X-ray diffraction study of Ca0.88Gd0.12F2.12 single crystals with a modified fluorite structure

The characteristic structural features of a single crystal of the anion-exc essive fluorite phase Ca0.88Gd0.12F2.12 have been studied by X-ray diffract ion analysis (MoKalpha radiation), sp, gr. Fm3m, Z = 4, a = 5.5000(4) Angst rom. The structure was refined in the anharmonic approximation of thermal a tomic vibrations to R-w = 0.0047. A total of 78 independent structure ampli tudes were measured; 14 parameters were refined; sin theta/lambda less than or equal to 1.13 Angstrom(-1). In the structure under study, the replaceme nt of the divalent Ca2+ cations by the trivalent Gd3+ cations in the positi on 4a (0, 0, 0) resulted in the statistical distribution of the fluorine at oms over three positions of the sp. gr. Fm3m. Thus, the fluorine atoms occu py the position 8c (1/4, 1/4, 1/4), the main position in the fluorite struc ture type, and the position 32f (v, v, v) with v = 0.417(1) on threefold sy mmetry axes. In addition, some fluorine atoms are shifted from the main 8c position along the threefold symmetry axis to the position 32f (w, w, w) wi th w = 0.313(2). (C) 2000 MAIK "Nauka/Interperiodica".