The Crystal Structure and Stoichiometry of the Ca2+xNd8-x(SiO4)6O2-1/2x System

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Abstract

A systematic study of Ca2+xNd8−x(SiO4)602−1/2x, one of the mineral-like phases formed by the rare earths in the “supercalcineceramic” nuclear waste forms, has been undertaken. Until now, the structure and stoichiometry of this apatite phase has only been inferred from the hexagonal symmetry revealed by its powder diffraction data (1). The goal of this study was to obtain a complete set of atomic coordinates and the temperature factor and occupation factor for each atom in this structure by applying the Rietveld profile analysis technique (2, 3) to powder x-ray diffraction data. Samples of several bulk compositions in the range 0≤ x≤ 4 were prepared in order to evaluate the solid solution limits of the apatite phase.