MICROSCOPIC ORIGIN OF THE PHASE TRANSITION IN LIQUID CHALCOGENS

已知Te、Se等液态硫化物在高温下发生液-液相变。这种结构转变伴随着半导体-金属（S-M）转变。本项目的主要目的是阐明观察到的液氢相变的微观起源。Hoshino， Maruyama和Endo考虑到扩展x射线吸收精细结构（EXAFS）数据的结构信息，对中子衍射（ND）测量得到的液体（1-）As-Te混合物的g(r)进行了一致的分析。我们的重要发现是，在1-As-Te混合物中，As位从三配位到双配位的变化与高温下的金属化有关，而在1-As-Te混合物中观察到的S-M转变是由短链分子组成的密集排列的转变引起的，而这主要是由体积收缩引起的。Yao等在较宽的温度和压力范围内测量了1-Te_<50>, Se_<50>的声衰减，发现S-M跃迁附近有一个尖锐的吸收峰，这可以解释为源于链间振荡运动的共振弛豫。计算机模拟研究是获取相变微观机理信息的必要手段。Oda和Hiwatari进行了1-氧分子动力学从头算模拟，1-氧是一种分子流体，其中单个o2单元携带分子磁矩。原子结构表现出对第一相邻分子平行排列的强烈偏好。磁性结构表现出较强的短程反铁磁相关性。Kimura用平均场理论研究了棒状和圆盘状分子的液晶混合物中的向列-各向同性相变。发现了从各向同性到两种单轴对称向列相之一的热致性相变；光学正N和负N，更少

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