2MgGaF7 (Chassain, 1969[Chassain, J. (1969). C. R. Hebd. Seances Acad. Sci. C, 268, 2188.]) Imma 4 7.42 7.16 10.16   1.18 0.67 1.761  
2O Na2MgScF7 (Chassain, 1969[Chassain, J. (1969). C. R. Hebd. Seances Acad. Sci. C, 268, 2188.]) Imma 4 7.55 7.34 10.43   1.18 0.733 1.611  
2O Na2MgVF7 (Chassain, 1969[Chassain, J. (1969). C. R. Hebd. Seances Acad. Sci. C, 268, 2188.]) Imma 4 7.45 7.24 10.30   1.18 0.68 1.735  
2O Na2NiFeF7 (Cosier et al., 1970[Cosier, R., Wise, A., Tressaud, A., Grannec, J., Olazcuag, R. & Portier, J. (1970). C. R. Hebd. Seances Acad. Sci. C, 271, 142-145.]; Laligant et al., 1989[Laligant, Y., Calage, Y., Heger, G., Pannetier, J. & Ferey, G. (1989). J. Solid State Chem. 78, 66-77.]; Thompson et al., 1992[Thompson, G. R., Pankhurst, Q. A. & Johnson, C. E. (1992). J. Magn. Magn. Mater. 104, 893-894.]) Imma 4 7.2338 (3) 10.3050 (3) 7.4529 (3)   1.18 0.62 1.903 Magnetic
2O Na2NiAlF7 (Tressaud et al., 1974[Tressaud, A., Dance, J. M., Portier, J. & Hagenmul, P. (1974). Mater. Res. Bull. 9, 1219-1226.]; Heger, 1973[Heger, G. (1973). Int. J. Magn. 5, 119-124.]) Imma 4 7.31 (2) 7.07 (2) 10.04 (2)   1.18 0.603 1.959 Magnetic
2O Na2NiCoF7 (Tressaud et al., 1974[Tressaud, A., Dance, J. M., Portier, J. & Hagenmul, P. (1974). Mater. Res. Bull. 9, 1219-1226.]; Cosier et al., 1970[Cosier, R., Wise, A., Tressaud, A., Grannec, J., Olazcuag, R. & Portier, J. (1970). C. R. Hebd. Seances Acad. Sci. C, 271, 142-145.]) Imma 4 7.40 (2) 7.20 (2) 10.24 (2)   1.18 0.618 1.911 Magnetic
2O Na2NiCrF7 (Hansler & Rudorff, 1970[Hansler, R. & Rudorff, W. (1970). Z. Naturforsch. B, 25, 1306.]; Laligant, Ferey et al., 1987[Laligant, Y., Ferey, G., Heger, G. & Pannetier, J. (1987). Z. Anorg. Allg. Chem. 553, 163-171.]) Imma 4 7.183 (1) 10.224 (1) 7.414 (1)   1.18 0.653 1.808 Magnetic
2O Na2CoGaF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3011 (6) 10.5436 (9) 7.3845 (7)   1.18 0.645 1.829  
2O Na2CoInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.4032 (6) 10.3892 (8) 7.5302 (9)   1.18 0.745 1.5849  
2O Na2CoScF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.431 (1) 10.546 (1) 7.544 (1)   1.18 0.718 1.645  
2O Na2MnTIF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.371 (1) 10.369 (3) 7.603 (1)   1.18 0.67 1.761  
2O Na2NiGaF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]; Dahlke et al., 1998[Dahlke, P., Peschel, B. & Babel, D. (1998). Z. Anorg. Allg. Chem. 624, 1003-1010.]) Imma 4 7.1805 (7) 10.2433 (9) 7.4256 (7)   1.18 0.655 1.802  
2O Na2NiInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3632 (5) 10.3490 (7) 7.5274 (6)   1.18 0.745 1.584  
2O Na2NiScF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3116 (7) 10.3278 (9) 7.4779 (7)   1.18 0.718 1.645  
2O Na2MgTIF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3756 (8) 10.418 (1) 7.5496 (8)   1.18 0.695 1.698  
2O Na2ZnFeF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.281 (1) 10.446 (2) 7.459 (1)   1.18 0.645 1.829  
  Na2ZnGaF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.2494 (7) 10.3283 (8) 7.3582 (6)   1.18 0.68 1.735  
2O Na2ZnInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.4077 (6) 10.4759 (9) 7.5732 (6)   1.18 0.77 1.532  
2O Na2ZnTIF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.338 (1) 10.300 (2) 7.539 (1)   1.18 0.705 1.674  
2O Na2MgFeF7 (Pankhurst et al., 1991[Pankhurst, Q. A., Johnson, C. E. & Wanklyn, B. M. (1991). J. Magn. Magn. Mater. 97, 126-130.]; Chassain, 1969[Chassain, J. (1969). C. R. Hebd. Seances Acad. Sci. C, 268, 2188.]) Imma 4 7.49 7.25 10.26   1.18 0.635 1.858 Magnetic
2O Na2MgAlF7 (Byström, 1944[Byström, A. (1944). Ark. Kemi Miner. Och Geol. B, 18, 1-7.]; Knop et al., 1982[Knop, O., Cameron, T. S. & Jochem, K. (1982). J. Solid State Chem. 43, 213-221.]) Imma 4 7.501 (1) 9.968 (2) 7.285 (1)   1.18 0.618 1.911  
2O Na2ZnAlF7 (Dahlke & Babel, 1994[Dahlke, P. & Babel, D. (1994). Z. Anorg. Allg. Chem. 620, 1692-1697.]) Imma 4 7.092 (1) 10.092 (1) 7.337 (1)   1.18 0.6375 1.851  
2O Ag2CuMnF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.5006 (9) 10.5025 (9) 7.6452 (8)   1.28 0.655 1.954  
2O Ag2CoAlF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.252 10.16 7.601   1.28 0.603 2.124  
2O Ag2CoGaF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.313 10.35 7.678   1.28 0.655 1.954  
2O Ag2CoInF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.544 10.72 7.851   1.28 0.745 1.718  
2O Ag2CoScF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.497 10.64 7.789   1.28 0.718 1.784  
2O Ag2MnAlF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.360 10.32 7.601   1.28 0.593 2.160  
2O Ag2MnGaF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.465 10.62 7.787   1.28 0.645 1.986  
2O Ag2MnScF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.634 10.78 7.802   1.28 0.708 1.809  
2O Ag2NiGaF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.255 10.28 7.650   1.28 0.655 1.954  
2O Ag2NiScF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.463 10.54 7.771   1.28 0.718 1.785  
2O Ag2MgAlF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.197 10.01 7.571   1.28 0.618 2.073  
2O Ag2MgGaF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.257 10.21 7.664   1.28 0.67 1.910  
2O Ag2MgInF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.495 10.62 7.832   1.28 0.76 1.684  
2O Ag2MgScF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.427 10.52 7.782   1.28 0.733 1.747  
2O Ag2CuAlF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.109 10.22 7.684   1.28 0.623 2.056  
2O Ag2CuGaF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.200 10.34 7.755   1.28 0.675 1.896  
2O Ag2ZnAlF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.237 10.14 7.590   1.28 0.6375 2.008  
2O Ag2ZnGaF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.303 10.32 7.688   1.28 0.68 1.882  
2O Ag2ZnInF7 (Koch et al., 1982[Koch, J., Hebecker, C. & John, H. (1982). Z. Naturforsch. B, 37, 1659-1660.]) Imma 4 7.531 10.71 7.841   1.28 0.77 1.662  
2O Ag2CoCrF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.349 (1) 10.376 (1) 7.683 (1)   1.28 0.653 1.962  
2O Ag2CoFeF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3711 (8) 10.437 (1) 7.7145 (8)   1.28 0.62 2.065  
2O Ag2MnFeF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.490 (1) 10.612 (2) 7.731(1)   1.28 0.61 2.098  
2O Ag2MnInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.6747 (8) 10.856 (1) 7.8641 (7)   1.28 0.735 1.742  
2O Ag2MgCrF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.2746 (6) 10.3128 (9) 7.7060 (7)   1.28 0.668 1.918  
2O Ag2MgFeF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3100 (7) 10.335 (1) 7.6972 (9)   1.28 0.635 2.016  
2O Ag2MgTIF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.2506 (9) 10.362 (2) 7.497 (1)   1.28 0.695 1.842  
2O Ag2CuCrF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.2103 (6) 10.454 (1) 7.7871 (8)   1.28 0.673 1.903  
2O Ag2CuFeF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.2435 (9) 10.474 (2) 7.769 (1)   1.28 0.64 2  
2O Ag2CuInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3461 (6) 10.7501 (9) 7.9098 (6)   1.28 0.765 1.673  
2O Ag2ZnCrF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.3165 (7) 10.362 (1) 7.6877 (8)   1.28 0.678 1.889  
2O Ag2ZnFeF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.359 (1) 10.409 (2) 7.706 (1)   1.28 0.645 1.985  
