A new form of Cd3TeO6 revealing dimorphism

A new modification of Cd3TeO6, denoted as the β-form, has been structurally determined, adopting the rhombohedral Mg3TeO6 structure type.


Chemical context
Various salts of meta-telluric acid, H 2 TeO 4 , have been reported as a result of high-pressure and high-temperature experiments (3000 atm; 973 K) aiming at various M II TeO 4 phases, where M = Mg, Ca, Sr, Ba, Cd or Pb (Sleight et al., 1972). Meanwhile, the crystal structures of the Ca, Sr and Ba salts were determined (Hottentot & Loopstra, 1979;Weil et al., 2016) whereas those of the other phases remain unknown to date. In a recent project on single-crystal growth of the Cd salt of meta-telluric acid, we used a CsCl/NaCl salt mixture (Ź emcźuźny & Rambach, 1909) at temperatures < 800 K as a flux. Instead of the target phase CdTeO 4 , we obtained a new form of Cd 3 TeO 6 . The previously reported Cd 3 TeO 6 polymorph crystallizes as a monoclinically distorted cryolite-type material in space-group type P2 1 /n (Burckhardt et al., 1982) while the new form adopts the rhombohedral Mg 3 TeO 6 structure type.
Prior to the current study, solid solutions Cd 3-x Mn x TeO 6 with x = 3, 2, 1.5 and 1 were prepared in polycrystalline form (Ivanov et al., 2012), but not the cadmium end member, i.e. where x = 0. We report here the crystal structure of the new polymorph of Cd 3 TeO 6 , together with a comparative discussion of isostructural solid solutions Cd 3-x Mn x TeO 6 . In the following, we refer to the previously reported monoclinic polymorph of Cd 3 TeO 6 (Burckhardt et al., 1982) as the -form, and the new rhombohedral polymorph as the -form of Cd 3 TeO 6 .

Structural commentary
The crystal structure of -Cd 3 TeO 6 ( Fig. 1) (Christy et al., 2016;Gagné & Hawthorne, 2018). Both unique O atoms are bonded to one Te and three Cd atoms in the form of a distorted tetrahedron.
Like -Cd 3 TeO 6 , Mn 3 TeO 6 (Weil, 2006) as well as phases with x = 2, 1.5 and 1 of the Cd 3-x Mn x TeO 6 solid-solution series (Ivanov et al., 2012) adopt the rhombohedral Mg 3 TeO 6 structure type. A comparison of the bond lengths of the [MO 6 ] (M = Cd, Mn) octahedra in the end members -Cd 3 TeO 6 and Mn 3 TeO 6 and the solid solution Cd 1.5 Mn 1.5 TeO 6 (mixed occupancy for the M site) shows intermediate values for the solid solution, consistent with the different ionic radii for sixcoordinate Cd II and Mn II of 0.95 and 0.83 (high-spin) Å , respectively (Shannon, 1976). For a quantitative structural comparison of the end members -Cd 3 TeO 6 and Mn 3 TeO 6 the program compstru (de la Flor et al., 2016) available at the Bilbao Crystallographic Server (Aroyo et al., 2006) was used. The degree of lattice distortion is 0.0204, the maximum distance between the atomic positions of paired atoms is 0.0680 Å for pair O2, the arithmetic mean of all distances is 0.0417 Å , and the measure of similarity is 0.011. All these values show a high similarity between the two crystal structures.
The structure of the monoclinic -form of Cd 3 TeO 6 (Burckhardt et al., 1982) comprises of two cadmium sites (one on a general position and one on an inversion centre), one tellurium site on an inversion centre and three oxygen sites in general positions. While the [TeO 6 ] octahedra in both Cd 3 TeO 6 polymorphs have nearly the same bond length distribution [2 Â 1.904 (4), 2 Â 1.924 (5), 2 Â 1.948 (4) Å in the -form; for the -form, see: Table 1], the set of coordination polyhedra around the two Cd II cations in the two structures is different. In -Cd 3 TeO 6 , the cadmium site has a coordination number (CN) of six with an octahedral oxygen environment whereas in -Cd 3 TeO 6 , only one site is octahedrally surrounded [range of Cd-O bond lengths: 2.211 (5)-2.350 (4) Å ] and the other site exhibits a distorted [4 + 4] coordination [range of Cd-O bond lengths: 2.237 (5)-3.010 (5) Å ].
As noted above, the end members -Cd 3 TeO 6 and Mn 3 TeO 6 crystallize in the same structure type, suggesting a full miscibility over the complete range of x for the solid-solution series Cd 3-x Mn x TeO 6 . However, the adopted structure type for the complete range of x appears to be dependent on the reaction temperature. Single crystals of -Cd 3 TeO 6 for structure analysis were grown from a 9 CdO: 11 TeO 2 mixture that was heated in air at 1350 K for three h (Burckhardt et al., 1982) while single crystals of -Cd 3 TeO 6 were obtained at much lower temperatures (793 K) using a flux method. This suggests that the high-temperature synthesis yields the thermodynamically stable modification. The rule of thumb that in the majority of cases the denser polymorph represents also the thermodynamically stable modification supports this assumption because -Cd 3 TeO 6 [D x = 7.490 (2) Table 1 Selected bond lengths (Å ) in rhombohedral -Cd 3 TeO 6 and in isotypic (Cd 1.5 Mn 1.5 )TeO 6 and Mn 3 TeO 6 .  (2006) on the basis of single-crystal X-ray data at room temperature. [Symmetry codes: (i) y À 1 3 , Àx + y + 1 3 , Àz + 1 3 ; (ii) Àx + 1 3 , y + 2 3 , Àz + 2 3 ; (iii) Ày, x À y, z; (iv) Ày + 1 3 , x À y + 2 3 , z À 1 3 .]

