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 nonmetal-to-metal (NM-to-M) transition observed in liquid Se when the system is expanded by increasing temperature in the liquid-vapor supercritical region. The NM-to-M transition accompanying the expansion of the system is unexpected since it is in contradiction to the established understanding of NM-to-M transitions. We introduce structural models for expanded Se and calculate the electronic states and the wave functions of these models by the simulated annealing method. From the results of our calculations, we show that, when some of Se-Se bonds are weakened in the process of expanding the system or equivalently in the process of decreasing the density: the splitting between the bonding and anti-bonding level is reduced and consequently the anti-bonding hand is relatively lowered, which leads to the disappearance of the band gap between the anti-bonding and lone-pair band, or in other words to the occurrence of the NM-to-M transition.