2O Ag2ZnMnF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.]) Imma 4 7.408 (1) 10.503 (1) 7.6972 (9)   1.28 0.66 1.939  
2O Ag2NiAlF7 (Dance et al., 1974[Dance, J. M., Grannec, J., Jacoboni, C. & Tressaud, A. (1974). C. R. Hebd. Seances Acad. Sci. C, 279, 601-604.]) Imma 4 7.564 (6) 7.210 (6) 10.139 (15)   1.28 0.603 2.124 Magnetic
2O Ag2NiCrF7 (Dance et al., 1974[Dance, J. M., Grannec, J., Jacoboni, C. & Tressaud, A. (1974). C. R. Hebd. Seances Acad. Sci. C, 279, 601-604.]) Imma 4 7.673 (6) 7.305 (6) 10.285 (9)   1.28 0.653 1.962 Magnetic
2O Ag2NiFeF7 (Dance et al., 1974[Dance, J. M., Grannec, J., Jacoboni, C. & Tressaud, A. (1974). C. R. Hebd. Seances Acad. Sci. C, 279, 601-604.]) Imma 4 7.692 (6) 7.345 (6) 10.345 (9)   1.28 0.62 2.065 Magnetic
2O Ag2NiInF7 (Dance et al., 1974[Dance, J. M., Grannec, J., Jacoboni, C. & Tressaud, A. (1974). C. R. Hebd. Seances Acad. Sci. C, 279, 601-604.]) Imma 4 7.822 (6) 7.499 (6) 10.622 (9)   1.28 0.745 1.718 Magnetic
2O-II Na2CuInF7 (Ruchaud et al., 1992[Ruchaud, N., Grannec, J., Gravereau, P., Nunez, P., Tressaud, A., Massa, W., Frenzen, G. & Babel, D. (1992). Z. Anorg. Allg. Chem. 610, 67-74.]) Pmnb 4 7.318 10.602 7.712   1.18 0.765 1.542 Magnetic
2O-II Na2CuCrF7 (Kummer et al., 1988[Kummer, S., Massa, W. & Babel, D. (1988). Z. Naturforsch. B, 43, 694-701.]; Koch & Hebecker, 1985[Koch, J. & Hebecker, C. (1985). Naturwissenschaften, 72, 431-432.]) Pmnb 4 7.100 (1) 10.338 (1) 7.518 (1)   1.18 0.673 1.755  
2O-II Na2CuScF7 (Dahlke & Babel, 1994[Dahlke, P. & Babel, D. (1994). Z. Anorg. Allg. Chem. 620, 1692-1697.]) Pmnb 4 7.260 (1) 10.534 (1) 7.658 (1)   1.18 0.7375 1.6  
2O-III Na2NiInF7 (Frenzen et al., 1992[Frenzen, G., Massa, W., Babel, D., Ruchaud, N., Grannec, J., Tressaud, A. & Hagenmuller, P. (1992). J. Solid State Chem. 98, 121-127.]) Pmnb 4 7.356 10.334 7.523   1.18 0.745 1.584 Magnetic
2O-III Na2MgInF7 (Caramanian et al., 2001[Caramanian, A., Souron, J. P., Gredin, P. & de Kozak, A. (2001). J. Solid State Chem. 159, 234-238.]) Pnma 4 10.435 (1) 7.345 (1) 7.533 (1)   1.18 0.76 1.553 Magnetic
2M Na2CuGaF7 (Ruchaud et al., 1992[Ruchaud, N., Grannec, J., Gravereau, P., Nunez, P., Tressaud, A., Massa, W., Frenzen, G. & Babel, D. (1992). Z. Anorg. Allg. Chem. 610, 67-74.]) C2/c 8 12.325 (5) 7.318 (1) 12.780 (5) 109.29 (2) 1.18 0.675 1.748 Magnetic
2M and 4M Na2CuFeF7 (Welsch & Babel, 1992[Welsch, M. & Babel, D. (1992). Z. Naturforsch. B, 47, 685-692.]; Yakubovich et al., 1990[Yakubovich, O. V., Urusov, V. S., Frenzen, G., Massa, W. & Babel, D. (1990). Eur. J. Solid State Inorg. Chem. 27, 467-475.]; Kummer et al., 1988[Kummer, S., Massa, W. & Babel, D. (1988). Z. Naturforsch. B, 43, 694-701.]) C2/c 8 12.46 (1) 7.363 (8) 12.93 (1) 109.36 (7) 1.18 0.64 1.844  
C2/c 16 12.444 (2) 7.343 (1) 24.672 (5) 99.27 (3)      
3T Na2MnAlF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.])   6 7.2854 (4)   17.844 (1)   1.18 0.593 1.992  
3T Na2MnInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.])   6 7.6006 (3)   18.617 (1)   1.18 0.735 1.605  
3T Na2MnScF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.])   6 7.5442 (4)   18.479 (1)   1.18 0.708 1.668  
3T Na2MnGaF7 (Courbion et al., 1988[Courbion, G., Ferey, G., Holler, H. & Babel, D. (1988). Eur. J. Solid State Inorg. Chem. 25, 435-447.]) P3121 6 7.421 (3)   18.166 (6)   1.18 0.645 1.829  
3T Na2MnCrF7 (Courbion et al., 1988[Courbion, G., Ferey, G., Holler, H. & Babel, D. (1988). Eur. J. Solid State Inorg. Chem. 25, 435-447.]) P3121 6 7.401 (1)   18.091 (2)   1.18 0.6425 1.837  
3T Na2MnVF7 (Peschel & Babel, 1997[Peschel, B. & Babel, D. (1997). Z. Anorg. Allg. Chem. 623, 1614-1620.]) P3121 6 7.467   18.216   1.18 0.655 1.802  
3T Na2MnFeF7 (Verscharen & Babel, 1978[Verscharen, W. & Babel, D. (1978). J. Solid State Chem. 24, 405-421.]; Cosier et al., 1970[Cosier, R., Wise, A., Tressaud, A., Grannec, J., Olazcuag, R. & Portier, J. (1970). C. R. Hebd. Seances Acad. Sci. C, 271, 142-145.]; Boireau et al., 1993[Boireau, A., Gravereau, P., Dance, J. M., Tressaud, A., Hagenmuller, P., Soubeyroux, J. L., Welsch, M. & Babel, D. (1993). Mater. Res. Bull. 28, 27-38.]) P3121 6 7.488 (2)   18.257 (6)   1.18 0.61 1.934 Magnetic
3T Ag2MnInF7 (Koch & Hebecker, 1988[Koch, J. & Hebecker, C. (1988). Naturwissenschaften, 75, 360.])   6 7.751 (1)   18.838 (4)   1.28 0.735 1.741  
4M Na2CoAlF7 (Boireau et al., 1993[Boireau, A., Gravereau, P., Dance, J. M., Tressaud, A., Hagenmuller, P., Soubeyroux, J. L., Welsch, M. & Babel, D. (1993). Mater. Res. Bull. 28, 27-38.]; Gravereau et al., 1992[Gravereau, P., Boireau, A., Dance, J. M., Trut, L. & Tressaud, A. (1992). Acta Cryst. C48, 2108-2111.]) C2/c 16 12.378 (4) 7.210 (3) 24.019 (9) 99.67 (2) 1.18 0.613 1.927 Magnetic
4M Na2ZnGaF7 (Dahlke et al., 1998[Dahlke, P., Peschel, B. & Babel, D. (1998). Z. Anorg. Allg. Chem. 624, 1003-1010.]) C2/c 16 12.519 7.303 24.353 99.74 1.18 0.68 1.735  
4M Na2ZnFeF7 (Dahlke et al., 1998[Dahlke, P., Peschel, B. & Babel, D. (1998). Z. Anorg. Allg. Chem. 624, 1003-1010.]) C2/c 16 12.610 7.359 24.538 99.70 1.18 0.645 1.829  
4M Na2FeVF7 (Peschel et al., 1995[Peschel, B., Molinier, M. & Babel, D. (1995). Z. Anorg. Allg. Chem. 621, 1573-1581.]) C2/c 16 12.710 (3) 7.429 (1) 24.716 (5) 100.03 (3) 1.18 0.625 1.888  
4M Na2CoVF7 (Peschel et al., 1995[Peschel, B., Molinier, M. & Babel, D. (1995). Z. Anorg. Allg. Chem. 621, 1573-1581.]) C2/c 16 12.703 (5) 7.391 (3) 24.651 (5) 100.02 (3) 1.18 0.645 1.829  
4M Na2FeCrF7 (Peschel et al., 1995[Peschel, B., Molinier, M. & Babel, D. (1995). Z. Anorg. Allg. Chem. 621, 1573-1581.]) C2/c 16 12.625 (3) 7.391 (1) 24.695 (5) 99.93 (3) 1.18 0.6125 1.927  
4M Na2Fe2F7(Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]; Cosier et al., 1970[Cosier, R., Wise, A., Tressaud, A., Grannec, J., Olazcuag, R. & Portier, J. (1970). C. R. Hebd. Seances Acad. Sci. C, 271, 142-145.]; Tressaud et al., 1974[Tressaud, A., Dance, J. M., Portier, J. & Hagenmul, P. (1974). Mater. Res. Bull. 9, 1219-1226.]) C2/c 16 12.676 (3) 7.422 (1) 24.710 (5) 99.97 (3) 1.18 0.58 2.034 Magnetic
4M Na2CoFeF7 (Welsch & Babel, 1992[Welsch, M. & Babel, D. (1992). Z. Naturforsch. B, 47, 685-692.]; Cosier et al., 1970[Cosier, R., Wise, A., Tressaud, A., Grannec, J., Olazcuag, R. & Portier, J. (1970). C. R. Hebd. Seances Acad. Sci. C, 271, 142-145.]; Tressaud et al., 1974[Tressaud, A., Dance, J. M., Portier, J. & Hagenmul, P. (1974). Mater. Res. Bull. 9, 1219-1226.]; Boireau et al., 1993[Boireau, A., Gravereau, P., Dance, J. M., Tressaud, A., Hagenmuller, P., Soubeyroux, J. L., Welsch, M. & Babel, D. (1993). Mater. Res. Bull. 28, 27-38.]) C2/c 16 12.622 (10) 7.360 (4) 24.516 (20) 99.71 (5) 1.18 0.62 1.903 Magnetic
4M Na2CoCrF7 (Boireau et al., 1993[Boireau, A., Gravereau, P., Dance, J. M., Tressaud, A., Hagenmuller, P., Soubeyroux, J. L., Welsch, M. & Babel, D. (1993). Mater. Res. Bull. 28, 27-38.]; Peschel et al., 1995[Peschel, B., Molinier, M. & Babel, D. (1995). Z. Anorg. Allg. Chem. 621, 1573-1581.]) C2/c 16 12.578 (3) 7.335 (1) 24.415 (5) 99.64 (3) 1.18 0.653 1.808 Magnetic
4M Na2FeAlF7 (Dahlke et al., 1998[Dahlke, P., Peschel, B. & Babel, D. (1998). Z. Anorg. Allg. Chem. 624, 1003-1010.]; Tressaud et al., 1974[Tressaud, A., Dance, J. M., Portier, J. & Hagenmul, P. (1974). Mater. Res. Bull. 9, 1219-1226.]) C2/c 16 12.426 7.278 24.206 99.99 1.18 0.563 2.098 Magnetic

