supplementary materials


HTML versionpdf versioncif file3d viewstructure factorssupplementary materialsCIF check reportsimilar papers Open access

Acta Cryst. (2009). E65, i91-i92    [ doi:10.1107/S1600536809048788 ]

The type IV polymorph of KEu(PO3)4

A. Oudahmane, M. Daoud, B. Tanouti, D. Avignant and D. Zambon

Abstract top

Single crystals of KEu(PO3)4, potassium europium(III) polyphosphate, were obtained by solid-state reactions. This monoclinic form is the second polymorph described for this composition and belongs to type IV of long-chain polyphosphates with general formula AIBIII(PO3)4. It is isotypic with its KEr(PO3)4 and KDy(PO3)4 homologues. The crystal structure is built of infinite helical chains of corner-sharing PO4 tetrahedra with a repeating unit of eight tetrahedra. These chains are further linked by isolated EuO8 square antiprisms, forming a three-dimensional framework. The K+ ions are located in pseudo-hexagonal channels running along [\overline{2}01] and are surrounded by nine O atoms in a distorted environment.

Comment top

Rare earth polyphosphates are interesting materials and bear potential applications (Rashchi & Finch, 2000; Barsukov et al., 2004). The title compound is a member of a large family of polyphosphates with general formula AIBIII(PO3)4 (where AI is a monovalent cation: Li, Na, K, Rb, Cs, Tl, NH4, Ag and BIII is a trivalent cation: Ln,Y, Bi). It is now well known that these compounds are classified into seven structural types usually labelled by roman numerals from I to VII. A short recapitulation of the main crystal chemical characteristics of these seven structural types has recently been given by Jaouadi et al. (2003). The KEu(PO3)4 polymorph described in this article belongs to the IV structural type.

In the crystal structure the Eu3+ ion is eight-coordinated by the oxygen atoms and its 8-coordination polyhedron is better described as a square antiprism than a dodecahedron according to the criteria of Porai-Koshits & Aslanov (1972) (δ1 = 10.37°, δ2 = 10.85°, δ3 = 47.97°, δ4 = 53.54°). The Eu—O distances range from 2.3199 (19) Å to 2.4827 (19) Å with an average <Eu—O> distance of 2.399 Å that is slightly shorter than the sum of the ionic radii i.e. 2.466 Å (Shannon, 1976). The structure of this type IV polymorph is built of infinite helical chains of corner-sharing PO4 tetrahedra further linked by isolated EuO8 square antiprisms. The (PO3)∞ chains exhibit a repeating unit of eight PO4 tetrahedra (Fig. 1) and are running along the [101] direction. The three-dimensional framework resulting from the edge-sharing between the PO4 tetrahedra and the EuO8 square antiprisms exhibits pseudo hexagonal channels where the K+ ions reside. The K+ ion is 9-coordinated by oxygen atoms with distances ranging from 2.789 (2) Å to 3.370 (3) Å. By sharing corners, the KO9 coordination polyhedra form corrugated chains running along the [010] direction (Fig. 2). Whereas the K atoms are separated by 6.599 (2) Å within the chain, the shortest K—K distance in the structure, 4.770 (2) Å, occurs between two adjacent (KO9)∞ chains. This shortest distance corresponds to the separation between two K+ ions within the channels of the structure running along the [201] direction. This separation distance AI—AI is strongly dependent on the nature of the AI element and decreases as the size of the AI element increases. For instance, in the AIGd(PO3)4 homologue series, where AI = K, Rb, Cs, this AI—AI shortest distance varies from 4.801 Å for K to 4.211 Å for Cs (4.524 Å for Rb). For CsEu(PO3)4 the shortest Cs—Cs distance is equal to 4.237Å (Zhu et al. 2009).

