Nanostructured crystals of the fluorite phases Sr1-xRxF2+x(R-rare-earth elements) and their ordering: II. Crystal structure of the ordered Sr4Lu3F17 phase

The crystal structure of the ordered phase Sr4Lu3F17 prepared by directed crystallization of the melt has been investigated. The crystals have a trigonally distorted fluorite lattice (space group R3-bar, Z = 6, a = 10.615(2) A, c = 19.547(6) A). The Sr4Lu3F17 phase is isostructural to Ba4R3F17 (R = Y, Yb). The distortions of the fluorite cation sublattice manifest themselves in the splitting of the only mixed position (Sr, Lu) into three positions: Sr(1), Sr(2), and Lu. All Lu3+ atoms are displaced from the center of the [Sr8{Lu6F36+1}F32] octacubic cluster in the [001]cub direction, and the Sr(1) cations are displaced toward the center of the octacubic cluster in the [111]cub direction. The coordination numbers of the Lu, Sr(1), and Sr(2) cations are 8, 10, and 11, respectively. The distortions of the fluorite anion sublattice are caused by the incorporation of additional anions into both the center of the octacubic cluster and the {F8} cubic holes outside the cluster.