Crystal Structure of Apatite Type Rare-Earth Silicate (Sr\u3csub\u3e2\u3c/sub\u3eRE\u3csub\u3e2\u3c/sub\u3e)(RE\u3csub\u3e6\u3c/sub\u3e)(SiO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e6\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e (RE=La, Pr, Tb, Tm, and Y)

The crystal structures of apatite-type (Sr2RE2)(RE6)(SiO4)6O2 (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)6O2 (RE=La, Pr, Tb, and Y) has a typical oxyapatite-type structure, AI4AII6(BO4)6O2 in space group P63/m (No. 176), where the AI site is shared equally and randomly by Sr and RE ions, AII 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)(Tm6)(SiO4)6O2 suggested that the hexagonal metric of apatite might not be sustained and the symmetry reduced to monoclinic, space group P21/m (No. 11), in order to compensate for the shorter Tm–O bond length. The P21/m model for (Sr2Tm2)(Tm6)(SiO4)6O2 also yields a better fit and improvement in bond valence as compared with P63/m model