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°C 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°C, 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.