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Crystal Structure of Apatite Type Rare‐Earth Silicate (Sr 2 RE 2 )(RE 6 )(SiO 4 ) 6 O 2 (RE=La, Pr, Tb, Tm, and Y)
Author(s) -
Leu LiiCherng,
Thomas Sherin,
Sebastian Mailadil Thomas,
Zdzieszynski Swavek,
Misture Scott,
Ubic Rick
Publication year - 2011
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2011.04388.x
Subject(s) - crystallography , apatite , monoclinic crystal system , crystal structure , materials science , silicate , mineralogy , rietveld refinement , chemistry , organic chemistry
The crystal structures of apatite‐type (Sr 2 RE 2 )(RE 6 )(SiO 4 ) 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 (Sr 2 RE 2 )(RE 6 )(SiO 4 ) 6 O 2 (RE=La, Pr, Tb, and Y) has a typical oxyapatite‐type structure, A I 4 A II 6 (BO 4 ) 6 O 2 in space group P 6 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 (Sr 2 Tm 2 )(Tm 6 )(SiO 4 ) 6 O 2 suggested that the hexagonal metric of apatite might not be sustained and the symmetry reduced to monoclinic, space group P 2 1 / m (No. 11), in order to compensate for the shorter Tm–O bond length. The P 2 1 / m model for (Sr 2 Tm 2 )(Tm 6 )(SiO 4 ) 6 O 2 also yields a better fit and improvement in bond valence as compared with P 6 3 / m model.