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Local and long-range order in ferroelastic lead phosphate at high pressure

Acta Cryst. (2004). B60, 1–9 [doi:10.1107/S0108768103026582]

[Lead phosphate]hk-½ reciprocal lattice section collected from a single crystal of Pb3(As0.52,P0.48O4)2 shows diffuse maxima that are indexed with half-integer indices of the R-3m unit cell.

Lead orthophosphate, Pb3(PO4)2, undergoes a phase transition from C2/c to R-3m symmetry at a pressure of 1.8 GPa at room temperature. Structures determined from neutron powder diffraction data indicate that the R-3m phase includes disordered positions for several atoms. Observation of X-ray diffuse scattering from the R-3m phase at high pressure suggests that this disorder is static and arises from the presence of several orientations of ordered microdomains of monoclinic local structure, and thus there is no significant change in the local structure at the phase transition. This is in contrast to the high-temperature R-3m phase in which the disorder is dynamic in nature.

R. J. Angel, U. Bismayer and W. G. Marshall