SEMICONDUCTOR-METAL TRANSITION IN FLUID SELENIUM NEAR THE CRITICAL-POINT

In the present paper we discuss the aspects and mechanism of the nonmetal-metal transition in fluid Se which occurs near the liquid-gas critical point. A brief review is presented on the unique and striking behavior in the electronic, structural and thermodynamic properties around the transition. New results of the optical absorption and X-ray diffraction measurements using techniques developed in our laboratory are also presented. The optical absorption coefficient of fluid Se was measured at temperatures up to 1700 °C and pressures up to 500 bar. Using the cell with the sample thickness of 3000 A we obtained the absorption coefficient in the wide range extending to the value of 2×105cm-1. The optical gap, derived from the analysis of the spectra, substantially decreases with temperature and pressure eventually to vanish near the critical point. X-ray diffraction measurements were performed at temperatures up to 1500 °C and pressures up to 510 bar by the energy dispersion method. 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. On the basis of the findings we speculate the existence of the small chain molecule with the planar zig-zag conformation which may cause the transition to the metallic state. © 1990.