Table 6
List of weberite oxides with RA/RB and the relative ionicity of the A-O bond

    Lattice parameters         Properties
Type   a (Å) b (Å) c (Å) [beta] (°) RA (Å) RB (Å) RA/RB IA/(IB + IA) investigated
2O Ca2Sb2O7 (Bystrom, 1945[Bystrom, A. (1945). Ark. Kemi Miner. Och Geol. A, 18, 1-8.]; Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]; Sato et al., 2002[Sato, J., Saito, N., Nishiyama, H. & Inoue, Y. (2002). J. Photochem. Photobiol. A, 148, 85-89.]; Lin et al., 2006[Lin, X. P., Huang, F. Q., Wang, W. D., Wang, Y. M., Xia, Y. J. & Shi, J. L. (2006). Appl. Catal. Gen. 313, 218-223.]) 7.28 7.44 10.18   1.12 0.6 1.86667 0.60636 Ferroelectric and photocatalytic
  7.3060 7.4627 10.2263  
  7.2900 10.2000 7.4500  
2O Cd2Sb2O7 (Brisse et al., 1972[Brisse, F., Stewart, D. J., Seidl, V. & Knop, O. (1972). Can. J. Chem. 50, 3648.]; Bystrom, 1945[Bystrom, A. (1945). Ark. Kemi Miner. Och Geol. A, 18, 1-8.]) 7.21 7.33 10.14   0.9 0.6 1.5 0.56093  
2O Sr2Sb2O7 (Knop et al., 1980[Knop, O., Demazeau, G. & Hagenmuller, P. (1980). Can. J. Chem. Rev. 58, 2221-2224.]; Groen & Ijdo, 1988[Groen, W. A. & IJdo, D. J. W. (1988). Acta Cryst. C44, 782-784.]; Lin et al., 2006[Lin, X. P., Huang, F. Q., Wang, W. D., Wang, Y. M., Xia, Y. J. & Shi, J. L. (2006). Appl. Catal. Gen. 313, 218-223.]; Sato et al., 2002[Sato, J., Saito, N., Nishiyama, H. & Inoue, Y. (2002). J. Photochem. Photobiol. A, 148, 85-89.]) 7.452 7.687 10.381   1.26 0.6 2.1 0.61038 Ferroelectric and photocatalytic
  7.4557 (2) 10.3708 (3) 7.6860 (1)  
  7.4557 10.3708 7.6860  
2O Pb2Sb2O7 (Ivanov & Zavodnik, 1990[Ivanov, S. A. & Zavodnik, V. E. (1990). Kristallografiya, 35, 842-846.]; Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]) 7.484 (1) 7.857 (1) 10.426 (2)   1.29 0.6 2.15 0.54792 Ferroelectric
7.4774 7.8549 10.4250  
2O Ca2Os2O7 (Reading et al., 2002[Reading, J., Knee, C. S. & Weller, M. T. (2002). J. Mater. Chem. 12, 2376-2382.]; Weller et al., 2003[Weller, M. T., Reading, J. & Knee, C. S. (2003). Solid State Chem. 90-91, 201-205.]) 7.2104 (2) 10.1211 (3) 7.3813 (2)   1.12 0.575 1.94783 0.55518 Electronic
2O Sr2Bi2O7 (Knop et al., 1980[Knop, O., Demazeau, G. & Hagenmuller, P. (1980). Can. J. Chem. Rev. 58, 2221-2224.]) 7.70 7.91 10.58   1.26 0.76 1.65789 0.58305  
2O Ba2U2O7 (Cordfunke & Ijdo, 1988[Cordfunke, E. H. P. & Ijdo, D. J. W. (1988). J. Phys. Chem. Solids, 49, 551-554.]) 8.1665 (15) 11.3081 (21) 8.1943 (16)   1.42 0.76 1.86842 0.5232  
2O Na2Te2O7 (Knop & Demazeau, 1981[Knop, O. & Demazeau, G. (1981). J. Solid State Chem. 39, 94-99.]) 7.233 (5) 10.104 (7) 7.454 (5)   1.18 0.56 2.10714 0.64905  
2O Ag2Te2O7 (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]) 7.266 (2) 10.1430 (9) 7.6021 (17)   1.28 0.56 2.28571 0.60811  
2O CaPbSb2O7 (Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]) 7.3577 7.5362 10.3521   1.205 0.6 2.00833 0.59219 Ferroelectric
2O DyNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.26 (6) 7.41 (5) 10.20 (6)   1.1035 0.6 1.83917 0.6051  
2O GdNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.29 (1) 7.47 (0) 10.20 (7)   1.1165 0.6 1.86083 0.60468  
2O EuNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.30 (0) 7.47 (2) 10.21 (4)   1.123 0.6 1.87167 0.6088  
2O SmNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.30 (8) 7.45 (7) 10.22 (7)   1.1295 0.6 1.8825 0.60636  
2O NdNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.32 (7) 7.49 (2) 10.24 (2)   1.1445 0.6 1.9075 0.60636  
2O PrNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.33 (7) 7.50 (6) 10.25 (5)   1.153 0.6 1.92167 0.58322  
2O LaNaSb2O7 (Desgardin et al., 1977[Desgardin, G., Robert, C. & Raveau, B. (1977). J. Inorg. Nucl. Chem. 39, 907-908.]) 7.37 (8) 7.50 (1) 10.28 (8)   1.17 0.6 1.95 0.60594  
2O KLuSb2O7 (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]; Sych, Kabanova & Andreeva, 1988[Sych, A. M., Kabanova, M. I. & Andreeva, S. G. (1988). Zh. Neorg. Khim. 33, 2756-2760.]; Sych, Kabanova, Garbuz & Kovalenko, 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) 7.23 10.23 7.39   1.2435 0.6 2.0725 0.60759  
2O KYbSb2O7 (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]; Sych, Kabanova & Andreeva, 1988[Sych, A. M., Kabanova, M. I. & Andreeva, S. G. (1988). Zh. Neorg. Khim. 33, 2756-2760.]; Sych, Kabanova, Garbuz & Kovalenko, 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) 7.24 10.25 7.40   1.2475 0.6 2.07917 0.61077  
2O KErSb2O7 (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]; Sych, Kabanova & Andreeva, 1988[Sych, A. M., Kabanova, M. I. & Andreeva, S. G. (1988). Zh. Neorg. Khim. 33, 2756-2760.]; Sych, Kabanova, Garbuz & Kovalenko, 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) 7.26 10.25 7.41   1.257 0.6 2.095 0.6088  
2O KHoSb2O7 (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]; Sych, Kabanova & Andreeva, 1988[Sych, A. M., Kabanova, M. I. & Andreeva, S. G. (1988). Zh. Neorg. Khim. 33, 2756-2760.]; Sych, Kabanova, Garbuz & Kovalenko, 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) 7.26 10.25 7.42   1.2625 0.6 2.10417 0.6092  
2O KYSb2O7 (Klein et al., 2006[Klein, W., Curda, J., Peters, E. M. & Jansen, M. (2006). Z. Anorg. Allg. Chem. 632, 1508-1513.]; Sych, Kabanova & Andreeva, 1988[Sych, A. M., Kabanova, M. I. & Andreeva, S. G. (1988). Zh. Neorg. Khim. 33, 2756-2760.]; Sych, Kabanova, Garbuz & Kovalenko, 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) 7.26 10.25 7.43   1.2645 0.6 2.1075 0.6088  
2O KYTa2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.78 10.82 7.50   1.2645 0.64 1.97578 0.5324  
2O KDyTa2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.80 10.88 7.70   1.2685 0.64 1.98203 0.53282  
2O KGdTa2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.84 10.86 7.72   1.2815 0.64 2.00234 0.5324  
2O KSmTa2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.86 10.82 7.76   1.2945 0.64 2.02266 0.53405  
2O NaDyV2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.53 10.94 7.44   1.1035 0.54 2.04352 0.53496  
2O NaGdV2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.56 10.88 7.46   1.1165 0.54 2.06759 0.54352  
2O NaSmV2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.58 10.86 7.48   1.1295 0.54 2.09167 0.54526  
2O NaNdV2O7 (Gade & Chincholkar, 1979[Gade, K. & Chincholkar, V. S. (1979). J. Chem. Soc. Dalton Trans. p. 1959.]) 7.62 10.82 7.50   1.1445 0.54 2.11944 0.54526  
2O NaSrSbTeO7 (Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]) Not reported       1.22 0.58 2.10345 0.62891  
2O NaCdSbTeO7 (Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]) Not reported       1.04 0.58 1.7931 0.60778  
2O NdCaSbTeO7 (Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]) Not reported       1.15 0.58 1.9828 0.62694  
2O Na0.5Cd1.5(Fe0.5Te1.5)O7 (Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]) 7.131 7.317 10.183   1.12 0.5575 2.00897 0.59584  
2O Na0.5Ca1.5(Fe0.5Te1.5)O7 (Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]) Not reported       1.135 0.5575 2.0358744 0.626874  
2O Ba0.5Ca1.5(Fe0.5Te1.5)O7 (Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]) 7.176 7.464 10.140   1.195 0.5725 2.08734 0.627674  
2M CaLa1.5Sb1.5O7 (Au et al., 2007[Au, Y. S., Fu, W. T. & Ijdo, D. J. W. (2007). J. Solid State Chem. 180, 3166-3171.]) 7.5753 (3) 10.6870 (5) 7.5482 (3) 90.346 (3) 1.15 0.7 1.6428571 0.57329  
2M CaPr1.5Sb1.5O7 (Au et al., 2007[Au, Y. S., Fu, W. T. & Ijdo, D. J. W. (2007). J. Solid State Chem. 180, 3166-3171.]) 7.5188 (3) 10.6111 (4) 7.4952 (2) 90.315 (2) 1.1245 0.7 1.6064286 0.569502  
2M CaNd1.5Sb1.5O7 (Au et al., 2007[Au, Y. S., Fu, W. T. & Ijdo, D. J. W. (2007). J. Solid State Chem. 180, 3166-3171.]) 7.5019 (2) 10.5890 (3) 7.4770 (2) 90.298 (2) 1.11175 0.7 1.5882143 0.569502  
2M CaY1.5Sb1.5O7 (Au et al., 2007[Au, Y. S., Fu, W. T. & Ijdo, D. J. W. (2007). J. Solid State Chem. 180, 3166-3171.]) 7.3905 (1) 10.4563 (2) 7.3894 (1) 90.049 (1) 1.04425 0.7 1.4917857 0.55821  
3T Ca1.5Mn0.5Sb2O7 (Bonazzi & Bindi, 2007[Bonazzi, P. & Bindi, L. (2007). Am. Mineral. 92, 947-953.]) 7.282 (2)   17.604 (4)   1.04 0.6 1.73333 0.59369  
3T Mn2Sb2O7 (Scott, 1990[Batanov, S. S. (1975). Zh. Neorg. Khim. 20, 2595-2600.]) 7.191   17.402   0.96 0.6 1.6 0.540094  
3T Ca1.92Ta1.92Nd0.08Zr0.08O7 (Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]) 7.356 (1)   18.116 (1)   1.11956 0.61728 1.8136988 0.529853  
3T Ca2Ta2O7 (Cava et al., 1998[Cava, R. J., Krajewski, J. J. & Roth, R. S. (1998). Mater. Res. Bull. 33, 527-532.]; Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.]) 7.355 (1)   18.09 (1)   1.12 0.64 1.75 0.52985 Dielectric
4M Ca1.92Ta1.92Nd0.08Zr0.08O7 (Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]) 12.761 (1) 7.358 (1) 24.565 (1) 100.17 1.12 0.64 1.75 0.52922  
5M Ca1.8Ta1.8Sm0.24Ti0.17O7 (Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]) 12.763 (1) 7.130 (1) 30.190 (1) 94.09 (1) 1.13635 0.62743 1.81113 0.52523  
5M Ca2Ta1.8Nb0.2O7 (Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]) 12.749 (1) 7.347 (1) 30.23 (1) 94.23 (1) 1.12 0.64 1.75 0.52866 Dielectric
6M Ca2Ta2O7 (Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.]) 7.348 (3) 12.727 (3) 36.44 (5) 95.9 (1) 1.12 0.64 1.75 0.52985  
6T Ca1.89Ta1.86Sm0.16Ti0.1O7 (Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]) 7.353 (1)   36.264 (1)   1.1792 0.63505 1.85686 0.52649  
7M Ca2Ta1.9Nb0.1O7 (Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]) 12.714 (1) 7.370 (1) 42.45 (1) 95.75 (1) 1.12 0.64 1.75 0.52925 Dielectric
8O Ca2Ta2O7 (Ebbinghaus et al., 2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]) 7.3690 (2) 12.7296 (3) 48.263 (1)   1.12 0.64 1.75 0.52985 Dielectric and optical
R is the ionic radius from Shannon (1976[Shannon, R. D. (1976). Acta Cryst. A32, 751-767.]), [chi] is the electronegativity from Allred & Rochow (1958[Allred, A. L. & Rochow, E. G. (1958). J. Inorg. Nucl. Chem. 5, 264-268.]), I is the bond ionicity.

