X-ray powder diffraction data and Rietveld refinement for Ln6WO12 (Ln=Y, Ho)

The crystal structure of the two isostructural rare earth tungstates Ln(6)WO(12) (Ln=Y, Ho) has been refined by the Rietveld method from X-ray powder diffraction data. They crystallize with a three-dimensional rhombohedral structure (S.G. R (3) over bar and Z=3 for the R-centered setting) closely related to that of the binary oxides Ln(7)O(12) and deriving from the ideal fluorite structure. Final refinements. with isotropic thermal motion for each atom, resulted in profile and structure factors R-wp=0.166, R-F=0.037 with Ln=Y and R-wp=0.121, R-F=0.040 with Ln=Ho. The rare earth element is sevenfold coordinated with Ln-O bond lengths ranging from 2.19 to 2.70 Angstrom for Y6WO12 and from 2.18 to 2.68 Angstrom for Ho6WO12; the coordination polyhedron may be described as a monocapped trigonal prism. The tungsten atom is located at the center of a WO6 octahedron with a unique W-O distance of 1.98 and 1.92 Angstrom for Y6WO12 and Ho6WO12 respectively. (C) 2000 International Centre for Diffraction Data. [S0885-7156(00)00504-2].