Struktur und Bindung in Übergangsmetall-Fluoriden MIIMeIVF6. Neutronenbeugungs-Strukturuntersuchungen an CaSnF6, FeZrF6, und CrZrF6</SBT>
Compounds MIIMe IVF 6 requently undergo phase transitions from the cubic ordered ReO 3 to the trigonal LiSbF 6 structure when lowering the temperature. In case of a strongly Jahn-Teller unstable cation in the MII position additional phases may occur. Results of powder neutron-diffraction studies on CaSnF 6, FeZrF 6, and CrZrF 6 at different temperatures are reported. The high-temperature phases have the space group Fm3 m; the F - ligands are either statistically displaced from the MIIMe IV directions or undergo a strong thermal motion perpendicular to these directions ( ∢M II FMe IV: 165-180°). The thermal ellipsoids of the CrF bonds are strongly indicative of a dynamical Jahn-Teller effect in addition. In the low-temperature phases of CaSnF 6 and FeZrF 6 (space group R overline3) the ∢M II FMe IV is more distinctly bent (⋍155-160°). CrZrF 6 undergoes two reversible phase transitions, which are determined to occur at 415 ± 5 K (cubic → tetragonal, dynamic to static Jahn-Teller distortion of CrF 6 octahedra and 150 ± 10 K (tetragonal → (pseudo)monoclinic).