A significant difference between 2O and non-2O weberites is that the AB3 and A3B layers are parallel to the (011) planes for 2O and parallel to the (001) planes for other weberites, except for NaCu3F7 and CaLn1.5Sb1.5O7 (Ln = La, Pr, Nd and Y). The formula unit (Z) of NaCu3F7 and CaLn1.5Sb1.5O7 is also consistent with 2O weberites, four rather than eight, the latter the formula unit for other 2M weberites. As in §2.2[link] the lattice parameters of 2O weberites are approximately 21/2a, 2a and 21/2a (a [asymptotically equal to] 5 Å, the lattice parameters for fluorite). The lattice parameters of 2M weberites are nearly 61/2a, 21/2a and 61/2a. The [[0\bar 11]], [100] and [011] vectors of 2O become [100], [010] and [001] of 2M. The lattice parameter difference between 2M, 4M, 5M, 6M and 7M is mainly on the c axis. The lattice parameters for nM (n = 2, 4, 5 and 7) are approximately 61/2a, 21/2a and [n(6)1/2/2]a and they are nearly 21/2a, 61/2a and 3(6)1/2/a for 6M (Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.], 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.], 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]). The 8O weberite is closely related to a monoclinic variant rather than 2O in both the orientation of the AB3 and A3B layers and the lattice parameters. The lattice parameters are nearly 21/2a, 61/2a and 4(6)1/2/a. As for 3T, the [100], [-0.5,-0.5, 0.5] and [012] vectors in 2O are transformed into the basal vectors. The resulting lattice parameters are approximately 21/2a, 21/2a and 2(3)1/2a. The relationship of 2O, 2M and 3T weberites is shown in Fig. 10[link]. Meanwhile, the lattice parameters of 6T are approximately 21/2a, 21/2a and 4(3)1/2a, just double the length of the basal vector in the c axis.

[Figure 10]
Figure 10
(a) Relationship between 2O, 2M and 3T weberites (origin at the A2 site); large spheres: A ions; small spheres: B ions; blue solid lines: multiple unit cells of 2O; red dotted lines: the unit cell of 2M; green dashed lines: the unit cell of 3T. (b) (001) plane of 3T (green dashed lines) and 2M [red dotted lines, also indicating (011) of 2O].

For 2O weberites there are two special types in which the body-center symmetry is lost. The first case is when Cu2+ is introduced into Na2B2+B3+F7 at B-1 sites such as Na2CuCrF7 and Na2CuInF7 (Kummer et al., 1988[Kummer, S., Massa, W. & Babel, D. (1988). Z. Naturforsch. B, 43, 694-701.]; Ruchaud et al., 1992[Ruchaud, N., Grannec, J., Gravereau, P., Nunez, P., Tressaud, A., Massa, W., Frenzen, G. & Babel, D. (1992). Z. Anorg. Allg. Chem. 610, 67-74.]). The common Jahn-Teller distortion (the CuF6 octahedra are elongated perpendicular to the B-1 chains) in CuF6 octahedra leads to the lowering of symmetry, while maintaining the orthorhombic lattice. The space group is reduced to Pmnb, a subgroup of Imma (Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]). Another case of losing I-centering symmetry happens when the ionic radius of B-2 is larger than that of B-1. In a classic 2O weberite structure the ionic radius of B-2 is equal to or smaller than that of B-1. When a larger B-2 ion appears in a weberite compound, the anions, which are shared by two B-1 octahedral neighbors, distort toward B-2 ions. As a result, the A-2 ions cannot hold eight-coordination and change to seven-coordination. The B-2 ion keeps octahedral coordination with a seventh anion relatively close to it. As in the case of Na2NiInF7, the distance between the distorted anion and B-2 (In3+) is only 1.3 times larger than the shortest In-F bond length in B-2 octahedra (Frenzen et al., 1992[Frenzen, G., Massa, W., Babel, D., Ruchaud, N., Grannec, J., Tressaud, A. & Hagenmuller, P. (1992). J. Solid State Chem. 98, 121-127.]). In a 2O weberite structure, the ratio of the two distances is higher, such as 1.97 in Na2MgAlF7 or 1.83 in Ca2Os2O7 (Wyckoff, 1963[Wyckoff, R. W. G. (1963). Crystal Structures. New York: Interscience Publishers.]; Reading et al., 2002[Reading, J., Knee, C. S. & Weller, M. T. (2002). J. Mater. Chem. 12, 2376-2382.]). The distortion of the anion excludes the I-centering of the structure and results in the space group Pnmb (Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]). The notation of 2O-II and 2O-III is used for the first and the second condition, respectively.

One extreme case for 2O-III is Ln2(B,Ln)O7 (or Ln3BO7, where Ln3+ is a rare-earth element, and B is Os5+, Re5+, Ru5+, Re5+, Mo5+, Ir5+, Sb5+, Nb5+ or Ta5+). B-2 ions are the same as A ions. As a result, the B-2 sites and A-2 sites are indistinguishable. The structure has an arrangement of BO6-LnO8 layers (much like weberites), but a different cation configuration with VII coordination between the layers (Fig. 11[link]). Due to the fact that this type of structure does not maintain the three-dimensional BO6 octahedral network, it is considered a weberite-type structure rather than the weberite structure, or sometimes it is reported as a La3NbO7-type structure (Rossell, 1979[Rossell, H. J. (1979). J. Solid State Chem. 27, 115-122.]; Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]; Rooksby & White, 1964[Rooksby, H. P. & White, E. A. D. (1964). J. Am. Ceram. Soc. 47, 94-96.]; Abe et al., 2004[Abe, R., Higashi, M., Zou, Z. G., Sayama, K., Abe, Y. & Arakawa, H. (2004). J. Phys. Chem. B, 108, 811-814.], 2006[Abe, R., Higashi, M., Sayama, K., Abe, Y. & Sugihara, H. (2006). J. Phys. Chem. B, 110, 2219-2226.]; Cai & Nino, 2007[Cai, L. & Nino, J. C. (2007). J. Eur. Ceram. Soc. 27, 3971-3976.]; Cai et al., 2007[Cai, L., Guzman, J., Perez, L. & Nino, J. C. (2007). Solid-State Chemistry of Inorganic Materials VI, Materials Research Society Symposium Proceeding, 998E, 0988-qq0901-0904.]; Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]; Groen et al., 1987[Groen, W. A., van Berkel, F. P. F. & IJdo, D. J. W. (1987). Acta Cryst. C43, 2262-2264.]; Wakeshima & Hinatsu, 2006[Wakeshima, M. & Hinatsu, Y. (2006). J. Solid State Chem. 179, 3575-3581.]; Nishimine et al., 2007[Nishimine, H., Doi, Y., Hinatsu, Y. & Sato, M. (2007). J. Ceram. Soc. Jpn, 115, 577-581.]; Gemmill et al., 2007[Gemmill, W. R., Smith, M. D. & zur Loye, H. C. (2007). J. Chem. Cryst. 37, 793-795.]; Khalifah et al., 1999[Khalifah, P., Erwin, R. W., Lynn, J. W., Huang, Q., Batlogg, B. & Cava, R. J. (1999). Phys. Rev. B, 60, 9573-9578.], 2000[Khalifah, P., Huang, Q., Lynn, J. W., Erwin, R. W. & Cava, R. J. (2000). Mater. Res. Bull. 35, 1-7.]; Wiss et al., 2000[Wiss, F., Raju, N. P., Wills, A. S. & Greedan, J. E. (2000). Int. J. Inorg. Mater. 2, 53-59.]; Bontchev et al., 2000[Bontchev, R. P., Jacobson, A. J., Gospodinov, M. M., Skumryev, V., Popov, V. N., Lorenz, B., Meng, R. L., Litvinchuk, A. P. & Iliev, M. N. (2000). Phys. Rev. B, 62, 12235-12240.]; Harada & Hinatsu, 2001[Harada, D. & Hinatsu, Y. (2001). J. Solid State Chem. 158, 245-253.]; Harada et al., 2001[Harada, D., Hinatsu, Y. & Ishii, Y. (2001). J. Phys. Condens. Matter, 13, 10825-10836.]; Nishimine et al., 2005[Nishimine, H., Wakeshima, M. & Hinatsu, Y. (2005). J. Solid State Chem. 178, 1221-1229.]; Gemmill et al., 2004[Gemmill, W. R., Smith, M. D. & zur Loye, H. C. (2004). Inorg. Chem. 43, 4254-4261.], 2005[Gemmill, W. R., Smith, M. D., Mozharivsky, Y. A., Miller, G. J. & zur Loye, H. C. (2005). Inorg. Chem. 44, 7047-7055.]; Vanberkel & Ijdo, 1986[Vanberkel, F. P. F. & Ijdo, D. J. W. (1986). Mater. Res. Bull. 21, 1103-1106.]; Kahnharari et al., 1995[Kahnharari, A., Mazerolles, L., Michel, D. & Robert, F. (1995). J. Solid State Chem. 116, 103-106.]; Wltschek et al., 1996[Wltschek, G., Paulus, H., Svoboda, I., Ehrenberg, H. & Fuess, H. (1996). J. Solid State Chem. 125, 1-4.]; Greedan et al., 1997[Greedan, J. E., Raju, N. P., Wegner, A., Gougeon, P. & Padiou, J. (1997). J. Solid State Chem. 129, 320-327.]; Lam et al., 2002[Lam, R., Wiss, F. & Greedan, J. E. (2002). J. Solid State Chem. 167, 182-187.]; Plaisier et al., 2002[Plaisier, J. R., Drost, R. J. & IJdo, D. J. W. (2002). J. Solid State Chem. 169, 189-198.]; Lam et al., 2003[Lam, R., Langet, T. & Greedan, J. E. (2003). J. Solid State Chem. 171, 317-323.]; Barrier & Gougeon, 2003[Barrier, N. & Gougeon, P. (2003). Acta Cryst. E59, i22-i24.]; Hinatsu et al., 2004[Hinatsu, Y., Wakeshima, M., Kawabuchi, N. & Taira, N. (2004). J. Alloys Compd, 374, 79-83.]; Vente & Ijdo, 1991[Vente, J. F. & Ijdo, D. J. W. (1991). Mater. Res. Bull. 26, 1255-1262.]). Table 7[link] lists examples of Ln3BO7 and their properties that have been investigated. Details on the dielectric properties of Ln3NbO7 have been reported and will be covered in §3.2.2[link].

