The Crystal Structure of Ba2CuAlF9: a New Structure Type in Copper Fluoride Chemistry

Ba2CuAlF9 is monoclinic: a = 5.374(2) A, b = 7.312(2) A, c = 9.371(3) A, β = 90.20(1)°, Z = 2, space group P21/c (n° 14). The crystal structure was solved from X-ray single crystal data using 1071 unique reflections (900 with Fo/σ(Fo) > 4, R factor = 0.075). It is built up from infinite isolated cis chains of [MF6] mixed occupied fluorine octahedra sharing each, one edge and one vertex (M is randomly Cu or Al). An analogous kind of linkage was already observed for two other compounds from the ternary system BaF2/CuF2/AlF3. Close structural relationships exist between the cationic subnetworks of γ-BaAlF5 and Ba2CuAlF9.