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 CdII 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.