Preparation and structural elucidation of the new anion-excess fluorite variant Ba_4Er_3F_(17)

摘要

Pink coloured crystals of the new complex fluoride Ba_4Er_3F_(17) were prepared via solid state reactions from mixed powders of BaF_2 and ErF_3 (4:3) in platinum crucibles under Ar (1973 K, 48 h). Ba_4Er_3F_(17) crystallizes in the space group R 3-bar m with a = 11.059(1) and c = 20.346(2) A. The structural parameters have been refined using single crystal data. The structure of Ba_4Er_3F_(17) can be described as an ordered anion-rich fluorite type structure. The excess fluorine content is directly correlated with occurrence of Er_6F_(36) clusters. Within these units the central F_(12) cuboctahedra are centered by one additional F which lies on a sixfold split position. The trivalent Er are square antiprismatically surrounded by 8 F with distances Er-F ranging from 2.24 to 2.44 A and an additional capping F at 2.59 A. The two crystallographically different Ba are coordinated by 10 and 11 F, respectively, at distances of 2.59-3.15 A. Calculations of the Madelung part of the lattice energy, MAPLE, were performed for the analysis of the chemical bonding.