The crystal structure of a new CdTe2O5 polymorph, isotypic with ∊-CaTe2O5

The crystal structure of a new polymorph of CdTe2O5, designated as the β-form, contains 2 ∞[Te2O5]2– (100) layers with an undulating shape.


Chemical context
Cadmium pentaoxidoditellurate(IV), better known under its common name cadmium ditellurite, CdTe 2 O 5 , has been the subject of numerous investigations during the past decades with different emphases. In this regard, the CdO-TeO 2 phase diagram was elucidated by Robertson et al. (1978), or the formation of glasses in the Cd-Te-O system by Karaduman et al. (2012). Other studies focused on electric and ferroelastic properties of CdTe 2 O 5 (Redman et al., 1970), with its ferroelasticity remaining up to the melting point (Sadovskaya et al., 1983;Gorbenko et al., 1990). Single crystals of CdTe 2 O 5 are usually grown from the melt utilizing the Czochralski or Bridgeman techniques as crystal growth methods (Nawash, 2015). Even though single crystals of CdTe 2 O 5 have been grown for decades this way, a satisfactory structure model for this phase had never been published so far, and only lattice parameters of a sub-cell were given (Redman et al., 1970). Other phases in the Cd-Te-O system that are compiled in the Inorganic Crystal Structure Database (ICSD; Zagorac et al., 2019) include two polymorphs of CdTe IV O 3 (Krä mer & Brandt, 1985;Poupon et al., 2017), two polymorphs of Cd 3 Te VI O 6 (Burckhardt et al., 1982;Weil & Veyer, 2018) and the mixed Te IV/VI -compounds Cd 2 Te 2 O 7 and Cd 2 Te 3 O 9 (Weil, 2004).
The lack of a reasonable structure model for CdTe 2 O 5 might be caused by the micaceous appearance of the grown crystals (Redman et al., 1970), as well as by its ferroelastic properties, ISSN 2056-9890 which often are correlated with the formation of twins or multiple domains. The new CdTe 2 O 5 phase discovered during the present study originally intended to synthesize new mixedvalent cadmium oxidotellurates(IV,VI), however, belongs to a different polymorph, hereafter referred to as the -form of CdTe 2 O 5 .
In this communication we report on the synthesis and crystal structure analysis of -CdTe 2 O 5 and compare it quantitatively with the isotypic structure of "-CaTe 2 O 5 (Weil & Stö ger, 2008;Barrier et al., 2009).

Figure 2
The [TeO 4 ] polyhedra in the crystal structure of -CdTe 2 O 5 . Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1.

Figure 1
The [CdO 7 ] polyhedron in the crystal structure of -CdTe 2 O 5 . Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1.
The [TeO 4 ] polyhedra are connected to each other to form layers oriented parallel to (100). These layers have a distinct undulating shape (Fig. 3)  electron lone pair situated at each of the two Te IV atoms. The space requirements of the non-bonding electron pairs lead to the undulating shape of the layer, which results in the presence of large channels in the structure, which are oriented parallel to [011] (Fig. 4). Smaller channels are also realized and propagate parallel to [010] (Fig. 5).
The arrangement of such an undulating 2 1 [Te 2 O 5 ] 2layer was reported for the first time for the "-polymorph of CaTe 2 O 5 (Weil & Stö ger, 2008), which is isotypic with -CdTe 2 O 5 . CaTe 2 O 5 is likewise reported to crystallize in a mica-like form from the melt (Redman et al., 1970). Although several hightemperature polymorphs have also been reported for this phase (Tripathi et al., 2001), "-CaTe 2 O 5 is the only polymorph for which a crystal-structure determination has been performed (Weil & Stö ger, 2008;Barrier et al., 2009). The close similarity between the two structures can be explained by the very similar ionic radii (Shannon, 1976) of Ca (CN 7: 1.20 Å ) and Cd (CN 7: 1.17 Å ). The corresponding bond lengths in the isotypic structures (Table 1) differ only slightly with one exception: the Te1-O4 bond, which is 0.165 Å longer in the Ca structure, shows by far the biggest difference. A quantitative comparison between -CdTe 2 O 5 and "-CaTe 2 O 5 was carried out using the compstru software (de la Flor et al., 2016), available at the Bilbao Crystallographic Server (Aroyo et al., 2006). The absolute distances between paired atoms are 0.0303 Å for Cd/Ca1, 0.0628 Å for Te1, 0.0178 Å for Te2, 0.1426 Å for O1, 0.0791 Å for O2, 0.0384 Å for O3, 0.0788 Å for O4 and 0.0635 Å for O5. The degree of lattice distortion is 0.0118, the arithmetic mean of the distance between paired atoms is 0.0642 Å , and the measure of similarity is 0.077. Large channels in the -CdTe 2 O 5 structure running parallel to [011]. Displacement ellipsoids are drawn at the 90% probability level.

Figure 5
Smaller channels in the -CdTe 2 O 5 structure running parallel to [010]. Displacement ellipsoids are drawn at the 90% probability level.

Figure 3
The crystal structure of -CdTe 2 O 5 in a projection along [001]. Displacement ellipsoids are drawn at the 90% probability level.
Cd(NO 3 ) 3 Á2H 2 O, 0.0484 g (0.303 mmol) TeO 2 , 0.0710 g (0.309 mmol) H 6 TeO 6 and 0.12 g (1.8 mmol) 25% wt NH 3(aq) were weighed into a small teflon vessel with a volume of ca 3 ml. Deionized water was added until the vessel was filled to about two thirds of its volume. Then the vessel was heated to 483 K in a steel autoclave for 7 d under autogenous pressure. Afterwards, the autoclave was cooled to room temperature within about 4 h. The reaction product was a light-yellow, almost white solid. In the X-ray powder pattern of the bulk, -Cd 3 TeO 6 (Burckhardt et al., 1982) and -CdTe 2 O 5 were found. Under a polarising microscope a few small shiny colourless blocks of -CdTe 2 O 5 were isolated for single-crystal measurements.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Atom labels and starting coord-inates for refinement were adopted from the isotypic "-CaTe 2 O 5 structure (Weil & Stö ger, 2008

Crystal data
CdTe 2  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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )