X-ray absorption fine structure studies on expanded fluid Se: from liquid to dense vapor

X-ray absorption fine structure (XAFS) measurements for expanded fluid Se a t high temperatures and pressures up to 1650 degrees C and 600 bar have bee n carried out to investigate the change of local structure with Volume expa nsion. The electronic property of fluid Se changes from a semiconducting on e to metallic with increasing temperature and pressure. When the volume is further expanded, the fluid becomes non-metallic vapor. XAFS spectra of the non-metallic vapor shows that the vapor rarer than the critical density co nsists mainly of Se dimers. When the volume contracts exceeding the critica l density, the first peak in the radial distribution function is shifted to wards larger distance and is increasingly asymmetrical. (C) 1999 Elsevier S cience B.V. All rights reserved.