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

液态硒在常压下具有二重配位链状结构，具有半导体性质。随着体积随着温度和压力的增加而膨胀，流体（f-）Se在超临界区域中经历超导体-金属（SC-M）转变。随着进一步的体积膨胀，金属流体变成由Se二聚体组成的绝缘蒸气。研究硒二聚体在蒸气中如何凝结成金属流体是一个有趣的问题。预计大的波动会影响冷凝过程。我们对超临界流体Se在临界点（临界温度1615 ℃，临界压力385 bar）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，研究了局部结构与密度涨落之间的相关性. XAFS测量是在欧洲同步辐射设施使用同步辐射进行的。在1650 ℃和600巴压力下获得了绝缘硒蒸气的XAFS谱。结果表明，密度小于临界密度的蒸气主要由Se二聚体组成，密度大于临界密度的蒸气主要由Se二聚体和链状分子组成。利用广岛大学自制的X射线源进行了小角X射线散射测量，首次观测到了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。根据Orenstein-Zernike公式计算出临界密度附近的相关长度约为20 A，密度涨落约为15。所得到的密度波动强烈表明，硒二聚体在致密区诱导的波动形式的链状分子。通过XAFS光谱观察到的二聚体和链分子的混合物被认为是来自这样的密度波动。

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.