Density measurements for expanded fluid selenium up to the liquid-vapour supercritical region by the x-ray absorption method

Density of expanded fluid selenium was measured in the temperature and pressure ranges up to 1650 degrees C and 700 bar including the liquid-vapour 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. The critical temperature, pressure and density of 1615+/-5 degrees C, 385+/-5 bar, 1.85+/-0.03 g cm(-3), respectively, were obtained. We found that isochores bend in the semiconductor-metal transition region. The shape of the isochore seems an elongated inverse-g letter. The diameter of the liquid-vapour coexistence curve is deviated from the law of the rectilinear diameter to the liquid side slightly around the SC-M transition region and strongly near the critical point. A distinct crossover was also found in the critical exponent beta; the value of beta drastically changes from 0.30 to 0.10 when the critical point is approached.