Synthesis and Crystal Structure of Fluoride K2(Ta0.9I0.1)F7 with the Micro-Isomorphic Substitution in the Ta Cationic Position

Fluoride crystals K2(Ta0.9I0.1)F7 (sp. gr. Р21/c) complementary to the K2TaF7 and K2NbF7 fluoride family have been obtained by hydrothermal synthesis. Their structure contains isolated (TaF7)2‒ polyhedral blocks, the charge of which is compensated by K+ cations located between them. A small amount of the I5+ impurity has been found in the Та5+ site, which is atypical of this pair of elements. An example of such isomorphism is the occurrence of I5+ in the Ge4+ site in the K4[Ge0.85I0.12 □ 0.03]Ge5O20 germanate octahedron. The synthesis conditions have led to the separation of the initial components into two phases, fluoride K2(Ta0.9I0.1)F7 and iodate KIO3, without formation of the expected iodate fluorides.