Structural and Optical Studies of High-Pressure Phases of Solid Oxygen

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1. By using high-energy and high-brilliance synchrotron radiation sources in Spring-8 and ESRF, an energy-dispersive powder X-ray diffraction study on solid oxygen has been carried out up to 220 GPa at low temperature due to 12 Pa at low temperature of 19 K. The phase transition to the ζmetallic high-pressure phase, which had been revealed by our previous experiments, was reconfirmed at 96 GPa in this study. The transition was sluggish, and was accompanied by the discontinuous band gap closure, which was observed by our infrared absorption measurement. The transition was first-order. The metallic phase was stable up to 220 GPa. The phase diagram in the low temperature and high pressure region was established. The magnetic ordering in the αand δphases was suggested from an enha cement of the anisotropy of lattice compression.2. Single-crystals of the ε-OィイD22ィエD2 phase were prepared by a strain anneal method under a condition of 25 GPa and 650 K and were used for infrared absorption experiments under pressure up to 92 GPa. The spectrum at 92 GPa exhibited a drastic change corresponding to the metallization. The frequency of the IR-active vibron fundamentals and libron were obtained as a function of pressure to 86 GPa at room temperature. Raman scattering measurements was also carried out at room temperature. From the spectroscopic data of molecular vibration, the inter- and intra molecular interaction, and further the structure of the εphase were discussed. These results suggested that a molecular state is rather stable in the metallic ζphase.3. Structural analysis of a single crystal of the β phase (R3m) suggested that the oxygen molecules were disordered under three-fold axis symmetry. The disorder state may be related to the magnetic structure of the phase.