Table 7
Examples of weberite-type Ln3BO7 and inverse weberites

    Space   Lattice parameters Properties
Type Compounds group Z a (Å) b (Å) c (Å) investigated
Weberite type La3NbO7 (Abe et al., 2004[Abe, R., Higashi, M., Zou, Z. G., Sayama, K., Abe, Y. & Arakawa, H. (2004). J. Phys. Chem. B, 108, 811-814.], 2006[Abe, R., Higashi, M., Sayama, K., Abe, Y. & Sugihara, H. (2006). J. Phys. Chem. B, 110, 2219-2226.]; Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]) Cmcm 4 11.167 (1) 7.629 (1) 7.753 (1) Dielectric and photocatalytic
Weberite type La3TaO7 (Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]; Abe et al., 2004[Abe, R., Higashi, M., Zou, Z. G., Sayama, K., Abe, Y. & Arakawa, H. (2004). J. Phys. Chem. B, 108, 811-814.], 2006[Abe, R., Higashi, M., Sayama, K., Abe, Y. & Sugihara, H. (2006). J. Phys. Chem. B, 110, 2219-2226.]) Cmcm 4 11.1863 (4) 7.6152 (3) 7.7556 (3) Catalytic and magnetic
Weberite type Pr3NbO7 (Vente et al., 1994[Vente, J. F., Helmholdt, R. B. & Ijdo, D. J. W. (1994). J. Solid State Chem. 108, 18-23.]) Cmcm 4 10.959 (1) 7.5240 (7) 7.6676 (7) Magnetic
Weberite type Pr3TaO7(Vente et al., 1994[Vente, J. F., Helmholdt, R. B. & Ijdo, D. J. W. (1994). J. Solid State Chem. 108, 18-23.]) Cmcm 4 10.973 (1) 7.5230 (7) 7.6721 (7) Magnetic
Weberite type Nd3NbO7 (Cai et al., 2007[Cai, L., Guzman, J., Perez, L. & Nino, J. C. (2007). Solid-State Chemistry of Inorganic Materials VI, Materials Research Society Symposium Proceeding, 998E, 0988-qq0901-0904.]; Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]) Cmcm 4 10.905 (2) 7.517 (2) 7.624 (1) Dielectric
Weberite type Gd3NbO7 (Cai et al., 2007[Cai, L., Guzman, J., Perez, L. & Nino, J. C. (2007). Solid-State Chemistry of Inorganic Materials VI, Materials Research Society Symposium Proceeding, 998E, 0988-qq0901-0904.]; Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]; Astafyev et al., 1985[Astafyev, A. V., Sirotinkin, V. P. & Stefanovich, S. Y. (1985). Kristallografiya, 30, 603-604.]) C2221 4 10.610 (1) 7.521 (1) 7.550 (1) Dielectric
Weberite type Gd3TaO7 (Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]) C2211 4 10.6259 (4) 7.5234 (3) 7.5446 (3) Magnetic
Weberite type Dy3TaO7 (Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]) C2221 4 10.5332 (3) 7.4447 (2) 7.4816 (2) Magnetic
Weberite type Ho3TaO7 (Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]) C2221 4 10.4873 (4) 7.4292 (3) 7.4499 (3) Magnetic
Inverse Fe2F5(H2O)2 (Laligant, Pannetier et al., 1986[Laligant, Y., Pannetier, J., Labbe, P. & Ferey, G. (1986). J. Solid State Chem. 62, 274-277.]; Greneche et al., 1988[Greneche, J. M., Linares, J., Varret, F., Laligant, Y. & Ferey, G. (1988). J. Magn. Magn. Mater. 73, 115-122.]; Hall et al., 1977[Hall, W., Kim, S., Zubieta, J., Walton, E. G. & Brown, D. B. (1977). Inorg. Chem. 16, 1884-1887.]) Imma 4 7.447 (1) 10.8623 (2) 6.652 (1) Magnetic
Inverse ZnFeF5(H2O)2 (Laligant, Calage et al., 1986[Laligant, Y., Calage, Y., Torrestapia, E., Greneche, J. M., Varret, F. & Ferey, G. (1986). J. Magn. Magn. Mater. 61, 283-290.]) Imma 4 7.475 (1) 10.766 (1) 6.594 (1) Magnetic
Inverse MnFeF5(H2O)2 (Laligant, Calage et al., 1986[Laligant, Y., Calage, Y., Torrestapia, E., Greneche, J. M., Varret, F. & Ferey, G. (1986). J. Magn. Magn. Mater. 61, 283-290.]; Laligant, Pannetier et al., 1987[Laligant, Y., Pannetier, J., Leblanc, M., Labbe, P., Heger, G. & Ferey, G. (1987). Z. Kristallogr. 181, 1-10.]; Greneche et al., 1988[Greneche, J. M., Linares, J., Varret, F., Laligant, Y. & Ferey, G. (1988). J. Magn. Magn. Mater. 73, 115-122.]) Imma 4 7.5635 (2) 10.901 (1) 6.7319 (3) Magnetic
Inverse MnAlF5(H2O)2 (Subramanian et al., 2006[Subramanian, M. A., Marshall, W. J., Hoffmann, R. D. & Sleight, A. W. (2006). Z. Naturforsch. B, 61, 808-812.]) Imma 4 7.229 (2) 10.487 (4) 6.816 (2)  
Inverse MgAlF5(H2O)2 (Subramanian et al., 2006[Subramanian, M. A., Marshall, W. J., Hoffmann, R. D. & Sleight, A. W. (2006). Z. Naturforsch. B, 61, 808-812.]; Weil & Werner, 2001[Weil, M. & Werner, F. (2001). Monatsh. Chem. 132, 769-777.]) Imma 4 7.057 (2) 10.125 (4) 6.798 (2) Electrical resistivity
Inverse MnVF5(H2O)2 (Subramanian et al., 2006[Subramanian, M. A., Marshall, W. J., Hoffmann, R. D. & Sleight, A. W. (2006). Z. Naturforsch. B, 61, 808-812.]; Leroux et al., 1995[Leroux, F., Mar, A., Guyomard, D. & Piffard, Y. (1995). C. R. Acad. Sci. 320, 147-153.]) Imma 4 7.607 (2) 10.912 (4) 6.728 (2) Magnetic
[Figure 11]
Figure 11
Weberite-type Ln3BO7, viewed in the [0.05, 0.05, 1] direction. The layers of LnO8 and BO6 lines are parallel to the (001) plane. Between the layers are Ln with a coordination number of 7.

There is a special type of weberite (B+2B+3F5·2H2O) named inverse weberite (please see Table 7[link] for examples). In this structure A cations are missing. In order to maintain charge neutrality, two H2O molecules take the place of two F- ions. This structure has the same characteristic B octahedral network, just like the 2O weberite structure. The space group and Wyckoff positions of B cations and anions are the same as in the 2O weberite. However, B2+ ions take B-2 sites while B3+ cations lie at B-1 sites in this structure, which is opposite to the classic weberite structure (Laligant, Calage et al., 1986[Laligant, Y., Calage, Y., Torrestapia, E., Greneche, J. M., Varret, F. & Ferey, G. (1986). J. Magn. Magn. Mater. 61, 283-290.]; Laligant, Leblanc et al., 1986[Laligant, Y., Leblanc, M., Pannetier, J. & Ferey, G. (1986). J. Phys. C, 19, 1081-1095.]; Laligant, Pannetier et al., 1986[Laligant, Y., Pannetier, J., Labbe, P. & Ferey, G. (1986). J. Solid State Chem. 62, 274-277.]; Laligant, Pannetier et al., 1987[Laligant, Y., Pannetier, J., Leblanc, M., Labbe, P., Heger, G. & Ferey, G. (1987). Z. Kristallogr. 181, 1-10.]; Weil & Werner, 2001[Weil, M. & Werner, F. (2001). Monatsh. Chem. 132, 769-777.]; Leroux et al., 1995[Leroux, F., Mar, A., Guyomard, D. & Piffard, Y. (1995). C. R. Acad. Sci. 320, 147-153.]; Subramanian et al., 2006[Subramanian, M. A., Marshall, W. J., Hoffmann, R. D. & Sleight, A. W. (2006). Z. Naturforsch. B, 61, 808-812.]).

The investigation of Na2B2+B3+F7 weberites indicates that the resulting structure type is determined by the size of the B2+ cations. With increasing ionic radius of B2+, the structure changes from 2O to 2M, 4M and to 3T, gradually (Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]). As for oxide weberites, it is clear that the A cation still plays an important role, for example, 2O Ca2Sb2O7, 3T Ca1.5Mn0.5Sb2O7 and 3T Mn2Sb2O7 (Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]; Bonazzi & Bindi, 2007[Bonazzi, P. & Bindi, L. (2007). Am. Mineral. 92, 947-953.]; Scott, 1990[Scott, H. G. (1990). Z. Kristallogr. 190, 41-46.]; Butler et al., 1950[Butler, K. H., Bergin, M. J. & Hannaford, V. M. B. (1950). J. Electrochem. Soc. 97, 117-122.]; Bystrom, 1945[Bystrom, A. (1945). Ark. Kemi Miner. Och Geol. A, 18, 1-8.]). The occurrence of the monoclinic and trigonal variants may be closely related to the ionic radius ratio of the A and B cations. However, due to the limited number of compounds reported in each weberite-like structure, it is difficult to define the factors that determine when the variants occur.

As discussed before, the unpaired vertex (terminal anions) of B-2 octahedra are in trans configurations in 2O weberites. The trigonal and monoclinic variants produce another type of B-2 octahedra with the cis configuration (see Figs. 2[link]b and c). Grey and co-workers (Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]; Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]) figured out that the relative position of the terminal anions is a characteristic of weberite polytypes. In 2M and 3T polytypes cis-B2 are only in A3B layers, while trans-B2 are only in AB3 layers (Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]). The 4M and 6T weberites have alternative cis and trans configurations in successive A3B layers (Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]; Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]).

Several research groups (Yakubovich et al., 1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.], 1990[Yakubovich, O. V., Urusov, V. S., Frenzen, G., Massa, W. & Babel, D. (1990). Eur. J. Solid State Inorg. Chem. 27, 467-475.]; Dahlke et al., 1998[Dahlke, P., Peschel, B. & Babel, D. (1998). Z. Anorg. Allg. Chem. 624, 1003-1010.]; Verscharen & Babel, 1978[Verscharen, W. & Babel, D. (1978). J. Solid State Chem. 24, 405-421.]; Welsch & Babel, 1992[Welsch, M. & Babel, D. (1992). Z. Naturforsch. B, 47, 685-692.]) proposed that the formation of B-1 chains is a characteristic of Na2B2+B3+F7 weberite polytypes. The detailed description about the different stacking sequence and the orientation of B-1 chains in 2O, 2M, 3T and 4M variants has been reported by Yakubovich et al. (1993[Yakubovich, O., Urusov, V., Massa, W., Frenzen, G. & Babel, D. (1993). Z. Anorg. Allg. Chem. 619, 1909-1919.]). Here, the discussion is expanded to include all 2O, 2M, 3T, 4M, 5M, 6M, 6T, 7M and 8O variants. There are three different orientations for B-1 chains: type [alpha], type [beta] and type [gamma]. These three orientations are correlated with each other by a threefold rotation. For 2O weberites (including 2O-II and 2O-III), the stacking sequence of B-1 chains is [alpha][alpha][alpha]... (the orientation is parallel to the [100] direction). The same applies for pseudo-2O NaCu3F7 and CaLn1.5Sb1.5O7 (Ln = La, Pr, Nd and Y), special cases of 2M (Renaudin et al., 1988[Renaudin, J., Leblanc, M., Ferey, G., Dekozak, A. & Samouel, M. (1988). J. Solid State Chem. 73, 603-609.]; Au et al., 2007[Au, Y. S., Fu, W. T. & Ijdo, D. J. W. (2007). J. Solid State Chem. 180, 3166-3171.]). The directions of the B-1 chains for 2M (except NaCu3F7 and CaLn1.5Sb1.5O7) and 4M weberites are either nearly parallel to [110] (type [beta]) or [[\bar 1]10] (type [gamma]). The stacking array for 2M is [beta][gamma][beta][gamma], while it is [beta][beta][gamma][gamma][beta][beta][gamma][gamma] for the 4M polytype. As for 3T and 6T, the directions of the B-1 chains are nearly parallel to [010], [100] or [110]. The sequence of B-1 chains is [alpha][beta][gamma] in 3T and [alpha][alpha][beta][beta][gamma][gamma][alpha][alpha][beta][gamma][gamma] in 6T weberites.

