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Crystal Structure of Apatite Type Rare-Earth Silicate (Sr2RE2)(RE6)(SiO4)6O2 (RE=La, Pr, Tb, Tm, and Y)
Article first published online: 14 MAR 2011
© 2011 The American Ceramic Society
Journal of the American Ceramic Society
Volume 94, Issue 8, pages 2625–2632, August 2011
How to Cite
Leu, L.-C., Thomas, S., Sebastian, M. T., Zdzieszynski, S., Misture, S. and Ubic, R. (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: 2625–2632. doi: 10.1111/j.1551-2916.2011.04388.x
P. Davies—contributing editor
This work has been supported by the U.S. Agency for International Development and the National Academy of Sciences under the Pakistan-U.S. Science and Technology Cooperative Program, award no. PGA-P280420. Additional support was from National Science Foundation Major Research Instrumentation Program, Award No. 0521315.
- Issue published online: 9 AUG 2011
- Article first published online: 14 MAR 2011
- Manuscript No. 28123. Received June 8, 2010; approved December 16, 2010.
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.