A new technique has been developed for optical studies of amorphous solids to very high pressures. Raman spectra of Si02 glass measured at 8 GPa indicate a significant reduction in the width of the Si-0-Si angle distribution, which has been associated with a number of anomalous properties of silica glass under ambient conditions. Between 8 and -30 GPa irreversible changes in the Raman spectrum occur that are consistent with a shift in ring statistics in densified glass. The spectra suggest a breakdown in intermediate-range order at higher pressure. PACS numbers: 78.30.6t, 61.40. +b, 62.50.+p The structure and properties of amorphous solids are of widespread interest because of their obvious importance in the design and synthesis of new materials. ' The structure of these materials, including the degree of short-range and intermediate order and the dependence of these quantities on pressure and temperature, are also of fundamental concern. The detailed structure of vitreous silica, a tetrahedral oxide glass, has in fact been the subject of much recent controversy. 2 'In the case of crystalline solids, the application of diffraction and spectroscopic techniques at high pressure for determining the effect of pressure on structure and for constraining assignments for phonon spectra has long been recognized.Similar studies of amorphous materials at high pressure, however, have been unsuccessful. 9 The problem with such measurements arises from limited access to samples in high-pressure devices, strong spurious scattering from the apparatus which interferes with the signal from the sample, and the generally weak and/or broad absorption or scattering cross sections of amorphous solids in comparison to those of crystals. In the present study we show that high-quality Raman spectra of weakly scattering amorphous solids at pressures of &30 GPa (&300 kbar) can be obtained with a sensitive micro-optical technique. We report the first Raman spectra of Si02 glass measured in situ at high pressure and find dramatic effects of pressure on its structure and vibrational properties.In addition to the controversy surrounding the structure of silica glass, there is fundamental interest in this material because it has a number of unusual properties, including anomalous behavior at high pressure. 9 " On compression, the bulk and longitudinal moduli of silica glass decrease, in contrast to the increase which is observed in most solids, and pass through a minimum at -2 GPa. 9'o In addition, silica glass can be densified or compacted by static high pressures (i.e. , &10 GPa), shock loading, and neutron irradiation.On the basis of Brillouin scattering measurements performed at high pressure and Raman measurements of the material quenched from high pressure, Grimsditch9 proposed that a new type of polymorphism between amorphous states occurs on static compression. However, the structural basis of such a transformation is unknown, as Raman spectra could not be measured in situ at high pressure because of strong interference from fluoresc...
The high-pressure behavior of nitrogen in NaN(3) was studied to 160 GPa at 120-3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N(3-) are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions.
Optical observations and x-ray diffraction measurements of the reaction between iron and hydrogen at high pressure to form iron hydride are described. The reaction is associated with a sudden pressure-induced expansion at 3.5 gigapascals of iron samples immersed in fluid hydrogen. Synchrotron x-ray diffraction measurements carried out to 62 gigapascals demonstrate that iron hydride has a double hexagonal close-packed structure, a cell volume up to 17% larger than pure iron, and a stoichiometry close to FeH. These results greatly extend the pressure range over which the technologically important iron-hydrogen phase diagram has been characterized and have implications for problems ranging from hydrogen degradation and embrittlement of ferrous metals to the presence of hydrogen in Earth's metallic core.
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