Correlation between density variation and electrical conductivity in supercritical selenium probed by small angle x-ray scattering

Small angle x-ray scattering experiments were performed in order to investigate the semiconductor-metal and metal-nonmetal transitions that take place in supercritical fluid selenium. At the semiconductor to metal transition, we do not observe any density fluctuations indicating the homogeneity of the fluid at this scale. On the other hand, we observe the onset of density fluctuations and a sharp density variation in the pressure-temperature range of the metal-nonmetal transition. We show that the supercritical domain can be separated into two regions: a crossover takes place between a liquidlike density region and a gaslike one. The onset of density fluctuations, i.e., the development of rare and dense domains, forbids any percolation and induces the loss of the metallic character. (C) 2003 American Institute of Physics.