Crystal structure and thermal expansion of the low- and high-temperature forms of BaMIV(PO4)(2) compounds (M = Ti, Zr, Hf and Sn)

摘要

The crystal structure of beta-BaZr(PO4)(2), archetype of the high-temperature forms of BaM(1304)2 phosphates (with M = Ti, Zr, Hf and Sn), has been solved ob 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 alpha-structure into a trigonal one (S.G. P (3) over bar m1) through a simple mechanism involving the unfolding of the [Zr(PO4)(2)](n)(2-) layers. The thermal expansion is very anisotropic (e.g., -4.1