Ternäre fluoride des typs MeIIMeIVF6 mit LaF3-struktur

High-temperature monoclinic α-SrHfF6, and isostructural α-SrZrF6: associating Hf2F12 bipolyhedra and SrF8 snub disphenoids

The structure of the high-temperature monoclinic variety α-SrHfF₆ (strontium hafnium hexafluoride) [and of isostructural α-SrZrF₆ (strontium zirconium hexafluoride)] associates Hf₂F₁₂ bipolyhedra and SrF₈ snub disphenoids, forming zigzag twisted [SrF₆]_n layers. The distribution of the Hf and Sr polyhedra forms a three-dimensional framework which can be related to the family of anion-excess ReO₃-related superstructures. α-SrHfF₆ corresponds to a new ABX₆ type and is compared to the other main families already described. A partial amorphization of this structure is observed in samples quenched from the melt.

Disorder in BaThF6--refinement of anharmonic displacement parameters from high-pressure single-crystal X-ray diffraction data

The structure of BaThF(6) has been investigated as a function of temperature and pressure with single-crystal X-ray diffraction using synchrotron radiation. The compound crystallizes in the tysonite structure, space group P6(3)/mmc (a = 4.296(1) Å and c = 7.571(1) Å at ambient conditions). It is stable at least down to 150 K and up to 4 GPa. In the entire range of pressures and temperatures studied here, the compound is characterized by a high degree of disorder, both on the cationic and anionic positions. Despite the different valence states and sizes, both cations occupy the same crystallographic site in the ideal tysonite structure. The cationic disorder is described by two alternative approaches. The first model corresponds to a split-atom position model in which Ba(2+) is maintained on a special position with site symmetry 6m2, while Th(4+) is slightly displaced from the respective position. In the second model, both cations are maintained on the ideal position and anharmonic displacement parameters using a tensor of third order are introduced. Anharmonic displacement parameters have been refined from high-pressure single-crystal X-ray data measured in situ in a diamond anvil cell for the first time. The feasibility and general problems of anharmonic refinements of high-pressure X-ray data are further commented.