For isotypic AB(PO3)4 structures, where A is an alkali metal, Tl or NH4+, and B is a rare earth element, see: Palkina et al. (1977) for TlNd, Maksimova et al. (1978) for RbNd, Dago et al. (1980) for KEr, Maksimova et al. (1981) for CsNd, Maksimova et al. (1982) for RbHo, Horchani et al. (2004) for RbEr, Rekik et al. (2004) for KGd, Naïli & Mhiri (2005) for CsGd, Ben Zarkouna et al. (2006) for (NH4)Gd, Khlissa & Férid (2006) for RbTb, Ettis et al. (2006) for RbGd, Chehimi-Moumen & Férid (2007) for KDy, Horchani-Naifer & Férid (2007) for CsPr, and Zhu et al. (2009) for CsEu. For a review on the crystal chemisty of polyphosphates, see: Durif (1995).

Related literature top

Besides crystals of the title compound, crystals of the type III polymorph (Hu et al., 1984)) have also been obtained. For isotypic AB(PO3)4 structures, where A is an alkali metal, Tl or NH4+, and B is a rare earth element, see: Palkina et al. (1977) for TlNd; Maksimova et al. (1978) for RbNd; Dago et al. (1980) for KEr; Maksimova et al. (1981) for CsNd; Maksimova et al. (1982) for RbHo; Horchani et al. (2004) for RbEr; Rekik et al. (2004) for KGd; Naïli & Mhiri (2005) for CsGd; Ben Zarkouna et al. (2006) for (NH4)Gd; Khlissa & Férid (2006) for RbTb; Ettis et al. (2006) for RbGd; Chehimi-Moumen & Férid (2007) for KDy; Horchani-Naifer & Férid (2007) for CsPr; Zhu et al. (2009) for CsEu. For a review on the crystal chemisty of polyphosphates, see: Durif (1995). Jaouadi et al. (2003) have discussed the main crystal chemical characteristics of the seven AB(PO3)4 structure types. For applications of rare earth polyphosphates, see: Rashchi & Finch (2000); Barsukov et al. (2004). For general background, see: Porai-Koshits & Aslanov (1972). For ionic radii, see: Shannon (1976).

Experimental top

Crystals of the title compound were synthesized by reacting Eu2O3 with (NH4)H2PO4 and K2CO3 in a platinum crucible. A mixture of these reagents in the molar ratio 34:57:9 was used for the synthesis. The mixture was heated at 473 K for 6 h, then at 573 K for 6 h and finally at 873 K for 24 h. The furnace was then cooled down first to 773 K at the rate of 2 K.h-1 and then to room temperature at the rate of K.h-1. Single crystals were extracted from the batch by washing with hot water. Besides crystals of the title compound, crystals of the type III polymorph (Hu et al., 1984)) have also been obtained.

Refinement top

The highest residual peak in the final difference Fourier map was located 0.68 Å from atom Eu and the deepest hole was located 0.45 Å from atom K.

