Structural and Electronic Properties of Expanded Selenium in the Liquid-Vapor Supercritical Region

Abstract

The purpose of this work is to give a theoretical explanationof 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 isunexpected since it is in contradiction to the established understandingof NM-to-M transitions. We introduce structural models for expanded Se and calculatethe 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--Sebonds are weakened in the process of expanding the system or equivalently in the processof decreasing the density, the splittingbetween the bonding and anti-bonding level is reduced and consequentlythe anti-bonding band is relatively lowered, which leads to the disappearance of theband gap between the anti-bonding and lone-pair band, or in other words to the occurrence of the NM-to-M transition.