Density Measurement for Expanded Fluid Selenium up to the Liquid-Vapor Supercritical Region by the X-ray Absorption Method

Density of expanded fluid selenium was measured in the temperature and pressure ranges up to 1650 •Ž and 700 bar including the liquid-vapor critical point by the x-ray absorption method . For the measurements, a new type of cell made of polycrystalline sapphire and a high-pressure vessel of authors' own design were developed We obtained the critical temperature, pressure and density of 1615•}5 •Ž, 385•}5 bar and 1.85±0.03 gcm3, respectively. The anomaly that isochores bend in the semiconductor-metal transition region was observed, and we found that the isochores again bend in the metal-insulator transition region near the critical point . The diameter in the liquid-vapor coexistence curve is deviated strongly from the law of the rectilinear diameter to the liquid side near the critical point.