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

Ba₂CuAlF₉ is monoclinic: a = 5.374(2) Å, b = 7.312(2) Å, c = 9.371(3) Å, 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 F_o/σ(F_o) > 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.