Renormalization of the phase transition in lead phosphate, Pb-3(PO4)(2), by high pressure: structure

The structure of the high-pressure phase of lead phosphate, Pb-3(PO4)(2), h as been determined from neutron powder time-of-flight diffraction data. Rie tveld refinement to data collected at a pressure of 1.93(1) GPa and room te mperature yielded unit-cell parameters a = 5.4613(3) Angstrom c = 20.069(1) Angstrom, V = 518.41(4) Angstrom, The space group is R (3) over barm. The structure consists of PO4 tetrahedra together with ph atoms in two symmetri cally distinct sites. Analysis of the anisotropic displacement parameters o f the Pb and O atoms indicates that they are spatially disordered within th e structure. A structural model including split sites for these atoms sugge sts that the local environments around the Pb atoms in the high-pressure ph ase are very similar to those found in the low-pressure phase. We conclude that the phase transition from C2/c to R (3) over barm 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 d isplacements, The evolution of the structure of the monoclinic phase of lea d phosphate with pressure was also followed. With increasing pressure there is an apparent decrease in the displacements of the Ph atoms from the symm etry points of the high-pressure phase, but that of Pb2 does not extrapolat e to zero at the phase transition pressure.