Crystal structures of (Mg1-x,Fe(x))SiO3 postperovskite at high pressures.

X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg(0.9)Fe(0.1))SiO(3) and (Mg(0.6)Fe(0.4))SiO(3) at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO(3)-type structure with space group Cmcm, whereas the latter crystallizes in a structure with the Pmcm (Pmma) space group. The latter has a significantly higher density (ρ = 6.119(1) g/cm(3)) than the former (ρ = 5.694(8) g/cm(3)) due to both the larger amount of iron and the smaller ionic radius of Fe(2+) as a result of an electronic spin transition observed by X-ray emission spectroscopy (XES). The smaller ionic radius for low-spin compared to high-spin Fe(2+) also leads to an ordered cation distribution in the M1 and M2 crystallographic sites of the higher density ppv structure. Rietveld structure refinement indicates that approximately 70% of the total Fe(2+) in that phase occupies the M2 site. XES results indicate a loss of 70% of the unpaired electronic spins consistent with a low spin M2 site and high spin M1 site. First-principles calculations of the magnetic ordering confirm that Pmcm with a two-site model is energetically more favorable at high pressure, and predict that the ordered structure is anisotropic in its electrical and elastic properties. These results suggest that interpretations of seismic structure in the deep mantle need to treat a broader range of mineral structures than previously considered.

New high-pressure tetragonal polymorphs of SrTiO3-molecular orbital and Raman band change under pressure.

The vibrational and structural properties of SrTiO3 were investigated using Raman spectroscopy, synchrotron x-ray powder diffraction up to 55 GPa at 300 K, and ab initio quantum chemical molecular orbital (MO) calculations. These measurements and calculations revealed the structure transformation from cubic to tetragonal phase at about 9 GPa. Above 9 GPa, sharper new peaks were associated with a tetragonal structure. At about 30 GPa some bands disappeared and several new bands emerged. Structure transformation from I4/mcm to a new structure of P4/mbm was found at above 30 GPa by Rietveld profile fitting analysis. The diffraction pattern gave no indication of a Cmcm orthorhombic phase. Ab initio MO calculation proved the change of the molecular orbital coupling with a structure transformation. The Mulliken charge of Ti is increased with increasing pressure in the cubic phase, but the Sr charge continuously decreased. The d-p-π hybridization of the Ti-O bond and localizing the electron density are decreased with increasing pressure. The Ti-O bond becomes shorted in the P4/mbm phase and the change in the Ti charge accelerated more. All present investigations by x-ray diffraction, Raman spectra study and MO calculation show consistent results.

A new polymorph of FeAlO3 at high pressure.

Synchrotron X-ray diffraction measurements confirmed that a new polymorph of FeAlO3 could be synthesized at about 1800 K and 72 GPa. This phase can be indexed on an orthorhombic cell and transforms into the trigonal form on release of pressure. The c/a ratio of about 2.71 of the trigonal phase suggests corundum structure of FeAlO3 rather than LiNbO3 or ilmenite structure. This conclusion also suggests that the high-pressure orthorhombic phase could be the Rh2O3(II) structure rather than the GdFeO3-type perovskite structure.

Pressure response of Raman spectra of water and its implication to the change in hydrogen bond interaction.

In situ Raman spectroscopic measurements of water in the region of OH vibration were conducted up to 0.4 GPa at 23 and 52 degrees C. The frequencies of the decomposed OH stretching bands initially decreased with increasing pressure, reached a minimum at 0.15 GPa and increased up to 0.3 GPa and then decreased, which corresponds to the variations of the strength of hydrogen bonding. This variation was observed at 23 degrees C, but not at 52 degrees C, which suggests a change in pressure dependence on the hydrogen bond interaction between these two temperatures. Based on the equilibration model between hydrogen-bonded and nonhydrogen-bonded molecules, the present experimental results indicate that the pressure variation of the viscosity depends on the ratio of hydrogen-bonded molecules, rather than the strength of hydrogen bonding between molecules.

Structural changes induced by lattice-electron interactions: SiO2 stishovite and FeTiO3 ilmenite.

The bright source and highly collimated beam of synchrotron radiation offers many advantages for single-crystal structure analysis under non-ambient conditions. The structure changes induced by the lattice-electron interaction under high pressure have been investigated using a diamond anvil pressure cell. The pressure dependence of electron density distributions around atoms is elucidated by a single-crystal diffraction study using deformation electron density analysis and the maximum entropy method. In order to understand the bonding electrons under pressure, diffraction intensity measurements of FeTiO3 ilmenite and gamma-SiO2 stishovite single crystals at high pressures were made using synchrotron radiation. Both diffraction studies describe the electron density distribution including bonding electrons and provide the effective charge of the cations. In both cases the valence electrons are more localized around the cations with increasing pressure. This is consistent with molecular orbital calculations, proving that the bonding electron density becomes smaller with pressure. The thermal displacement parameters of both samples are reduced with increasing pressure.