Die Kristallstruktur der Verbindungen BaCaGaF7 und BaCaCrF7

Crystal Structure of the Compounds BaCaGaF₇ and BaCaCrF₇ The isostructural fluorides BaCaGaF₇ and BaCaCrF₇ (values in parentheses) crystallize monoclinically in space group P2/n, Z = 4: a = 539.0 (539.8), b = 541.0 (542.2), c = 1897.8 (1900.6) pm, β = 92.33 (92.10)°. Complete X-ray single crystal structure determinations showed, that the compounds are built up from triple layers CaF_8/2 · MF₃]^2?, which consist of a central sheet of edge-sharing CaF_8/2^2? polyhedra (distorted square antiprisms) with M^IIIF₆octahedra condensed on it at both sides and which are held together by 12-coordinated barium ions. The resulting average distances are: Ga? F = 187.8 (Cr? F = 189.4) pm, Ca ? F = 236.2 (236.2) pm, Ba? F = 289.4 (289.4) pm. Some relations to the structures of other fluorides are discussed.