Crystal structure and stoichiometry of the Ca/sub 2+x/Nd/sub 8-x/(SiO4)6O/sub 2-1/2x/ system

A systematic study of Ca/sub 2+x/Nd/sub 8-x/(SiO4)6O/sub 2-1/2x/, one of the mineral-like phases formed by the rare earths in the supercalcine-ceramic 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. 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 to powder x-ray diffraction data. Samples of several bulk compositions in the range 0 less than or equal to x less than or equal to 4 were prepared in order to evaluate the solid solution limits of the apatite phase. The structure of Ca2Nd8(SiO4)6O2 has been shown to be isostructural with Ca10(PO4)6F2. The metal ions occupying the 4f site are coordinated by six silicate oxygens at an average distance of 2.56 A. The Nd(+3) ions occupying the 6h site are coordinated by six silicate oxygens at an average distance of 2.49 A and by one anionic oxygen at 2.27 A. The anionic oxygen in 2a is coordinated by three Nd(+3) ions. The structure of Ca3Nd7(SiO4)6O15 is similar except that five Nd(+3) and one Ca(+2) ions occupy the 6h site and 1/4 of the anionic oxygens are missing in a random fashion