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

Experimental results on the structural properties of expanded fluid Se are described. Energy-dispersive X-ray diffraction measurements have been carried out up to the supercritical region including the semiconductor-metal transition region. It has been established that the twofold coordinated structure is largely preserved even in the metallic region near the critical point. The results of XAFS, XSAS and density measurements up to the supercritical region of fluid Se are also presented. On the basis of these results, the existence of small chain molecules with planar zig-zag conformation which may cause the transition to the metallic state in fluid Se is postulated.