Crystal Structure and Thermal Expansion of the Low- and High- temperature Forms of BaMIV(PO4)2 Compounds (M ¼ Ti, Zr,HfandSn)

The crystal structure of b-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 a-structure into a trigonal one(S.G. P3m1) through a simple mechanism involving the unfolding of the [Zr(PO4)2]n2- layers. The thermal expansion is very anisotropic (e.g., -4.1 < ai < 34.0 x 10-6 K-1 in the case of a-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¿¿.

BREGIROUX D.;  JARDIN R.;  POPA Karin;  RAISON Philippe;  WALLEZ G;  QUARTON M;  BRUNELLI M.; 

2009-07-01

ACADEMIC PRESS INC ELSEVIER SCIENCE

JRC52957

0022-4596,   

www.elsevier.com/locate/jssc,    https://publications.jrc.ec.europa.eu/repository/handle/JRC52957,   

10.1016/j.jssc.2009.02.012,