It is important to note that the close-packed cation layers are stacked the same as f.c.c. (face-centered cubic: cubic stacking in which the stacking sequence is ABCABC...) in 2O, 2M, 3T, 4M and 6T polytypes. The cation layers in 5M, 6M, 7M and 8O polytypes (Ca2Ta2O7-based compounds) are a mixture of cubic stacking and hexagonal stacking. The hexagonal stacking layers act as mirror glide planes for the cations, for example, the stacking sequence of 5M is ABC[\underline{A}]CBA[\underline{C}]AB in a unit cell (the underline letters indicate a hexagonal stacking; Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.], 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]). Therefore, 5M, 6M, 7M and 8O polytypes are not pure weberite. The weberite blocks are separated by h-stacking layers. A simpler approach is to describe the stacking of c as cubic and h as hexagonal. The stacking sequence is ccchccchcc for 5M [simplified as (3c)h(3c)h(2c), the integers indicating the number of c stacking layers], (5c)h(5c)h for 6M, (5c)h(7c)h for 7M and (4c)h(7c)h(3c) for 8O (Ebbinghaus et al., 2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]; Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.], 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.], 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]). There are several structural features resulting from the introduction of h stacking:

  • (i) h stacking only occurs in the CaTa3 cation layers;

  • (ii) h stacking causes the appearance of B-3 octahedra on the CaTa3 cation layers and the neighboring Ca3Ta cation layers, which have only one unpaired vertex (B-1 octahedra have no unpaired vertex and B-2 have two);

  • (iii) in h stacking layers B-3 chains are formed, but no B-1 chains.

In these cases type [alpha] is nearly parallel to [100] for 8O or [010] for 5M and 7M. Type [beta] is nearly parallel to [110] and type [gamma] is nearly parallel to [[\bar 110]]. The directions of the B chains are [beta][\underline{\beta}][alpha][\underline{\gamma}][gamma] for 5M, [\underline{\gamma}][beta][gamma][\underline{\beta}][gamma][beta] for 6M, [gamma][\underline{\alpha}][gamma][alpha][beta][\underline{\alpha}][beta] for 7M and [alpha][beta][\underline{\alpha}][gamma][alpha][beta][\underline{\alpha}][gamma] for 8O (B-3 chains are underlined). In summary, the stacking sequence of B chains changes with different weberite variants (see Fig. 12[link]) and the formation of B chains is a characteristic of all weberites.
[Figure 12]
Figure 12
B chains in different weberite-like structures. The vectors across the octahedra indicate the orientations of the B chains. (a) 2O, (b) 2M, (c) 3T, (d) 4M, (e) 5M, (f) 6T, (g) 6M, (h) 7M and (i) 8O.

2.4. Stability field

Both pyrochlores and weberites have BX6 octahedral networks. Owing to the fact that B-2 octahedra have two unpaired vertices, the BX6 octahedral network in weberite is typically less compact. Therefore, weberite has more potential to permit larger radii of A ions. Fig. 13[link] shows a diagram of RA versus RB for 159 pyrochlore oxides and 131 weberite compounds (83 weberite fluorides and 48 weberite oxides, see Tables 5[link] and 6[link] for a complete list and references). The 159 pyrochlore oxides are taken from two articles (Subramanian et al., 1983[Subramanian, M. A., Aravamudan, G. & Rao, G. V. S. (1983). Prog. Solid State Chem. 15, 55-143.]; Isupov, 2000[Isupov, V. A. (2000). Ferroelectr. Rev. 2, 115-168.]). Fig. 13[link] indicates that the majority of pyrochlores have RA ranging from 0.97 to 1.13 Å, while most weberites have RA values ranging from 1.10 to 1.30 Å. Weberite Ba2U2O7 has the highest RA value of 1.42 Å (Cordfunke & Ijdo, 1988[Cordfunke, E. H. P. & Ijdo, D. J. W. (1988). J. Phys. Chem. Solids, 49, 551-554.]; Shannon, 1976[Shannon, R. D. (1976). Acta Cryst. A32, 751-767.]). This clearly shows that larger RA values prefer the formation of the weberite (Brisse et al., 1972[Brisse, F., Stewart, D. J., Seidl, V. & Knop, O. (1972). Can. J. Chem. 50, 3648.]). The ratio of RA/RB for the weberite is between 1.5 and 2. The two end-members are Cd2Sb2O7 and Ag2Te2O7. However, the range of RA/RB for weberite greatly overlaps with the stability field for pyrochlore: 1.46-1.8 for A23+B24+O7 and 1.4-2.2 for A22+B25+O7 (Subramanian et al., 1983[Subramanian, M. A., Aravamudan, G. & Rao, G. V. S. (1983). Prog. Solid State Chem. 15, 55-143.]). Therefore, the ionic radius ratio is not the only determining factor in the structural stability.

[Figure 13]
Figure 13
Summary of RA versus RB for weberites (including oxide and fluorine) and pyrochlore oxides.

Electronegativity ([chi]) is another important factor in the field of existence, because the formation of weberites is closely related to the covalent character of the bonds (Sych, Kabanova, Garbuz et al., 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]; Lopatin et al., 1985[Lopatin, S. S., Averyanova, L. N. & Belyaev, I. N. (1985). Zh. Neorg. Khim. 30, 867-872.]; Weller et al., 2003[Weller, M. T., Reading, J. & Knee, C. S. (2003). Solid State Chem. 90-91, 201-205.]; Burchard & Rudorff, 1979[Burchard, G. & Rudorff, W. (1979). Z. Anorg. Allg. Chem. 454, 107-112.]). Weller et al. (2003[Weller, M. T., Reading, J. & Knee, C. S. (2003). Solid State Chem. 90-91, 201-205.]) used only [chi]A and [chi]B to picture the stability field of the weberite, but their study only included a limited number of compounds. Lopatin et al. (1982[Lopatin, S. S., Averyanova, L. N., Belyaev, I. N., Zvyagintsev, B. I. & Dyatlov, E. V. (1982). Zh. Neorg. Khim. 27, 2751-2755.], 1985[Lopatin, S. S., Averyanova, L. N. & Belyaev, I. N. (1985). Zh. Neorg. Khim. 30, 867-872.]) successfully utilized [chi]A and RA/RB to distinguish pyrochlores and weberites, and [chi]B and RA/RB to determine the different regions of the weberite and the layered perovskite. They chose Allred-Rochow (Allred & Rochow, 1958[Allred, A. L. & Rochow, E. G. (1958). J. Inorg. Nucl. Chem. 5, 264-268.]) electronegativities (which were completed by Little & Jones, 1960[Little, E. J. & Jones, M. M. (1960). J. Chem. Educ. 37, 231-233.]) because Allred-Rochow electronegativities are more precise when measuring the degree of covalent character of the bonds. Sych et al. (Sych, Kabanova, Garbuz et al., 1988[Sych, A. M., Kabanova, M. I., Garbuz, V. V. & Kovalenko, E. N. (1988). Inorg. Mater. 24, 1316-1320.]) introduced RA/RB versus the relative ionicity of the A-O bond, which is the ratio of the ionicity of the A-O bond to the sum of the ionicity of the A-O and B-O bonds. The ionicity of the A-O bond is calculated as

[I_{A{\rm -}{\rm O}} = 1-\exp[-0.25(\chi_A - \chi_{\rm O})^2]. \eqno(4)]

They used the electronegativities for the crystalline state calculated by Batanov (1975[Batanov, S. S. (1975). Zh. Neorg. Khim. 20, 2595-2600.]). The advantage of relative ionicity is that it contains the information for both A-O and B-O bonds. Therefore, the relative ionicity of A-O versus RA/RB is used to determine the stability field in this study, as shown in Fig. 14[link]. Here, the electronegativities of Allred-Rochow (Allred & Rochow, 1958[Allred, A. L. & Rochow, E. G. (1958). J. Inorg. Nucl. Chem. 5, 264-268.]) and Little-Jones (Little & Jones, 1960[Little, E. J. & Jones, M. M. (1960). J. Chem. Educ. 37, 231-233.]) were used in calculating the ionicity. In Fig. 14[link](a) there is no obvious separation between weberites and pyrochlores. The reason for this may be that both A22+B25+O7 and A23+B24+O7 pyrochlore compounds are plotted. There are very few, if any, A23+B24+O7 weberites reported. Most weberites are A22+B25+O7 or (A, A')22+(B, B')25+O7 and several A21+B26+O7 (Na2Te2O7 and Ag2Te2O7). The inclusion of A23+B24+O7, particularly high-pressure phases, complicates the stability field, therefore, Fig. 14[link](b) only contains A22+B25+O7, (A, A')22+(B, B')25+O7 compounds Na2Te2O7 and Ag2Te2O7 (all weberites points in Fig. 14[link]b are listed in Table 6[link]). Observing the plotted data in Fig. 14[link](b), there is a clear separation between weberites and pyrochlores. The dashed line is for visual effect - above the line is the weberite region. Weberites prefer a higher ratio of IA-O/(IA-O + IB-O) and a higher ratio of RA/RB than pyrochlores.

[Figure 14]
Figure 14
Stability field for weberite.

It is worth mentioning four specific compounds: Cd2Sb2O7 (RCd 2+ = 0.9 Å) in the pyrochlore region, and Ca2Sb2O7, Ca2Os2O7 (RCa2+ = 1.12 Å) and Pb2Sb2O7 (RPb2+ = 1.29 Å) in the weberite region (Shannon, 1976[Shannon, R. D. (1976). Acta Cryst. A32, 751-767.]). A high-pressure study has been performed on the first three compounds to investigate the transformation of the pyrochlore and weberite phases. Cd2Sb2O7 can form a metastable phase of weberite, which can be fully converted to pyrochlore under high pressure. Ca2Sb2O7 weberite is more stable than Cd2Sb2O7 weberite. The same high-pressure condition only results in mixed phases of Ca2Sb2O7 pyrochlore and weberite (Knop et al., 1980[Knop, O., Demazeau, G. & Hagenmuller, P. (1980). Can. J. Chem. Rev. 58, 2221-2224.]). At one atmosphere, Ca2Sb2O7 crystallizes as a pyrochlore below 973 K, above which it transforms to a weberite (Brisse et al., 1972[Brisse, F., Stewart, D. J., Seidl, V. & Knop, O. (1972). Can. J. Chem. 50, 3648.]). Meanwhile, Ca2Os2O7 weberite is stable and the synthesis of pyrochlore Ca2Os2O7 under pressure leads to calcium-deficient Ca1.7Os2O7 (Reading et al., 2002[Reading, J., Knee, C. S. & Weller, M. T. (2002). J. Mater. Chem. 12, 2376-2382.]; Weller et al., 2003[Weller, M. T., Reading, J. & Knee, C. S. (2003). Solid State Chem. 90-91, 201-205.]). The reported crystal structure of Pb2Sb2O7 also strongly depends on the synthesis conditions. Low-temperature firing or wet chemical synthesis resulted in a cubic pyrochlore phase. The cubic phase was metastable and readily transformed into weberite or rhombohedrally distorted pyrochlore (Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]; Brisse et al., 1972[Brisse, F., Stewart, D. J., Seidl, V. & Knop, O. (1972). Can. J. Chem. 50, 3648.]). These four compounds can crystallize as different polymorphs depending on the processing history, as presented in Fig. 14[link](b). It is worth noting that although the ionic radii ratio and bond ionicity are two major factors, there may be some additional crystallochemical characteristics or parameters that play a role in determining the prevalence of weberite over pyrochlore or vice versa. It would be interesting to perform some computational calculations (e.g. density functional theory) to shed light on the comparative lattice energy and stability field of pyrochlore and weberite.

