Anomalous compression behaviour of GdPO4-monazite

Many AB2O4 spinels transform to post-spinel structures without decomposition.Three structures of CaMn2O4 (Pmab), CaFe2O4 (Pnam) and CaTi2O4 (Bbmm) have been reported as post-spinel phases.Structure studies were conducted under high pressure using SR.CaMn2O4 transforms to CaTi2O4 type structure at about 30GPa.MnO6 octahedron is distorted by the Jahn-Teller effect induced from Mn 3+ (3d4) in the octahedral site.X-ray emission study at APS has been executed to discloses high-low spin transition of CaMn2O4 up to 100GPa.The spin transition gradually proceeds from 50GPa and then mixed spin states at 80GPa.Low-spin state is confirmed at 85GPa.A new phase is found by powder diffraction at 82 GPa.This structure (Pnam) is produced via martensic transformation by displacing atoms in every three layers perpendicular to the c-axis.Four independent sites of Mn atoms yield the different spin transition pressures.PbTiO3 tetragonal perovskite structure (P4mm) at ambient conditions transforms cubic (Pm3m) at about 12GPa, which induces the transition from ferroelectric to paraelectric.Electron density distribution analysis is performed by maximum entropy method (MEM) using single-crystal diffraction intensities up to 12GPa taken at PF and up to 35GPa by two-circle diffractometer with IP at APS. Pressure dependence of the static dipole moment is elucidated by charge density analysis.MEM provides the split atom model of Ti and oxygen atoms in the direction to the c-axis and in the cubic phase no more splitting was observed.The split is generated from the statistical distribution of atoms or domain structures.The split becomes gradually smaller with increasing pressure.This is an intrinsic phenomenon in PbTiO3.Polarization in PbTiO3 is partly canceled.