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. ORTEP-3 view of the repeating unit with eight PO4 tetrahedra, leading to helical (PO3)∞ chains. Displacement ellipsoids are drawn at the 50 % probability level. Symmetry codes: (a) -1/2+x, 3/2-y, 1/2+z; (b) -1+x, -1+y, z; (c) x, -1+y, 1+z; (d) -1+x, y, z; (e) -1/2+x, 1/2-y, 1/2+z; (f) 3-x, 1-y, 1-z; (g) 2-x, 1-y, 1-z; (h) 5/2-x, -1/2+y, 1/2-z; (i) -3/2+x, 1/2-y, -1/2+z; (j) 3/2-x, -1/2+y, 1/2-z; (k) 2-x, 1-y, -z.
[Figure 2] Fig. 2. Partial view of infinite chains of corner-sharing KO9 polyhedra.
potassium europium(III) polyphosphate top
Crystal data top
KEu(PO3)4F(000) = 952
Mr = 506.94Dx = 3.476 Mg m−3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 6203 reflections
a = 10.3723 (1) Åθ = 2.6–40.6°
b = 8.9721 (1) ŵ = 7.63 mm−1
c = 10.8320 (1) ÅT = 296 K
β = 106.053 (1)°Hexagonal prism, colourless
V = 968.73 (2) Å30.12 × 0.11 × 0.10 mm
Z = 4
Data collection top
Bruker APEXII CCD
diffractometer		6201 independent reflections
Radiation source: fine-focus sealed tube5023 reflections with I > 2σ(I)
graphiteRint = 0.045
Detector resolution: 8.3333 pixels mm-1θmax = 40.6°, θmin = 3.1°
ω and φ scansh = −18→18
Absorption correction: multi-scan
(SADABS; Bruker, 2008)		k = −16→16
Tmin = 0.466, Tmax = 0.513l = −19→5
24299 measured reflections
Refinement top
Refinement on F20 constraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.030Secondary atom site location: difference Fourier map
wR(F2) = 0.072 w = 1/[σ2(Fo2) + (0.0311P)2 + 1.1689P]
where P = (Fo2 + 2Fc2)/3
S = 1.04(Δ/σ)max = 0.002
6201 reflectionsΔρmax = 1.72 e Å−3
163 parametersΔρmin = −2.04 e Å−3
0 restraints
Crystal data top
KEu(PO3)4V = 968.73 (2) Å3
Mr = 506.94Z = 4
Monoclinic, P21/nMo Kα radiation
a = 10.3723 (1) ŵ = 7.63 mm−1
b = 8.9721 (1) ÅT = 296 K
c = 10.8320 (1) Å0.12 × 0.11 × 0.10 mm
β = 106.053 (1)°
Data collection top
Bruker APEXII CCD
diffractometer		6201 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2008)		5023 reflections with I > 2σ(I)
Tmin = 0.466, Tmax = 0.513Rint = 0.045
24299 measured reflectionsθmax = 40.6°
Refinement top
R[F2 > 2σ(F2)] = 0.030163 parameters
wR(F2) = 0.0720 restraints
S = 1.04Δρmax = 1.72 e Å−3
6201 reflectionsΔρmin = −2.04 e Å−3
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
K0.79467 (11)0.57090 (14)0.04227 (10)0.0395 (2)
Eu0.500385 (11)0.772652 (13)0.184899 (12)0.00645 (3)
P10.85428 (6)0.90558 (7)0.24010 (7)0.00615 (10)
P20.54001 (6)0.82848 (7)−0.14102 (7)0.00607 (10)
P30.24938 (6)1.02436 (7)0.22927 (6)0.00609 (10)
P40.17657 (6)0.89469 (7)−0.01962 (6)0.00626 (10)
O10.14215 (17)0.9549 (2)0.10673 (19)0.0085 (3)
O20.35389 (19)0.9131 (2)0.29039 (19)0.0093 (3)
O3−0.02278 (19)0.7943 (2)0.2518 (2)0.0103 (3)
