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, TiTeO(3)F(2), and divanadium(IV) ditellurium(IV) heptaoxide difluoride, V(2)Te(2)O(7)F(2), have been characterized and present, respectively, titanium and vanadium in the tetravalent state. The TiTeO(3)F(2) structure is based on linear double rows of TiO(3)F(3) polyhedra sharing vertices. These rows are connected to adjacent rows via two vertices of Te(2)O(5) 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 V(2)Te(2)O(7)F(2) structure consists of zigzag chains of VO(4)F(2) octahedra alternately sharing O-O and F-F edges. These chains are connected via Te(2)O(5) 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 \overline{1} 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 V(2)Te(2)O(7)F(2), but in TiTeO(3)F(2) 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 MTeO(3)F, 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.