New vanadium(IV) and titanium(IV) oxyfluorotellurates(IV): V2Te2O7F2 and TiTeO3F2

As part of a continuing study of oxyfluorotellurates(IV), materials likely to present interesting nonlinear optical properties, two new phases, titanium(IV) tellurium(IV) trioxide difluoride, TiTeO₃F₂, and divanadium(IV) ditellurium(IV) heptaoxide difluoride, V₂Te₂O₇F₂, have been characterized and present, respectively, titanium and vanadium in the tetravalent state. The TiTeO₃F₂ structure is based on linear double rows of TiO₃F₃ polyhedra sharing vertices. These rows are connected to adjacent rows via two vertices of Te₂O₅ bipolyhedra. The Te, Ti, one F and two O atoms are on general positions, with one O and F statistically occupying the same site with half‐occupancy for each anion. One O and one F occupy sites with .m. symmetry. The V₂Te₂O₇F₂ structure consists of zigzag chains of VO₄F₂ octahedra alternately sharing O—O and F—F edges. These chains are connected via Te₂O₅ bipolyhedra, forming independent mixed layers. The Te, V, one F and three O atoms are on general positions while one O atom occupies a site of symmetry. In both phases, the electronic lone pair E of the Te^IV atom is stereochemically active. A full O/F anionic ordering is observed in V₂Te₂O₇F₂, but in TiTeO₃F₂ one of the six anionic sites is occupied by half oxygen and half fluorine, all the others being strictly ordered. These compounds represent new members of a growing family of oxyfluorotellurates(IV), including the recently characterized members of formula MTeO₃F, M being a trivalent cation. As was true for the previous members, they are characterized by an unusually high thermal and chemical stability in relation to the absence of direct Te—F bonds.