Comparison of the crystal chemistry of tellurium (VI), molybdenum (VI), and tungsten (VI) in double perovskite oxides and related materials

A comprehensive structural comparison of 56 Te⁶⁺-, Mo⁶⁺-, and W⁶⁺-containing oxides with the double perovskite stoichiometry (A₂BB′O₆) is presented. This work shows that much like d⁰ Mo⁶⁺- and W⁶⁺-containing perovskites, p⁰ Te⁶⁺-containing compositions are strongly affected by the tolerance factor and identities of the A- and B-cations. To make this comparison more complete, the ambient temperature crystal structures of five A₂BTeO₆ (A = Ca²⁺, Sr²⁺, or Ba²⁺; B = Zn²⁺ or Cd²⁺) perovskites were determined via powder diffraction and their vibronic and electronic structures were probed via infrared and diffuse reflectance spectroscopy. The new structural information reported here coupled with a thorough review of relevant literature demonstrates that Te⁶⁺, with its sigma bonding preference and lack of allowed orbital mixing gives rise to additional structure types that are not commonly observed in the Mo⁶⁺ or W⁶⁺ analogues. Analysis of double perovskites containing the hexavalent cations comparing the tolerance factor to the difference in ionic radii of the cations with octahedral coordination is presented. Additionally, examination of the Coulombic repulsions between the B and Te⁶⁺ cations plotted as a function of difference in the twelve- and seven-coordinate ionic radii for the A- and B-cations respectively provides new insight on why A₂BTeO₆ and A₂BWO₆ (A = B = Sr²⁺ or Ba²⁺) adopt perovskite structures with non-cooperative octahedral tilting distortions, while cooperative octahedral distortions are observed when the A and B sites are occupied by smaller cations like Ca²⁺ and Cd²⁺.