Lithium starts to lose its high reflectivity at 19 GPa and turns black like graphite. The color change is reversible with large hysteresis. The black color does not change to 200 GPa. At 75 GPa, it is not transparent to visible light, nor to a laser beam at 0. 785 eV, nor to a laser beam at 0. 12 eV. This behavior means it is either a semiconductor with a gap below 0. 12 eV, or a semi-metal like graphite although such behavior could arise from multiinternal reflections as a result of the anisotropy of the permittivity in the single crystal. There is no vibron peak from a lithium molecule up to 150 GPa. The Raman spectrum at 160 GPa has a broad peak around 600 cm-1 on a large background, which is considered to be caused by the fluorescence of diamond.
Oxygen is popular but unique among diatomic molecules in that it behaves magnetically at low temperature. Under high pressure, however, we expect the insulator-metal transition. This expectation has been suggested by measuring its optical reflectivity under high pressures around 95 GPa (1 Mbar). A new structural transition is considered to be accompanied by metallization. Obviously, the most direct method of detecting metallization is to measure the electrical resistance. We measured the resistance of oxygen at pressures of over 100 GPa and identified the metallic state from a change in the slope of dR/dT. In this paper, we present our findings on the superconducting transition of oxygen under high pressures of around 100 GPa and at temperatures of under 0. 6 K. The superconducting transition is indicated by a drop in resistance. We confirmed this by observing the magnetic field dependence of the drop and by detecting the Meissner demagnetization signal.
A molecular solid consisting of low-Z elements tends to crystallize into a plastic crystal, in which molecules rotate freely on lattice points. By the application of pressure, rotation of molecules freezes and the order-disorder transition takes place. The structural phase transition of these molecular crystals has been frequently studied by spectrographic techniques. The recent progress of synchrotron radiation sources has enabled us to investigate crystal structures of low-Z materials through x-ray diffraction analysis. In this article, pressure-induced structural phase transition of CX4 (X=H, F, Cl) solids is reviewed.
High-pressure studies of tin tetraiodide SnI4 are reviewed. Recent x-ray diffraction studies have shown that SnI4 undergoes successive phase transitions from an insulator crystalline phase (I; Pa3) to a metallic crystalline phase (II) at 7 GPa, to the amorphous state at about 15 GPa, and to a non-molecular crystalline phase (III; Fm3m) at 61 GPa. The crystal structure of phase III has been determined to be a substitutional disordered structure in which both iodine and tin atoms are randomly located at the fcc sites.
Hexaiodobenzene (C6I6) is one of the organic monomolecular crystals indicating pressure-induced metallization. Our recent studies on x-ray powder diffraction experiments and Raman scattering spectral measurements of C6I6 under high pressure are reviewed: pressurization in a preliminary process of the pressure-induced metallization causes six iodine atoms to be suspended from a benzene-ring accompanied by a decrease in intermolecular I-I distances. It follows from this that charge transfer interaction is mainly generated by the overlapping of a 5pz orbital of iodine and a π-orbital of carbon among the adjacent molecules which accelerates the molecular deformation.
In this article, a recent discovery of a liquid-liquid phase transition in phosphorus is reviewed. By an in situ X-ray diffraction method, we have observed an abrupt pressure-induced structural change between a low-pressure molecular liquid and a high-pressure polymeric liquid at about 1 GPa. Experimental results strongly support that it is a first-order transition between two thermodynamically stable liquid phases. This is the first in-situ observation of such a transition in a liquid of pure substance.
The single-crystal x-ray diffraction method using a diamond anvil cell is an important method for crystal structure determination under high-pressure. The x-ray absorption spectroscopy under high-pressure provides the detailed information on local structure around particular kinds of atoms, even if the crystal structure is unknown. Extended x-ray absorption fine structure (EXAFS) spectroscopy is useful as a probe of vibration dynamics under pressure. An anharmonic effective pair potential can be investigated using the EXAFS technique. The knowledge of the local structure is of primary importance for the understanding of the complicated physical properties of solid solutions. The change of the spin state of the Co4+ ion in the perovskite-type Sr (Co, Mn) O3 solid solution whose magnetic properties varies according to the compositions have been explained based on the local structure analysis.
The effective and reliable sealing with metal gaskets for a high-pressure optical window is described. A practical method for polishing a window plug is explained. We have developed a pair of jigs to shape Au gaskets, which are explained in detail. The shaping of Au gaskets with the jigs is also demonstrated. A window plug gripping jigs to fasten the high-pressure optical window is also introduced.
We have developed an experimental method using emulsions to measure the metastable melting curves of ice phases at low temperatures and high pressures. For the melting experiment, we constructed a special hydraulic press with two oil-pressure systems, and reduced error in the sample pressure.