X-Ray and TEM Studies of CdTeMoO6 and CoTeMoO6: A New Superstructure of Fluorite Type with Cation and Anion Deficiencies (▪CoTeMo)(□2O6)

The crystal structures of two telluromolybdates CdTeMoO₆ and CoTeMoO₆ have been solved ab initio. CdTeMoO₆ crystallizes in a tetragonal cell (space group No. 113, P2₁m, Z=2) with a=5.2840(1) Å, c=9.0595(2) Å whereas CoTeMoO₆ crystallizes in orthorhombic space group P2₁2₁2 (No. 18) with two formula units in the unit cell of dimensions a=5.2545(1) Å, b=5.0653(1) Å, and c=8.8589(2) Å. The X-ray powder diffraction pattern data were refined by the Rietveld profile technique and led respectively to R_Bragg=0.07 for CdTeMoO₆ and R_Bragg=0.07 for CoTeMoO₆. The crystal structure of CdTeMoO₆ is built up from corner-sharing distorted CdO₄ tetrahedra which build a layer in the a,b plane, while in CoTeMoO₆ cobalt atoms exhibited an octahedral distorted surrounding. Both compounds are simultaneously cation- and anion-deficient 1.1.2 superstructures of fluorite in which the electron lone pairs of tellurium are stereo-chemically active. High-resolution electron microscopy images of CdTeMoO₆ showed well-ordered crystals fragments, but in some crystals defects have also been detected.