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.

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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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Structurally integrated optical fiber sensors form the basis for smart structure technology. Over the past decade a variety of sensor configurations have been developed for measurement of strains and deformations in structures. Strains and deformations alter the refractive index and the geometry of the optical fiber material. These changes perturb the intensity, phase, and polarization of the light-wave propagating along the probing fiber. The optical perturbations are detected for the determination of strain. The research presented here describes the development of a new optical fiber sensor system for measurement of structural strains based on white light interferometry. An optical switch provides for multiplexing of strain signals from various locations in the structure. Redundant Bragg grating type fiber optic sensors as well as strain gauges were employed for comparison and verification of strain signals as measured by the new system. The system provides capability for distributed sensing of strains in large structures.