3. Interesting properties and potential applications

For fluorine-based weberites, the magnetic properties attract most of the attention. The triangular network formed by B2+ and B3+ cations in the HTB-like planes generally support three different magnetically ordered systems:

  • (i) the diamagnetic B3+ ions separate linear chains of paramagnetic B2+ ions, for example, antiferromagnetic Na2NiAlF7, Na2FeAlF7 and Na2NiInF7 (Frenzen et al., 1992[Frenzen, G., Massa, W., Babel, D., Ruchaud, N., Grannec, J., Tressaud, A. & Hagenmuller, P. (1992). J. Solid State Chem. 98, 121-127.]; Heger, 1973[Heger, G. (1973). Int. J. Magn. 5, 119-124.]);

  • (ii) both B2+ and B3+ are paramagnetic ions, like ferromagnetic Na2NiFeF7 and antiferromagnetic Na2NiCrF7 (Laligant et al., 1989[Laligant, Y., Calage, Y., Heger, G., Pannetier, J. & Ferey, G. (1989). J. Solid State Chem. 78, 66-77.]; Thompson et al., 1992[Thompson, G. R., Pankhurst, Q. A. & Johnson, C. E. (1992). J. Magn. Magn. Mater. 104, 893-894.]; Heger, 1973[Heger, G. (1973). Int. J. Magn. 5, 119-124.]; Frenzen et al., 1992[Frenzen, G., Massa, W., Babel, D., Ruchaud, N., Grannec, J., Tressaud, A. & Hagenmuller, P. (1992). J. Solid State Chem. 98, 121-127.]);

  • (iii) diamagnetic B2+ forming linear chains which isolate the paramagnetic B3+ such as antiferromagnetic Na2MgFeF7 (Pankhurst et al., 1991[Pankhurst, Q. A., Johnson, C. E. & Wanklyn, B. M. (1991). J. Magn. Magn. Mater. 97, 126-130.]).

As for weberite oxide, various properties have been investigated including the photocatalytic properites (Abe et al., 2004[Abe, R., Higashi, M., Zou, Z. G., Sayama, K., Abe, Y. & Arakawa, H. (2004). J. Phys. Chem. B, 108, 811-814.], 2006[Abe, R., Higashi, M., Sayama, K., Abe, Y. & Sugihara, H. (2006). J. Phys. Chem. B, 110, 2219-2226.]; Sato et al., 2002[Sato, J., Saito, N., Nishiyama, H. & Inoue, Y. (2002). J. Photochem. Photobiol. A, 148, 85-89.]; Lin et al., 2006[Lin, X. P., Huang, F. Q., Wang, W. D., Wang, Y. M., Xia, Y. J. & Shi, J. L. (2006). Appl. Catal. Gen. 313, 218-223.]), the resistivity of Ca2Os2O7 weberite (Reading et al., 2002[Reading, J., Knee, C. S. & Weller, M. T. (2002). J. Mater. Chem. 12, 2376-2382.]), magnetic properties (Khalifah et al., 1999[Khalifah, P., Erwin, R. W., Lynn, J. W., Huang, Q., Batlogg, B. & Cava, R. J. (1999). Phys. Rev. B, 60, 9573-9578.]; Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]; Wakeshima & Hinatsu, 2006[Wakeshima, M. & Hinatsu, Y. (2006). J. Solid State Chem. 179, 3575-3581.]), ferroelectric properties (Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]; Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]) and dielectric properties (Cai & Nino, 2007[Cai, L. & Nino, J. C. (2007). J. Eur. Ceram. Soc. 27, 3971-3976.]; Cava et al., 1998[Cava, R. J., Krajewski, J. J. & Roth, R. S. (1998). Mater. Res. Bull. 33, 527-532.]; Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]). The interest in the properties is first due to the fact that the weberite structure is considered more favorable for the realisation of a ferroelectric state than the pyrochlore structure (Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]). The Sb-based compounds are the most investigated weberites for ferroelectric properties. Ten years ago, Cava et al. (1998[Cava, R. J., Krajewski, J. J. & Roth, R. S. (1998). Mater. Res. Bull. 33, 527-532.]) found that the temperature coefficients of the dielectric constant (TC[epsilon]r) of the Ca2Ta2O7-Ca2Nb2O7 system can be close to zero. A series of investigations on Ca2Ta2O7-based weberites have followed (Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.], 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.], 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]; Ebbinghaus et al., 2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]). Ln3BO7 (where Ln = rare-earth elements and B is Nb or Ta) are also interesting weberite-type compounds. The crystal structure is related to the ionic radius of Ln3+, which provides a stage for the study of structure-dielectric properties relationships. This section will focus on the ferroelectric properties and dielectric properties of weberite oxides.

3.1. Ferroelectric properties

A2Sb2O7 (A = Ca2+, Pb2+ and Sr2+) are perhaps the most studied weberites owing to their ferroelectric properties. Second-harmonic generation and heat-capacity measurements indicated a possible ferroelectric phase transition in Pb2Sb2O7. Dielectric constants showed a thermal hysteresis around the Curie temperature (Tc) in Pb2Sb2O7 (Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]; Milyan & Semrad, 2005[Milyan, P. M. & Semrad, E. E. (2005). Russ. J. Inorg. Chem. 50, 1599-1604.]). Single-crystal X-ray and powder neutron diffraction were performed in detailed crystallographic studies (Ivanov & Zavodnik, 1990[Ivanov, S. A. & Zavodnik, V. E. (1990). Kristallografiya, 35, 842-846.]; Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]). Below Tc, there is a slight distortion from a centrosymmetric structure and ionic displacements cause spontaneous polarization in this structure. The results indicated a non-centrosymmetric (space group I2cm) to centrosymmetric (space group Imam, another setting of Imma) phase transition (Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]; Ivanov et al., 1998[Ivanov, S., Tellgren, R. & Rundlof, H. (1998). European Powder Diffraction Conference 5, Pts 1 and 2 278-2, pp. 768-772.]; Ivanov & Zavodnik, 1990[Ivanov, S. A. & Zavodnik, V. E. (1990). Kristallografiya, 35, 842-846.]). Tc depends on the A cation: 510 K for Pb2Sb2O7,110 K for Ca2Sb2O7 and 90 K for Sr2Sb2O7. The substitution of Ca by Pb in Ca2Sb2O7 causes a shift of Tc towards a higher temperature: 200 K for CaPbSb2O7 weberite. Therefore, the A sublattice seems more likely to be the ferroelectrically active one.

It is worth noting that Pb2Sb2O7 can also form rhombohedrally distorted pyrochlore (Brisse et al., 1972[Brisse, F., Stewart, D. J., Seidl, V. & Knop, O. (1972). Can. J. Chem. 50, 3648.]). The pyrochlore phase is paraelectric even at room temperature. Actually, Pb2Sb2O7 weberite has a higher Tc than most Pb-based pyrochlores. These facts may serve as evidence that the weberite structure is more suitable for the appearance of the ferroelectric state (Astafev et al., 1985[Astafev, A. V., Bush, A. A., Stefanovich, S. Y. & Venevtsev, Y. N. (1985). Inorg. Mater. 21, 560-563.]; Isupov, 2000[Isupov, V. A. (2000). Ferroelectr. Rev. 2, 115-168.]).

3.2. Dielectric properties

3.2.1. Ca2Ta2O7-based compounds

According to Grey and co-workers (Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.]) the structure of pure Ca2Ta2O7 is 3T weberite up to 1673 K, where it transforms to the 7M polytype. The structure can be easily modified by doping and different synthesis routines (Grey et al., 1999[Grey, I. E., Roth, R. S., Mumme, G., Bendersky, L. & Minor, D. (1999). Solid State Chemistry of Inorganic Materials II, Materials Research Society Symposium Proceedings, Vol. 547, pp. 127-138. Boston, MA: Materials Research Society.], 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.], 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]; Ebbinghaus et al., 2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]). One of the most interesting dielectric properties of Ca2Ta2O7 is that the temperature coefficient of the dielectric constant (TC[epsilon]r) is 0 when mixing with 18 mol% of Ca2Nb2O7, meeting the requirement for the application of microwave dielectrics (Cava et al., 1998[Cava, R. J., Krajewski, J. J. & Roth, R. S. (1998). Mater. Res. Bull. 33, 527-532.]). TC[epsilon]r [asymptotically equal to] 0 can be easily understood because TC[epsilon]r is negative for Ca2Ta2O7 (-444 p.p.m. K-1 at 295 K) and positive for Ca2Nb2O7 (231 p.p.m. K-1 at 295 K). Extensive studies on the structure of the (1-x)Ca2Ta2O7 - xCa2Nb2O7 system have been performed by powder and single-crystal X-ray diffraction, and powder neutron diffraction (Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]). The system forms 7M weberite solid solutions up to x = 0.1 and Ca2Nb2O7-type solid solutions from x = 0.2-1. When x = 0.1, the structure transforms into 5M. The solubility limit is reached when the substitution of Ca2Nb2O7 increases to 15 mol% and Ca2Nb2O7 forms as a second phase. The presence of Ca2Nb2O7 thus results in TC[epsilon]r compensation, making it approximately zero.

