Structural changes in REMnO 3 (RE= Y, Ho, Lu) under high pressure were examined by synchrotron x-ray diffraction methods at room temperature. Compression occurs more readily in the ab plane than along the c-axis. With increased pressure, a pressure-induced hexagonal to orthorhombic phase transition was observed starting at ~ 22 GPa for Lu(Y)MnO 3 . When the pressure is increased to 35 GPa, a small volume fraction of Lu(Y)MnO 3 is converted to the orthorhombic phase and the orthorhombic phase is maintained on pressure release. High pressure IR absorption spectroscopy and Mn K-edge near edge x-ray absorption spectroscopy confirm that the hexagonal P6 3 cm structure is stable below ~20 GPa and the environment around Mn ion is not changed. Shifts in the unoccupied p-band density of states with pressure are observed in the Mn K-Edge spectra. A schematic pressure-temperature phase diagram is given for the small ion REMnO 3 system.
We have combined temperature-dependent local structural measurements with first-principles densityfunctional calculations to develop a three-dimensional local structure model of the misfit system ͓Ca 2 CoO 3 ͔͓CoO 2 ͔ 1.61 ͑referred to as Ca 3 Co 4 O 9 ͒ which has a rocksalt structure stacked incommensurately on a hexagonal CoO 2 lattice. The local structural measurements reveal a low coordination of Co͑2͒-O bonds in the rocksalt layer with large static structural disorder. The temperature dependence of the Co͑1͒-Co͑1͒ bond correlations in the CoO 2 layer is found to be normal above ϳ75 K and has very small static disorder. An anomalous enhancement in the Co͑1͒-Co͑1͒ correlations occurs at the onset of long-range magnetic order. Density-functional computations suggest that the reduction in the coordination of Co͑2͒ is due to the formation of chains of Co͑2͒O x in the a-b plane linked to the Ca-O layers by c-axis Co͑2͒-O bonds. The reduced dimensionality introduced by the chainlike structure in the rocksalt layer and high atomic order in the CoO 2 layer may enable low thermal conductivity and high electrical conductivity in the respective layers.
High-pressure resistivity and x-ray diffraction measurements were conducted on La 0.85 MnO 3-δ to ~6 GPa and ~7 GPa, respectively. At low pressures the metal-insulator transition temperature (T MI ) increases linearly up to a critical pressure, P* ~ 3.4 GPa, followed by reduction of T MI at higher pressure. Analysis of the bond distances and bond angles reveal that a bandwidth increase drives the increase of T MI below P*. The reduction of T MI at higher pressures is found to result from Jahn-Teller distortions of the MnO 6 octahedra. The role of anharmonic interatomic potentials is discussed.
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