Density fluctuations and cluster formation for conductive fluids near liquid-vapor supercritical region.

液态硒在常压下具有双重配位链结构，并具有半导体性质。当体积随着温度和压力的增加而膨胀时，流体（f-） Se在超临界区域发生半导体-金属（SC-M）转变。随着体积进一步膨胀，金属流体变成由硒二聚体组成的绝缘蒸汽。研究水蒸气中的硒二聚体如何凝聚成金属流体是一个有趣的问题。预计大的波动将影响冷凝过程。本文对超临界流体Se在临界点（临界温度：1615 C，临界压力：385bar）附近进行了小角x射线散射（SAXS）和x射线吸收精细结构（XAFS）测量，研究了Se局部结构与密度波动的相关性。XAFS测量是在欧洲同步辐射设施使用同步辐射进行的。得到了绝缘Se蒸气在l650℃、高达600bar压力下的XAFS光谱。结果表明，密度小于临界密度的蒸汽主要由硒二聚体组成，而密度大于临界密度的蒸汽主要由二聚体和链状分子组成。SAXS测量是在广岛大学使用内部x射线源进行的，我们首次观察到f-Se的临界乳光。以高压He气体的SAXS强度为参考，得到了f-Se的SAXS强度的绝对值。利用Orenstein-Zernike公式，从观测到的临界密度附近的光谱推导出了约20A的相关长度和约15的密度波动。得到的密度波动强烈表明，稠密区的Se二聚体是由波动引起的链状分子。XAFS光谱观察到的二聚体和链状分子的混合物被认为来自于这种密度波动。

Liquid Se has two-fold coordinated chain structure at normal pressure and has semi conducting properties. As the volume is expanded with increasing temperature and pressure, fluid (f-) Se undergoes a semiconductor-metal (SC-M) transition in the supercritical region. With further volume-expansion, the metallic fluid becomes insulating vapor consisting of Se dimers. It is interesting to study how the Se dimers in the vapor condense to be the metallic fluid. Large fluctuations are expected to affect the condensation process. We have carried out small angle x-ray scattering (SAXS) and x-ray absorption fine structure (XAFS) measurements for supercritical fluid Se near the critical point (critical temperature ; 1615ﾟC, critical pressure ; 385bar) to study the correlation between the local structure and density fluctuations. XAFS measurements were carried out using synchrotron radiation at the European Synchrotron Radiation Facility. XAFS spectra for insulating Se vapor were obtained at l65OﾟC and at pressures up to 600bar. The results show that the vapor with smaller density than the critical density consists mainly of Se dimers arid that with larger density than the critical density consists of the dimers and chain molecules. The SAXS measurements were carried out using in-house x-ray source at Hiroshima University and we have observed the critical opalescence of f-Se for the first time. Using the SAXS intensity of a high pressure He gas as a reference, we have obtained the absolute values of the SAXS intensity of f-Se. The correlation length of about 20A and the density fluctuation of about 15 are deduced from the observed spectra near the critical density using Orenstein-Zernike formula. The obtained density fluctuation strongly suggests that Se dimers in the dense region induced by the fluctuation form chain molecules. The mixture of the dimers and chain molecules observed by the XAFS spectroscopy is considered to come from such density fluctuations.