STRUCTURAL AND ELECTRONIC-PROPERTIES OF EXPANDED SELENIUM IN THE LIQUID-VAPOR SUPERCRITICAL REGION

The purpose of this work is to give a theoretical explanation of the n onmetal-to-metal (NM-to-M) transition observed in liquid Se when the s ystem is expanded by increasing temperature in the liquid-vapor superc ritical region. The NM-to-M transition accompanying the expansion of t he system is unexpected since it is in contradiction to the establishe d understanding of NM-to-M transitions. We introduce structural models for expanded Se and calculate the electronic states and the wave func tions of these models by the simulated annealing method. From the resu lts of our calculations, we show that, when some of Se-Se bonds are we akened in the process of expanding the system or equivalently in the p rocess of decreasing the density: the splitting between the bonding an d anti-bonding level is reduced and consequently the anti-bonding hand is relatively lowered, which leads to the disappearance of the band g ap between the anti-bonding and lone-pair band, or in other words to t he occurrence of the NM-to-M transition.