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

Pure lead phosphate, Pb3(PO4)2, undergoes a phase transition from C2/c to R\bar 3m symmetry at a pressure of approximately 1.8 GPa and room temperature. Single-crystal X-ray diffraction measurements of the unit-cell parameters of a sample doped with 1.6% Ba2+ for the Pb2+ indicates that the doping reduces the transition pressure by approximately 0.1 GPa. The structural evolution of both samples through the phase transition has been determined by Rietveld refinement of neutron powder diffraction data collected to pressures of 6.3 and 3.3 GPa, respectively. There is no evidence for any significant change in the local structure at the phase transition at high pressures; the structure of the R\bar 3m phase at pressures just above the phase transition includes disordered positions for several atoms. The observation of diffuse scattering from the R\bar 3m phase at high pressure by single-crystal X-ray diffraction suggests that the disorder is static and arises from the presence of several orientations of the ordered microdomains of the monoclinic local structure. The macroscopic transition from monoclinic to trigonal symmetry therefore appears to correspond to the pressure at which the coherency strains between the locally monoclinic microdomains are sufficient to create a dimensionally trigonal lattice within which local displacements of atoms are still significant. A further pressure increase then decreases the magnitude of these displacements until at 3.5 GPa or higher they are not detectable by our current experimental probes, and the structure appears to have true local and global trigonal symmetry.