Renormalization of the phase transition in lead phosphate, Pb₃(PO₄)₂, by high pressure: structure

The structure of the high-pressure phase of lead phosphate, Pb₃(PO₄)₂, has been determined from neutron powder time-of-flight diffraction data. Rietveld refinement to data collected at a pressure of 1.93(1) GPa and room temperature yielded unit-cell parameters a = 5.4613(3) Å, c = 20.069(1) Å, V = 518.41(4) ų. The space group is Rm. The structure consists of PO₄ tetrahedra together with Pb atoms in two symmetrically distinct sites. Analysis of the anisotropic displacement parameters of the Pb and O atoms indicates that they are spatially disordered within the structure. A structural model including split sites for these atoms suggests that the local environments around the Pb atoms in the high-pressure phase are very similar to those found in the low-pressure phase. We conclude that the phase transition from C2/c to Rm symmetry occurs as a result of the disordering of the static displacements of the Pb2 atoms and not, to pressures of 1.93 GPa, as the result of the elimination of these displacements. The evolution of the structure of the monoclinic phase of lead phosphate with pressure was also followed. With increasing pressure there is an apparent decrease in the displacements of the Pb atoms from the symmetry points of the high-pressure phase, but that of Pb2 does not extrapolate to zero at the phase transition pressure.