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

Authors

Lii-Cherng Leu, Boise State University
Sherin Thomas, National Institute for Interdisciplinary Science and Technology (CSIR)
Mailadil Thomas Sebastian, National Institute for Interdisciplinary Science and Technology (CSIR)
Swavek Zdzieszynski, Alfred University
Rick Ubic, Boise State UniversityFollow

Document Type

Article

Publication Date

8-1-2011

Abstract

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, A^I₄A^II₆(BO₄)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 (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.

Publication Information

Leu, Lii-Cherng; Thomas, Sherin; Sebastian, Mailadil Thomas; Zdzieszynski, Swavek; and Ubic, Rick. (2011). "Crystal Structure of Apatite Type Rare-Earth Silicate (Sr₂RE₂)(RE₆)(SiO₄)₆O₂ (RE=La, Pr, Tb, Tm, and Y)". Journal of the American Ceramic Society, 94(8), 2625–2632. https://doi.org/10.1111/j.1551-2916.2011.04388.x