Theoretical study of expanded selenium in the supercritical region

Abstract

By means of theoretical calculations, we elucidate the mechanism for the nonmetal-to-metal (NM-to-M) transition that occurs with density decrease in supercritical Se. We first show from energetical considerations that some of the bonds in Se chains are disrupted when the density is decreased, and secondly we clarify that the bond disruption causes a drastic reduction of the splitting between the bonding and anti-bonding levels. As a consequence, the energy gap separating occupied and unoccupied levels decreases and eventually disappears, thus altering the nature of the system from nonmetallic to metallic. A remarkable point is that, although the bandwidth (W) reduces on decrease of the density, the degree of the reduction in is so marked that the ratio increases. This feature is in contrast with the traditional Wilson transition, in which the decrease of density has no significant influence on the differences between the corresponding energy levels, such as the spacing between the s level and the p level in the case of expanded Hg, while the decrease of the density narrows the bandwidth W and reduces the ratio as well, and accordingly a metal-to-nonmetal transition is induced. Our calculations also show that structural changes such as shortening of the bond lengths take place at the NM-to-M transition.