X-RAY-DIFFRACTION MEASUREMENTS FOR EXPANDED FLUID SELENIUM UP TO THE SUPERCRITICAL REGION

The X-ray diffraction measurements for fluid Se have been carried out in the temperature and pressure range up to 1500-degrees-C and 815 bar. We have developed a new type of high-pressure vessel and a new sample cell made of single-crystal sapphire for the X-ray diffraction measurements using the energy-dispersive method. We have obtained the structure factor and the pair distribution function as functions of temperature and pressure. From the detailed analysis of the pair distribution function we conclude that the twofold coordinated structure is largely preserved in the region where the optical gap disappears and the progressive increase of conductivity becomes evident. On the basis of the findings we speculate on the existence of the small chain molecule with the planer zig-zag conformation which may cause the transition to the metallic state in fluid Se.