Synchrotron Mossbauer spectroscopy (SMS) was performed on an hcp-phase alloy of composition Fe92Ni8 at a pressure of 21 GPa and a temperature of 11 K. Density functional theoretical calculations predict antiferromagnetism in both hcp Fe and hcp Fe-Ni. For hcp Fe, these calculations predict no hyperfine magnetic field, consistent with previous experiments. For hcp Fe-Ni, however, substantial hyperfine magnetic fields are predicted, but these were not observed in the SMS spectra. Two possible explanations are suggested. First, small but significant errors in the generalized gradient approximation density functional may lead to an erroneous prediction of magnetic order or of erroneous hyperfine magnetic fields in antiferromagnetic hcp Fe-Ni. Alternately, quantum fluctuations with periods much shorter than the lifetime of the nuclear excited state would prohibit the detection of moments by SMS.

The local phonon density of states (DOS) at the Sn site in tin monoxide (SnO) is studied at pressures up to 8 GPa with Sn-119 nuclear resonant inelastic x-ray scattering (NRIXS) of synchrotron radiation at 23.88 keV. The preferred orientation (texture) of the SnO crystallites in the investigated samples is used to measure NRIXS spectra preferentially parallel and almost perpendicular to the c axis of tetragonal SnO. A subtraction method is applied to these NRIXS spectra to produce projected local Sn DOS spectra as seen parallel and perpendicular to the c axis of SnO. These experimentally obtained local Sn DOS spectra, both in the polycrystalline case as well as projected parallel and perpendicular to the c axis, are compared with corresponding theoretical phonon DOS spectra, derived from dispersion relations calculated with a recently developed shell model. Comparison between the experimental projected Sn DOS spectra and the corresponding theoretical DOS spectra enables us to follow the pressure-induced shifts of several acoustic and optic phonon modes. While the principal spectral features of the experimental and theoretical phonon DOS agree well at energies above 10 meV, the pressure behavior of the low-energy part of the DOS is not well reproduced by the theoretical calculations. In fact, they exhibit, in contrast to the experimental data, a dramatic softening of two low-energy modes, their energies approaching zero around 2.5 GPa, clearly indicating the limitations of the applied shell model. These difficulties are obviously connected with the complex Sn-O and Sn-Sn bindings within and between the Sn-O-Sn layers in the litharge structure of SnO. We derived from the experimental and theoretical DOS spectra a variety of elastic and thermodynamic parameters of the Sn sublattice, such as the Lamb-Mossbauer factor, the mean force constant, and Debye temperatures, as well as the vibrational contributions to the Helmholtz free energy, specific heat, entropy, and internal energy. We found, in part, good agreement between these values, for instance, for the Gruneisen parameters for some selected phonon modes, especially for some optical modes studied recently by Raman spectroscopy. We discuss in detail a possible anisotropy in the elastic parameters resulting from the litharge-type structure of SnO, for instance for the Lamb-Mossbauer factor, where we can compare with existing data from Sn-119-Mossbauer spectroscopy.

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.