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Crystal structure and thermal expansion of the low- and high-temperature forms of BaM(PO4)2 compounds (M=Ti, Zr, Hf and Sn) |
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| Authors: | D. Bregiroux K. Popa P.E. Raison M. Quarton C. Ferrero |
| Affiliation: | a UPMC Univ. Paris 06, CNRS-UMR 7574, ENSCP-ParisTech, Laboratoire de Chimie de la Matière Condensée de Paris, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b ‘‘Al.I. Cuza’’ University, Department of Chemistry, 11-Carol I Blvd., 700506 Iasi, Romania c European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany d ESRF, 6 rue Jules Horowitz, BP 220-38043 Grenoble Cedex, France |
| Abstract: | The crystal structure of β-BaZr(PO4)2, archetype of the high-temperature forms of BaM(PO4)2 phosphates (with M=Ti, Zr, Hf and Sn), has been solved ab initio by Rietveld analysis from synchrotron X-ray powder diffraction data. The phase transition appears as a topotactic modification of the monoclinic (S.G. C2/m) lamellar α-structure into a trigonal one (S.G.  ) through a simple mechanism involving the unfolding of the  layers. The thermal expansion is very anisotropic (e.g., −4.1<αi<34.0×10−6 K−1 in the case of α-BaZr(PO4)2) and quite different in the two forms, as a consequence of symmetry. It stems from a complex combination of several mechanisms, involving bridging oxygen rocking in M-O-P linkages, and “bond thermal expansion”. |
| Keywords: | Thermal expansion Crystal structure Phosphate-based materials |
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