Structural changes and the metal-non-metaltransition in supercritical fluids

Abstract

Energy-dispersive x-ray diffraction (XD), small-angle x-rayscattering (SAXS) and x-ray absorption fine-structure (XAFS)measurements for metallic fluids up to the supercritical region werecarried out using synchrotron radiation. We obtained the structurefactor S(k) and the pair distribution functions g(r) forexpanded fluid Hg from the liquid to the dense vapour regionincluding the metal-non-metal (M-NM) transition region in thedensity range from 13.6 to 1.9 g cm-3 by means of XD measurements atSPring-8 in Japan. The density variations of the interatomic distance(r1) and coordination number (N1) obtained are discussed inrelation to the M-NM transition in fluid Hg. To investigatestructural change in the semiconductor-metal (SC-M) transition inexpanded fluid Se, XD measurements, at SPring-8, and XAFS measurements,at the European Synchrotron Radiation Facility (ESRF) in France, were carried out at high temperatures and high pressures.It was found that the twofold-coordinated chain structure ispreserved and contraction of the covalent bond occurs on theSC-M transition. XAFS measurements for dense Se vapour near thecritical point were also carried out to study how dimers in therarefied vapour condense to the metallic fluid. SAXS measurementswere carried out to obtain information on the density fluctuation offluid Se near the critical point. On the basis of the structuraldata obtained by XD, XAFS and SAXS measurements for fluid Se, wediscuss how the density fluctuation affects the local structure andelectronic properties of fluid Se near the critical point.