Crystal Structure of Apatite Type Rare-Earth Silicate (Sr2RE2)(RE6)(SiO4)6O2 (RE = La, Pr, Tb, Tm, and Y)

The crystal structures of apatite-type (Sr2RE2)(RE6)(SiO4)(6)O-2 (RE = La, Pr, Tb, Tm, and Y) ceramics prepared by conventional solid-state processing has been examined. The phase and structure analysis was carried out using powder X-ray diffraction (XRD) and transmission electron microscopy. Electron diffraction and Rietveld structure refinement of XRD data indicated that (Sr2RE2)(RE6)(SiO4)(6)O-2 (RE = La, Pr, Tb, and Y) has a typical oxyapatite-type structure, A(4)(I)A(6)(II)(BO4)(6)O-2 in space group P6(3)/m (No. 176), where the A(I) site is shared equally and randomly by Sr and RE ions, A(II) is occupied by RE ions only, and B is occupied by Si. As the metaprism twist angle in this lanthanide series should increase as the size of RE decreases, the unrealistically low metaprsim twist angle for (Sr2Tm2)(Tm-6)(SiO4)(6)O-2 suggested that the hexagonal metric of apatite might not be sustained and the symmetry reduced to monoclinic, space group P2(1)/m (No. 11), in order to compensate for the shorter Tm-O bond length. The P2(1)/m model for (Sr2Tm2)(Tm-6)(SiO4)(6)O-2 also yields a better fit and improvement in bond valence as compared with P6(3)/m model.