MICROSCOPIC ORIGIN OF THE PHASE TRANSITION IN LIQUID CHALCOGENS

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It is known that the liquid chalcogens such as Te and Se undergo the liquid-liquid phase transition at high temperature. This structural transition is accompanied by the semiconductor-metal (S-M) transition.The main purpose of the present project is to elucidate the microscopic origin of the phase transition observed for the liquid chalcogen. A consistent analysis of g(r) for liquid (1-) As-Te mixtures obtained from the neutron diffraction (ND) measurements has been performed by Hoshino, Maruyama and Endo taking the structural information on the extended X-ray absorption fine structure (EXAFS) data into consideration. Our important findings are that in the 1-As-Te mixtures the change of As sites from the threefold coordinated to twofold coordinated configuration is associated with the metallization at high temperature and the S-M transition observed for 1-As-Te mixtures arises from the transformation to densely packed configuration composed of shortened chain- molecules which is caused … More by volume contraction.Yao et al. have measured the sound attenuation in 1-Te_<50>, Se_<50> in a wide temperature and pressure range and found a sharp absorption peak near the S-M transition, which may be interpreted as a resonance relaxation originating from the inter-chain oscillating motion.The computer simulation study is indispensable for obtaining information on the microscopic mechanism of the transition. Oda and Hiwatari have carried out an ab initio molecular dynamics simulation of 1-oxygen, a molecular fluid in which the individual O_2 units carry a molecular magnetic moment. The atomic structure shows a strong preference for parallel alignment of the first neighbor molecules. The magnetic structure shows strong short range antiferromagnetic correlations.Kimura has studied the nematic-isotropic phase transition in the liquid crystalline mixtures of rod-like and disk-like molecules by a mean field theory. The thermotropic phase transition have been found from the isotropic to one of the two types of uniaxially symmetric nematic phases; optically positive N and negative N. Less