When subjected to high pressure and extensive x-radiation, water (H2O) molecules cleaved, forming O - O and H - H bonds. The oxygen (O) and hydrogen (H) framework in ice VII was converted into a molecular alloy of O-2 and H-2. X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrated that this crystalline solid differs from previously known phases. It remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the hydrogen-oxygen binary system.

When subjected to high pressure and extensive x-radiation, water (H2O) molecules cleaved, forming O - O and H - H bonds. The oxygen (O) and hydrogen (H) framework in ice VII was converted into a molecular alloy of O-2 and H-2. X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrated that this crystalline solid differs from previously known phases. It remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the hydrogen-oxygen binary system.

The HS -> LS spin crossover effect (high-spin -> low-spin transition) induced by high pressure in the range 45-53 GPa is observed in trivalent Fe3+ ions in the paramagnetic phase of a (GdFe3)-Fe-57(BO3)(4) gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K-beta high-resolution x-ray emission spectroscopy (YES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.

The HS -> LS spin crossover effect (high-spin -> low-spin transition) induced by high pressure in the range 45-53 GPa is observed in trivalent Fe3+ ions in the paramagnetic phase of a (GdFe3)-Fe-57(BO3)(4) gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K-beta high-resolution x-ray emission spectroscopy (YES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.

Applying hard x-ray photon radiation to a mixture of liquid N-2 and O-2 contained under pressure in a diamond-anvil cell, we break the strong covalent bonding of the molecules and form ionic compounds of complex nitrogen oxide ions at a pressure as low as 0.5 GPa previously expected for molecular phases. A new ionic NO(+)NO3(-) phase has been discovered at around 2 GPa. Structural characterization of the high-pressure ionic NO(+)NO3(-) phase with Rietveld refinement reveals an interesting layered monoclinic P2(1)/m structure with large elastic anisotropy, offering promises for generating materials with interesting properties and providing the basis for future theoretical studies.

Applying hard x-ray photon radiation to a mixture of liquid N-2 and O-2 contained under pressure in a diamond-anvil cell, we break the strong covalent bonding of the molecules and form ionic compounds of complex nitrogen oxide ions at a pressure as low as 0.5 GPa previously expected for molecular phases. A new ionic NO(+)NO3(-) phase has been discovered at around 2 GPa. Structural characterization of the high-pressure ionic NO(+)NO3(-) phase with Rietveld refinement reveals an interesting layered monoclinic P2(1)/m structure with large elastic anisotropy, offering promises for generating materials with interesting properties and providing the basis for future theoretical studies.

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near K-edge spectra of SiO2 glass to 51 GPa obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below similar to 10 GPa are consistent with those of quartz and coesite, whereas the spectra above similar to 22 GPa are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in SiO2 glass between 10 GPa and 22 GPa. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed SiO2 glass.

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near K-edge spectra of SiO2 glass to 51 GPa obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below similar to 10 GPa are consistent with those of quartz and coesite, whereas the spectra above similar to 22 GPa are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in SiO2 glass between 10 GPa and 22 GPa. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed SiO2 glass.

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe-57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mossbauer spectroscopy (nuclear forward scattering NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe-57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mossbauer spectroscopy (nuclear forward scattering NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.