A new form of Cd₃TeO₆ revealing dimorphism

Phase-formation studies in the system CdO-TeO₃ using a CsCl/NaCl melt at comparatively low temperatures revealed that tricadmium orthotellurate(VI), Cd₃TeO₆, is dimorphic. The new modification of Cd₃TeO₆ is denoted as the β-form and adopts the rhombohedral Mg₃TeO₆ structure type with one Cd and two O sites in general positions, and two Te sites with site symmetry each. In comparison with the previously reported monoclinic cryolite-type α-form that was prepared at higher temperatures, β-Cd₃TeO₆ has a much lower density and most likely represents a metastable modification. Whereas the [TeO₆] octa-hedra in both polymorphs are very similar and show only minor deviations from ideal values, the polyhedra around the Cd^II sites are different, with a distorted [CdO₆] octa-hedron in both modifications but an additional [CdO₈] polyhedron with a [4 + 4] coordination in the α-form.