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 at high temperatures and pressures up to 1650 degrees C and 600 bar have been carried out to investigate the change of local structure with Volume expansion. The electronic property of fluid Se changes from a semiconducting one 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 consists mainly of Se dimers. When the volume contracts exceeding the critical density, the first peak in the radial distribution function is shifted towards larger distance and is increasingly asymmetrical. (C) 1999 Elsevier Science B.V. All rights reserved.