Another interesting aspect is that most of the (1-x)Ca2Ta2O7 - xCa2Nb2O7 compounds have higher dielectric constants (above 30) than pure Ca2Ta2O7 and Ca2Nb2O7 at 1 MHz (Cava et al., 1998[Cava, R. J., Krajewski, J. J. & Roth, R. S. (1998). Mater. Res. Bull. 33, 527-532.]). However, another set of published dielectric constants of 5M Ca2Ta1.8Nb0.2O7 and 7M Ca2Ta1.9Nb0.1O7 are approximately 18 and 20 at 1 MHz, respectively, which are lower than the previous publication (Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]). It is not clear what causes the discrepancy in dielectric constant measurement. It may be due to different firing conditions and measurement methods. Dielectric properties at radio frequency have also been investigated. The dielectric constants of 3T Ca1.6Nd0.4Ta1.6Zr0.4O7, 5M Ca2Ta1.8Nb0.2O7 and 7M Ca2Ta1.9Nb0.1O7 are approximately stable (18-19) from 100 kHz up to 5 GHz and reach a maximum (22, 24.5 and 26.1, respectively) at ~ 8 GHz. The dielectric constant is comparable for some important microwave dielectrics, such as BaMg1/3Ta2/3O3 (~ 24; Reaney & Iddles, 2006[Reaney, I. M. & Iddles, D. (2006). J. Am. Ceram. Soc. 89, 2063-2072.]). However, the problem with these systems is that they have low quality factors (Q [asymptotically equal to] 200) for technical applications (Grey et al., 2001[Grey, I. E., Roth, R. S., Mumme, W. G., Planes, J., Bendersky, L., Li, C. & Chenavas, J. (2001). J. Solid State Chem. 161, 274-287.]).

It is interesting to see that 8O Ca2Ta2O7, which is synthesized by the optical floating zone melting method from 3T Ca2Ta2O7 powder, has a relatively high dielectric constant (~ 60) at room temperature (Ebbinghaus et al., 2005[Ebbinghaus, S. G., Kalytta, A., Kopf, J., Weidenkaff, A. & Reller, A. (2005). Z. Kristallogr. 220, 269-276.]). And [epsilon]r increases to 90 at 50 K. The high dielectric constant may result from a net dipole created by the off-center Ta5+ in the TaO6 octahedra of the Ca3Ta layers. The shifting of Ta5+ also produces two short Ta-O bonds and two long Ta-O bonds, leading to the distortion of TaO6 octahedra. The high dielectric constant and the ability to tailor it are interesting for scientific study and possible electronic applications.

In addition, the crystallographic study of Nd2Zr2O7 and Sm2Ti2O7 doping Ca2Ta2O7 has been conducted by Grey et al. (Grey et al., 2003[Grey, I. E., Mumme, W. G., Ness, T. J., Roth, R. S. & Smith, K. L. (2003). J. Solid State Chem. 174, 285-295.]; Grey & Roth, 2000[Grey, I. E. & Roth, R. S. (2000). J. Solid State Chem. 150, 167-177.]). The resulting phases include 3T, 4M, 5M and 6T weberites. The great structural flexibility of Ca2Ta2O7-based compounds is interesting in a crystallographic study and may have potentials in technical applications.

3.2.2. Weberite-type Ln3NbO7

As stated in §2.3[link], Ln2(Nb,Ln)O7 (or Ln3NbO7, where Ln is La3+, Pr3+, Nd3+ and Gd3+) is a weberite-type structure. It is an extreme case of 2O-III weberite structure, in which A-2 and B-2 are the same. Our recent study on the dielectric properties of Ln3NbO7 (Ln = La3+, Nd3+ and Gd3+) showed some interesting results (Cai et al., 2007[Cai, L., Guzman, J., Perez, L. & Nino, J. C. (2007). Solid-State Chemistry of Inorganic Materials VI, Materials Research Society Symposium Proceeding, 998E, 0988-qq0901-0904.]; Cai & Nino, 2007[Cai, L. & Nino, J. C. (2007). J. Eur. Ceram. Soc. 27, 3971-3976.]). As shown in Fig. 15[link], all three compounds exhibited a dielectric relaxation behavior similar to that observed in pyrochlore compounds (Roth et al., 2008[Roth, R. S., Vanderah, T. A., Bordet, P., Grey, I. E., Mumme, W. G., Cai, L. & Nino, J. C. (2008). J. Solid State Chem. 181, 406-414.]; Nino et al., 2001[Nino, J. C., Lanagan, M. T. & Randall, C. A. (2001). J. Appl. Phys. 89, 4512-4516.]). The permittivity increases sharply with increasing temperature until a maximum is reached. After that, the permittivity decreases slightly with an increase in temperature. The permittivity is between 35 and 60 for La3NbO7 and 34-47 for Gd3NbO7 from 113 to 473 K, and is between 34 and 62 for Nd3NbO7 from 113 to 673 K at 1 MHz. These three compounds have close permittivity at 113 K. The relaxation temperatures are different, 183, 473 and 323 K for La3NbO7, Nd3NbO7 and Gd3NbO7, respectively. The difference in relaxation temperature indicates the possibility of tailoring the temperature at which dielectric relaxation occurs through variations in compositions. As for the origin of different dielectric relaxation temperatures, a possible explanation is provided by comparing the structure. The main difference between the structure is that in Gd3NbO7 the polyhedra are more distorted (Wakeshima et al., 2004[Wakeshima, M., Nishimine, H. & Hinatsu, Y. (2004). J. Phys. Condens. Matter, 16, 4103-4120.]; Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]; Rossell, 1979[Rossell, H. J. (1979). J. Solid State Chem. 27, 115-122.]). In Table 8[link] polyhedral distortions are quantified in the deviation of bond length and bond angle. The calculation is based on the atomic positions after Rossell (1979[Rossell, H. J. (1979). J. Solid State Chem. 27, 115-122.]). Nd3NbO7 has nearly perfect NbO6 octahedra, while Gd3NbO7 has the most distorted octahedra. The LnO8 cube in Nd3NbO7 is the least distorted while LaO8 and GdO8 cubes have comparable distortion. These distortions are attributed to the `openness' of the structure which causes an easier polarization of the material and results in a lower relaxation temperature (Astafyev et al., 1985[Astafyev, A. V., Sirotinkin, V. P. & Stefanovich, S. Y. (1985). Kristallografiya, 30, 603-604.]). Additional details on the relationship of dielectric relaxation and polyhedral distortions are ongoing and will be a matter for future publications.

Table 8
Structural comparison of three weberite-type structures (Allpress & Rossell, 1979[Allpress, J. G. & Rossell, H. J. (1979). J. Solid State Chem. 27, 105-114.]; Rossell, 1979[Rossell, H. J. (1979). J. Solid State Chem. 27, 115-122.])

  La3NbO7 Nd3NbO7 Gd3NbO7
[Delta]dLn-O/(dLn-O)av in LnO8 cube (%) 13.33 7.14 13.06
Deviation of O-Ln-O angle from perfect cube (°) -9.87-6.46 -2.37-2.47 -5.77-8.88
[Delta]dNb-O/(dNb-O)av in NbO6 octahedra (%) 1.64 0.41 4.86
Deviation of O-Nb-O angle from perfect octahedra -1.58-1.58 0.27-0.42 -5.63-5.63
dLn-O and dNb-O represents the bond length between Ln3+ and O2- and the bond length between Nb5+ and O2-, respectively. [Delta]dLn-O means the difference of longest and shortest bond length between Ln3+ and O2- in LnO8 distorted cubes. [Delta]dNb-O is the difference of the longest and shortest bond length between Nb5+ and O2- in NbO6 distorted octahedra. (dLn-O)av is the average bond length between Ln3+ and O2- in LnO8 distorted cubes. (dNb-O)av is the average bond length between Nb5+ and O2- in NbO6 distorted octahedra.
[Figure 15]
Figure 15
Dielectric properties between 1 kHz and 1 MHz of (a) La3NbO7, (b) Nd3NbO7, (c) Gd3NbO7 and (d) Arrhenius analysis of Gd3NbO7.

Owing to the interesting dielectric loss behavior observed in Gd3NbO7 its characterization includes more frequencies (1, 4, 6, 10, 30, 80, 100, 300, 800 kHz and 1 MHz), as shown in Fig. 15[link](c). The temperature (Tm), at which the loss peak occurs increases with increasing measuring frequency. To better understand the phenomena, the Arrhenius function is used to model the relaxation behaviour of Gd3NbO7

[v = v_0 \exp\left(-{E_{\rm a}\over k_{\rm B}T_{\rm m}}\right), \eqno(5)]

where v is the measuring frequency, the pre-exponential v0 is the attempt-jump frequency, Ea is the activation energy and kB is Boltzmann's constant. Tm is determined for each measuring frequency by fitting the loss peak to a Gaussian function. The non-symmetric tails of loss peaks are cut off during fitting. The resulting Arrhenius plot is presented in Fig. 15[link](d). From the linear fit, v0 = 1.51 × 1011 Hz, and the activation energy Ea is 0.45 eV, which is larger than typical values observed in Nb-based pyrochlores, for example 0.32 eV in Ca-Ti-Nb-O pyrochlore and 0.14 eV in Bi-Zn-Nb-O pyrochlore (Roth et al., 2008[Roth, R. S., Vanderah, T. A., Bordet, P., Grey, I. E., Mumme, W. G., Cai, L. & Nino, J. C. (2008). J. Solid State Chem. 181, 406-414.]; Nino, 2002[Nino, J. C. (2002). PhD dissertation. The Pennsylvania State University.]). However, other ionic and dipolar compounds systems have even higher activation energies; for example, 0.53 eV for CaF2-doped NaF and 1.02 eV for (Ba0.8Sr0.2)(Ti1-xZrx)O3 (Johnson et al., 1969[Johnson, H. B., Tolar, N. J., Miller, G. R. & Cutler, I. B. (1969). J. Phys. Chem. Solids, 30, 31-42.]; Cheng et al., 2004[Cheng, B. L., Wang, C., Wang, S. Y., Button, T. W., Lu, H. B., Zhou, Y. L., Chen, Z. H. & Yang, G. Z. (2004). Appl. Phys. Lett. 84, 5431-5433.]). Thus, the calculated Ea is acceptable.

The dielectric study of Ln3NbO7 compounds points to the possibility of tailoring the dielectric relaxation and develop further a paradigm for the compositional design of fluorite-related ceramics with optimized dielectric properties.

4. Conclusions

There are a considerable number of weberite compounds (A2B2X7) that have been studied to date. Here the stability field in terms of the ratio of RA and RB, and the relative bond ionicity has been established. This structure can be interpreted in different ways. As an anion-deficient fluorite structure, it has similar close-packed cationic networks as fluorite and pyrochlore. It is presented here that the cation sublattices of the weberite and the pyrochlore structures are correlated by an axial transformation and that the different stacking inside an AB3 and A3B cation slab leads to a different coordination environment of anions in weberite and pyrochlore. There are various types of weberite-like structures. They can be distinguished by the number of AB3 and A3B slabs and the crystal system. B chains are a characteristic of all weberite structures even for weberite polytypes with N > 4. The stacking sequence and the orientation of B-chains changes with respect to monoclinic and trigonal variants.

So far, investigations have primarily focused on the crystallographic aspects of weberites and some weberite compounds are reported to have interesting properties. It is clear that they are of great scientific interest. However, few studies have concentrated on the properties and possible applications. There is no doubt that weberite compounds possess various useful properties that can be tailored owing to the fact that many metal cations can be introduced into this structure as well as that a large diversity of variants exist in this structure. The realisation of the potential of weberites for electrical applications will grow as more extensive studies are conducted and knowledge of the structures increases.

Acknowledgements

The authors would like to thank the financial support by the US National Science Foundation for funding CAREER grant (DMR-0449710).

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Acta Cryst (2009). B65, 269-290   [ doi:10.1107/S0108768109011355 ]