Crystal structure of apatite type rare-earth silicate (Sr ₂RE₂)(RE₆)(SiO₄)₆O ₂ (RE=La, Pr, Tb, Tm, and Y)

The crystal structures of apatite-type (Sr₂RE ₂)(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, A^I₄A^II₆(BO ₄)₆O₂ in space group P6₃/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 (Sr ₂Tm₂)(Tm₆)(SiO₄)₆O ₂ 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 P2 ₁/m model for (Sr₂Tm₂)(Tm₆)(SiO ₄)₆O₂ also yields a better fit and improvement in bond valence as compared with P6₃/m model.