Studies on Molecular Dissociation and Metallization of Molecular crystals under High Pressure

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1. Structural Phase Transition and Metallization of Solid OxygenThe insulator-metal transition of solid oxygen has been suggested from the slight increase of the refrectivity at pressure around 100 GPa. We have observed the isostructural phase transition from the epsilon phase to the zeta phase at 96 GPa. The both phases are assigned to a monoclinic lattice. The transition is responsible for the metallization of solid oxygen. At the transition, the monoclinic angle and the molar volume were scarcely changed. To reveal the mechanism of the metallization, we have performed Raman studies on solid oxygen under pressure of up to 110 GPa. At pressure of below the transition, a vibrational mode and librational modes were clearly observed and the frequency of the vibrational mode monotonically increased, but any peak of the Raman active modes was not observed at pressure of above the transition. The monotonical increase of the frequency suggests that oxygen molecules were not dissociated at the transition. Now we conclude that the metallization of solid oxygen is realized through the mechanism of the band overlapping. We also proposed the space group of the epsilon phase as C2/m.2. Pressure-induced structural phase transition of Se and CX_4 (X=F,Cl, Br)Pressure-induced structural phase transitions of CX_4 compounds (X=F,C1 and Br) have been investigated. Selenium has several allotropic modifications at ambient conditions. The molecular dissociation and the metallization of these modifications were also studied.