O40.6017 (2)0.5368 (2)0.1754 (2)0.0117 (3)
O50.5648 (2)0.7615 (2)−0.0114 (2)0.0117 (3)
O60.17180 (19)1.0919 (2)0.3112 (2)0.0106 (3)
O70.73783 (19)0.8192 (2)0.2547 (2)0.0134 (4)
O80.5691 (2)0.7074 (2)0.4090 (2)0.0114 (3)
O90.31707 (19)0.8419 (2)0.0175 (2)0.0121 (3)
O100.53947 (19)1.0334 (2)0.1765 (2)0.0121 (3)
O110.31589 (17)1.1548 (2)0.1668 (2)0.0092 (3)
O120.8309 (2)0.9497 (2)0.0936 (2)0.0125 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
K0.0363 (5)0.0581 (6)0.0243 (4)0.0047 (4)0.0085 (4)−0.0045 (4)
Eu0.00596 (4)0.00617 (4)0.00701 (5)0.00051 (3)0.00144 (3)0.00084 (4)
P10.0054 (2)0.0053 (2)0.0068 (2)−0.00011 (17)0.0002 (2)0.00105 (19)
P20.0060 (2)0.0053 (2)0.0071 (2)−0.00059 (17)0.0021 (2)−0.00025 (19)
P30.0057 (2)0.0065 (2)0.0062 (3)0.00066 (17)0.0020 (2)−0.00024 (19)
P40.0052 (2)0.0077 (2)0.0050 (2)−0.00047 (17)0.00004 (19)0.00081 (19)
O10.0071 (6)0.0106 (7)0.0078 (7)−0.0029 (5)0.0023 (6)−0.0027 (6)
O20.0094 (7)0.0093 (7)0.0087 (8)0.0032 (5)0.0015 (6)0.0013 (6)
O30.0108 (7)0.0104 (7)0.0101 (8)0.0053 (6)0.0036 (6)0.0047 (6)
O40.0163 (8)0.0086 (7)0.0117 (8)0.0013 (6)0.0063 (7)0.0015 (6)
O50.0135 (8)0.0155 (8)0.0074 (8)0.0021 (6)0.0048 (7)0.0014 (6)
O60.0114 (7)0.0126 (7)0.0095 (8)0.0022 (6)0.0059 (7)−0.0018 (6)
O70.0073 (7)0.0148 (8)0.0173 (10)−0.0029 (6)0.0019 (7)0.0051 (7)
O80.0114 (7)0.0125 (8)0.0093 (8)0.0046 (6)0.0011 (6)0.0038 (6)
O90.0083 (7)0.0193 (9)0.0080 (8)0.0053 (6)0.0011 (6)0.0024 (7)
O100.0099 (7)0.0060 (6)0.0212 (10)0.0017 (5)0.0059 (7)0.0020 (7)
O110.0060 (6)0.0077 (6)0.0146 (9)−0.0005 (5)0.0040 (6)0.0009 (6)
O120.0191 (9)0.0087 (7)0.0069 (8)−0.0009 (6)−0.0009 (7)0.0026 (6)
Geometric parameters (Å, °) top
K—O42.789 (2)P1—O71.480 (2)
K—O52.860 (2)P1—O4vi1.483 (2)
K—O6i2.874 (2)P1—O121.588 (2)
K—O2i2.961 (2)P1—O3iii1.5968 (19)
K—O10ii3.075 (3)P1—Kvi3.4868 (13)
K—O3iii3.221 (2)P2—O10iv1.4795 (19)
K—O7ii3.234 (3)P2—O51.483 (2)
K—O11iv3.326 (2)P2—O11iv1.6015 (18)
K—O73.370 (3)P2—O3i1.602 (2)
K—P3i3.4008 (12)P3—O61.482 (2)
K—P1ii3.4868 (13)P3—O21.4873 (19)
Eu—O92.3199 (19)P3—O111.6005 (19)
Eu—O42.3775 (19)P3—O11.6033 (19)
Eu—O102.3799 (18)P3—Kvii3.4008 (12)
Eu—O52.401 (2)P4—O91.4784 (19)
Eu—O72.4045 (19)P4—O8viii1.484 (2)
Eu—O82.406 (2)P4—O11.601 (2)
Eu—O6v2.4214 (18)P4—O12iv1.601 (2)
Eu—O22.4827 (19)
O4—K—O559.57 (6)O5—Eu—O2141.68 (7)
O4—K—O6i100.70 (7)O7—Eu—O2118.10 (7)
O5—K—O6i89.02 (7)O8—Eu—O272.99 (6)
O4—K—O2i147.48 (7)O6v—Eu—O277.51 (6)
O5—K—O2i99.06 (6)O7—P1—O4vi118.08 (13)
O6i—K—O2i51.47 (5)O7—P1—O12109.51 (12)
O4—K—O10ii76.15 (6)O4vi—P1—O12110.78 (11)
O5—K—O10ii118.26 (7)O7—P1—O3iii108.73 (11)
O6i—K—O10ii143.04 (7)O4vi—P1—O3iii110.13 (12)
O2i—K—O10ii135.75 (6)O12—P1—O3iii97.66 (11)
O4—K—O3iii94.01 (6)O10iv—P2—O5121.60 (13)