Figure 1
The crystal structure of -Cd 3 TeO 6 in polyhedral view in a projection preparation of monoclinic -Cd 3 TeO 6 and the given solid solutions Cd 3-x Mn x TeO 6 (1270 K following a ceramic route; Ivanov et al., 2012), it appears likely that the rhombohedral -Cd 3 TeO 6 end member can be prepared only at lower temperatures whereas certain amounts of manganese substituting cadmium in the Cd 3-x Mn x TeO 6 solid-solution series stabilize the Mg 3 TeO 6 structure type at higher temperatures. Unfortunately, because of the scarcity of -Cd 3 TeO 6 material, a detailed investigation of the thermal behaviour of this phase, e.g. in terms of stability and a possible phase transition to -Cd 3 TeO 6 , could not be undertaken.

Database survey
According

Synthesis and crystallization
The rhombohedral -form of Cd 3 TeO 3 was obtained as one of the products from a flux synthesis using a CsCl/NaCl salt mixture (molar ratio 0.65/0.35). To 1.7 g of the salt mixture were added CdO (0.13 g) and TeO 3 (0.18 g). TeO 3 had previously been prepared by heating H 6 TeO 6 at 573 K for 8 h.
The reaction mixture was evacuated and sealed in a silica ampoule, heated from room temperature within 3 h to 793 K, kept at that temperature for 90 h and cooled within 10 h back to room temperature. The silica ampoule was subsequently broken and the solidified melt leached out with water for 2 h. The off-white product was filtered off, washed with water and was air-dried. The title compound was present in the form of a few nearly spherical colourless crystals. Other phases identified by single-crystal X-ray diffraction measurements of selected crystals were -Cd 3 TeO 6 (Burckhardt et al., 1982), the mixed-valent Te IV/VI compound Cd 2 Te 2 O 7 (Weil, 2004) and a new form of incommensurately modulated CdTe 2 O 5 (Weil & Stö ger, 2018). Estimated on optical inspection with a microscope, all these phases represent minor by-products. Powder X-ray diffraction measurements of the bulk additionally revealed triple-perovskite-type CsCdCl 3 (Siegel & Gebert, 1964) as the main phase and the Te IV compound CdTeO 3 (Krä mer & Brandt, 1985) as a minority phase. Some additional reflections in the X-ray powder diffraction pattern of the bulk could not be assigned to the phases mentioned above or to any other known phase(s).

Crystal data
Cd 3  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.