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

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. P3-barm1) through a simple mechanism involving the unfolding of the [Zr(PO4)2]n2- layers. The thermal expansion is very anisotropic (e.g., -4.1<αi<34.0x10-6 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'. - Graphical abstract: The layered high-temperature form of BaM(PO4)2, only expands along the c-axis.