O5—K—O3iii93.77 (6)O10iv—P2—O11iv110.86 (11)
O6i—K—O3iii164.29 (7)O5—P2—O11iv106.08 (11)
O2i—K—O3iii112.83 (6)O10iv—P2—O3i107.58 (12)
O10ii—K—O3iii46.46 (5)O5—P2—O3i109.67 (12)
O4—K—O7ii49.24 (5)O11iv—P2—O3i98.65 (10)
O5—K—O7ii108.54 (6)O6—P3—O2117.21 (12)
O6i—K—O7ii97.63 (6)O6—P3—O11108.85 (11)
O2i—K—O7ii138.36 (6)O2—P3—O11109.42 (10)
O10ii—K—O7ii52.04 (5)O6—P3—O1106.70 (11)
O3iii—K—O7ii96.13 (6)O2—P3—O1111.17 (11)
O4—K—O11iv105.76 (6)O11—P3—O1102.44 (11)
O5—K—O11iv46.23 (5)O9—P4—O8viii119.05 (12)
O6i—K—O11iv78.27 (6)O9—P4—O1108.15 (11)
O2i—K—O11iv57.13 (5)O8viii—P4—O1109.93 (11)
O10ii—K—O11iv138.47 (6)O9—P4—O12iv108.86 (12)
O3iii—K—O11iv92.53 (6)O8viii—P4—O12iv110.62 (12)
O7ii—K—O11iv153.96 (6)O1—P4—O12iv98.17 (11)
O4—K—O755.48 (6)P4—O1—P3124.84 (11)
O5—K—O756.64 (6)P3—O2—Eu126.82 (12)
O6i—K—O7144.25 (6)P3—O2—Kvii93.80 (9)
O2i—K—O7135.83 (6)Eu—O2—Kvii139.28 (8)
O10ii—K—O763.35 (5)P1ix—O3—P2vii130.06 (14)
O3iii—K—O744.54 (5)P1ix—O3—Kix91.84 (9)
O7ii—K—O785.78 (3)P2vii—O3—Kix97.17 (9)
O11iv—K—O783.30 (6)P1ii—O4—Eu138.08 (12)
O9—Eu—O4118.75 (7)P1ii—O4—K105.27 (10)
O9—Eu—O1079.54 (7)Eu—O4—K108.28 (7)
O4—Eu—O10142.30 (6)P2—O5—Eu143.49 (12)
O9—Eu—O571.81 (7)P2—O5—K110.57 (10)
O4—Eu—O571.94 (7)Eu—O5—K105.40 (8)
O10—Eu—O585.13 (7)P3—O6—Eux144.03 (13)
O9—Eu—O7138.27 (7)P3—O6—Kvii97.49 (9)
O4—Eu—O775.03 (7)Eux—O6—Kvii118.48 (8)
O10—Eu—O770.79 (7)P1—O7—Eu148.46 (13)
O5—Eu—O776.95 (8)P1—O7—Kvi87.05 (10)
O9—Eu—O8143.51 (7)Eu—O7—Kvi92.57 (7)
O4—Eu—O879.39 (7)P1—O7—K88.34 (10)
O10—Eu—O8105.75 (7)Eu—O7—K91.59 (7)
O5—Eu—O8143.72 (7)Kvi—O7—K175.37 (7)
O7—Eu—O874.50 (7)P4xi—O8—Eu130.26 (12)
O9—Eu—O6v75.16 (7)P4—O9—Eu146.27 (13)
O4—Eu—O6v75.02 (7)P2iv—O10—Eu138.15 (12)
O10—Eu—O6v142.48 (6)P2iv—O10—Kvi106.53 (11)
O5—Eu—O6v112.01 (7)Eu—O10—Kvi97.15 (7)
O7—Eu—O6v143.80 (7)P3—O11—P2iv132.40 (12)
O8—Eu—O6v80.43 (7)P3—O11—Kiv136.15 (9)
O9—Eu—O275.52 (7)P2iv—O11—Kiv88.55 (8)
O4—Eu—O2143.69 (7)P1—O12—P4iv133.66 (14)
O10—Eu—O269.57 (7)
Symmetry codes: (i) x+1/2, −y+3/2, z−1/2; (ii) −x+3/2, y−1/2, −z+1/2; (iii) x+1, y, z; (iv) −x+1, −y+2, −z; (v) −x+1/2, y−1/2, −z+1/2; (vi) −x+3/2, y+1/2, −z+1/2; (vii) x−1/2, −y+3/2, z+1/2; (viii) x−1/2, −y+3/2, z−1/2; (ix) x−1, y, z; (x) −x+1/2, y+1/2, −z+1/2; (xi) x+1/2, −y+3/2, z+1/2.
Table 1
Selected geometric parameters (Å) top
P1—O71.480 (2)P3—O61.482 (2)
P1—O4i1.483 (2)P3—O21.4873 (19)
P1—O121.588 (2)P3—O111.6005 (19)
P1—O3ii1.5968 (19)P3—O11.6033 (19)
P2—O10iii1.4795 (19)P4—O91.4784 (19)
P2—O51.483 (2)P4—O8v1.484 (2)
P2—O11iii1.6015 (18)P4—O11.601 (2)
P2—O3iv1.602 (2)P4—O12iii1.601 (2)
Symmetry codes: (i) −x+3/2, y+1/2, −z+1/2; (ii) x+1, y, z; (iii) −x+1, −y+2, −z; (iv) x+1/2, −y+3/2, z−1/2; (v) x−1/2, −y+3/2, z−1/2.
references
References top

