We have used X-ray Raman spectroscopy (XRS) to study benzene up to similar to 20 GPa in a diamond anvil cell at ambient temperature. The experiments were performed at the High-Pressure Collaborative Access Team's 16 ID-D undulator beamline at the Advanced Photon Source. Scanned monochromatic X-rays near 10 keV were used to probe the carbon X-ray edge near 284 eV via inelastic scattering. The diamond cell axis was oriented perpendicular to the X-ray beam axis to prevent carbon signal contamination from the diamonds. Beryllium gaskets confined the sample because of their high transmission throughput in this geometry. Spectral alterations with pressure indicate bonding changes that occur with pressure because of phase changes (liquid: phase 1, 11, 111, and III') and possibly due to changes in the hybridization of the bonds. Changes in the XRS spectra were especially evident in the data taken when the sample was in phase III', which may be related to a rate process observed in earlier shock wave studies.

We report measurements of the atomic form factor of lithium, beryllium, and aluminum single crystals at low-momentum transfers (Q=1.6-50 nm(-1)) from the intensity of phonons observed by inelastic x-ray scattering. Comparing to Hartree-Fock calculations, the form factor deviates significantly in the case of lithium and beryllium around k(F). These deviations can be mostly understood on the basis of electron redistribution by a pseudopotential. The influence of multiple scattering due to coherent phonon scattering and possible deviations from the adiabatic approximation are also discussed.

We report measurements of the atomic form factor of lithium, beryllium, and aluminum single crystals at low-momentum transfers (Q=1.6-50 nm(-1)) from the intensity of phonons observed by inelastic x-ray scattering. Comparing to Hartree-Fock calculations, the form factor deviates significantly in the case of lithium and beryllium around k(F). These deviations can be mostly understood on the basis of electron redistribution by a pseudopotential. The influence of multiple scattering due to coherent phonon scattering and possible deviations from the adiabatic approximation are also discussed.

The two widely accepted mechanisms of the insulator-metal Mott-Hubbard transitions which have been considered up until now are driven by the band-filling or bandwidth effects. We found a different mechanism of the Mott-Hubbard insulator-metal transition, which is controlled instead by the changes in the Mott-Hubbard energy U. In contrast to the changes in the bandwidth W in the "bandwidth control" scenario or to the variations of the band-filling n parameter in the "band-filling" scenario, a dramatic decrease in the Mott-Hubbard energy U plays the key role in this mechanism. We have experimentally observed this type of the insulator metal transition in the transition metal oxide BiFeO3. The decrease in the Mott-Hubbard energy is caused by the high-spin-low-spin crossover in the electronic d shell of 3d transition metal ion Fe3+ with d(5) configuration under high pressure. The pressure-induced spin crossover in BiFeO3 was investigated and confirmed by synchrotron x-ray diffraction, nuclear forward scattering, and x-ray emission methods. The insulator-metal transition at the same pressures was found by the optical absorption and dc resistivity measurements.

The two widely accepted mechanisms of the insulator-metal Mott-Hubbard transitions which have been considered up until now are driven by the band-filling or bandwidth effects. We found a different mechanism of the Mott-Hubbard insulator-metal transition, which is controlled instead by the changes in the Mott-Hubbard energy U. In contrast to the changes in the bandwidth W in the "bandwidth control" scenario or to the variations of the band-filling n parameter in the "band-filling" scenario, a dramatic decrease in the Mott-Hubbard energy U plays the key role in this mechanism. We have experimentally observed this type of the insulator metal transition in the transition metal oxide BiFeO3. The decrease in the Mott-Hubbard energy is caused by the high-spin-low-spin crossover in the electronic d shell of 3d transition metal ion Fe3+ with d(5) configuration under high pressure. The pressure-induced spin crossover in BiFeO3 was investigated and confirmed by synchrotron x-ray diffraction, nuclear forward scattering, and x-ray emission methods. The insulator-metal transition at the same pressures was found by the optical absorption and dc resistivity measurements.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earth's interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earth's interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; 1310) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earth's lower mantle.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earth's interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earth's interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; 1310) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earth's lower mantle.

High-resolution x-ray-absorption (XAS) experiments in the partial fluorescence yield mode (PFY) and resonant inelastic x-ray emission (RXES) measurements were performed on the intermediate-valence compound YbAl3 under pressure of up to 38 GPa. The results of the YbAl3 PFY-XAS and RXES studies show that the valence of Yb increases smoothly from 2.75 at ambient pressure to 2.93 at 38 GPa. In situ angle-dispersive synchrotron high-pressure x-ray-diffraction experiments carried out using a diamond cell at room temperature show that the ambient pressure cubic phase is stable up to 40 GPa. The results obtained from self-interaction corrected local spin density-functional calculations to understand the pressure effect on the Yb valence and compressibility are in good agreement with the experimental results.

High-resolution x-ray-absorption (XAS) experiments in the partial fluorescence yield mode (PFY) and resonant inelastic x-ray emission (RXES) measurements were performed on the intermediate-valence compound YbAl3 under pressure of up to 38 GPa. The results of the YbAl3 PFY-XAS and RXES studies show that the valence of Yb increases smoothly from 2.75 at ambient pressure to 2.93 at 38 GPa. In situ angle-dispersive synchrotron high-pressure x-ray-diffraction experiments carried out using a diamond cell at room temperature show that the ambient pressure cubic phase is stable up to 40 GPa. The results obtained from self-interaction corrected local spin density-functional calculations to understand the pressure effect on the Yb valence and compressibility are in good agreement with the experimental results.

The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O-2 molecules interact predominantly through the half-filled 1 pi(g)* orbital < 10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1 pi(g)* orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O-2 molecules at the transition to the E-phase. The E-phase, which consists of (O-2)(4) clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O-2)(4) clusters develop upon compression of the E-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.