X-ray Raman scattering experiments were performed on C-60 fullerenes and multi-wall carbon nanotubes (MWCNT) up to 20 GPa and 25 GPa using a diamond anvil cell and synchrotron radiation at HPCAT, Advanced Photon Source. The intensity of the near edge peaks representing sp(2) hybridization (pi*) decreases with increasing pressure in both materials. Around 13 GPa the X-ray Raman spectra of C-60 completely transformed to sp(3) type bonding leading to diamond-like phase. Similar features have been observed in MWCNT at 16 GPa indicating the formation of strong interlayer covalent bonds. Our experiments are in excellent agreement with recent theoretical simulations reported on the mechanical behaviour of compressed nanotubes and provide direct experimental evidence for bond switching of these carbonaceous systems at high pressures. (c) 2006 Elsevier B.V. All rights reserved.

X-ray Raman scattering experiments were performed on C-60 fullerenes and multi-wall carbon nanotubes (MWCNT) up to 20 GPa and 25 GPa using a diamond anvil cell and synchrotron radiation at HPCAT, Advanced Photon Source. The intensity of the near edge peaks representing sp(2) hybridization (pi*) decreases with increasing pressure in both materials. Around 13 GPa the X-ray Raman spectra of C-60 completely transformed to sp(3) type bonding leading to diamond-like phase. Similar features have been observed in MWCNT at 16 GPa indicating the formation of strong interlayer covalent bonds. Our experiments are in excellent agreement with recent theoretical simulations reported on the mechanical behaviour of compressed nanotubes and provide direct experimental evidence for bond switching of these carbonaceous systems at high pressures. (c) 2006 Elsevier B.V. All rights reserved.

Single wall carbon nanotubes (SWCNT) were ball milled with Ti and TiH2. Samples collected at different milling times were characterized by X-ray diffraction, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray Raman spectroscopy (XRS). While the intensity of the pi* excitonic transition of the Ti+SWCNT samples remains unchanged with milling time, a doublet feature observed around 285.4 eV in the X-ray Raman spectra of the 5 h milled sample shows increasing hybridization of the Ti states with the nanotubes above the Fermi level. On the other hand, in addition to a double peak nature, the XRS spectra of TiH2+SWCNT samples show a decrease in the pi* intensity which provides clear evidence for increasing sp(3) hybridization with milling time. (c) 2007 Elsevier B.V. All rights reserved.

Single wall carbon nanotubes (SWCNT) were ball milled with Ti and TiH2. Samples collected at different milling times were characterized by X-ray diffraction, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray Raman spectroscopy (XRS). While the intensity of the pi* excitonic transition of the Ti+SWCNT samples remains unchanged with milling time, a doublet feature observed around 285.4 eV in the X-ray Raman spectra of the 5 h milled sample shows increasing hybridization of the Ti states with the nanotubes above the Fermi level. On the other hand, in addition to a double peak nature, the XRS spectra of TiH2+SWCNT samples show a decrease in the pi* intensity which provides clear evidence for increasing sp(3) hybridization with milling time. (c) 2007 Elsevier B.V. All rights reserved.

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.