Barsukov, I. V., Syťko, V. V. & Umreiko, D. S. (2004). J. Appl. Spectrosc. 71, 676–680.

Ben Zarkouna, E., Driss, A. & Férid, M. (2006). Acta Cryst. C62, i64–i66.

Brandenburg, K. (1999). DIAMOND. Crystal Impact GbR, Bonn, Germany.

Bruker (2008). SADABS, APEX2 and SAINT. Bruker AXS Inc., Madison,Wisconsin, USA.

Chehimi-Moumen, F. & Férid, M. (2007). Acta Cryst. E63, i129–i130.

Dago, A. M., Pushcharovskii, D. Yu., Pobedimskaya, E. A. & Belov, N. V. (1980). Sov. Phys. Dolk. 25, 231–233.

Durif, A. (1995). Crystal Chemistry of Condensed Phosphates. New York and London: Plenum Press.

Ettis, H., Naïli, H. & Mhiri, T. (2006). Acta Cryst. E62, i166–i168.

Horchani, K., Amami, J., Merle, D. & Férid, M. (2004). J. Phys. IV, 122, 123–128.

Horchani-Naifer, K. & Férid, M. (2007). Acta Cryst. E63, i131–i132.

Hu, N., Lin, Y., Zhou, Q.-L. & Liu, S.-Z. (1984). Yingyong Huaxue, 1, 47–50.

Jaouadi, K., Naïli, H., Zouari, N., Mhiri, T. & Daoud, A. (2003). J. Alloys Compd, 354, 104–114.

Khlissa, F. & Férid, M. (2006). Acta Cryst. E62, i272–i273.

Maksimova, S. I., Palkina, K. K. & Chibiskova, N. T. (1982). Izv. Akad. Nauk SSSR Neorg. Mater. 18, 653–700.

Maksimova, S. I., Palkina, K. K. & Loshchenov, V. V. (1981). Izv. Akad. Nauk SSSR Neorg. Mater. 17, 116–120.

Maksimova, S. I., Palkina, K. K., Loshchenov, V. B. & Kuznetsov, V. G. (1978). Zh. Neorg. Khim. 23, 2959–2965.

Naïli, H. & Mhiri, T. (2005). Acta Cryst. E61, i204–i207.

Palkina, K. K., Saifuddinov, V. Z., Kuznetsov, V. G. & Chudinova, N. N. (1977). Sov. Phys. Dolk. 237, 837–839.

Porai-Koshits, M. A. & Aslanov, L. A. (1972). J. Struct. Chem. 13, 244–253.

Rashchi, F. & Finch, J. A. (2000). Miner. Eng. 13, 1019–1035.

Rekik, W., Naïli, H. & Mhiri, T. (2004). Acta Cryst. C60, i50–i52.

Shannon, R. D. (1976). Acta Cryst. A32, 751–767.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Zhu, J., Cheng, W.-D. & Zhang, H. (2009). Acta Cryst. E65, i70.