Electronic and structural properties of fluid selenium at high pressures and temperatures – a new mechanism of non-metal-to-metal transition

Abstract

By theoretical calculations, we describe the mechanism of the non-metal-to-metal transition on the volume expansion of fluid selenium (Se) in the supercritical region. We first show from energetic considerations that some of the bonds in Se chains are weakened when the volume is expanded, and secondly we show that the bond weakening causes a reduction in the splitting, ΔEσ∗−σ≡Eσ∗−Eσ between the bonding level, Eσ and the anti-bonding level, Eσ∗. In spite of the fact that the band width, W, is decreased on the volume expansion, the degree of the reduction in ΔEσ∗−σ is so large that the ratio (W/ΔEσ∗−σ) increases. At some critical volume, the weakened bonds are disrupted and the energy gap separating the occupied and unoccupied bands disappears, thus bringing the system from non-metal to metal. This feature is in contrast with the well-established mechanisms for the Bloch–Wilson transition and the Mott–Hubbard transtion, in which the volume expansion does not affect the characteristic quantity (the level difference ΔE for the former and the electron correlation energy, U, for the latter, respectively), while the band widths are narrowed, so that both (W/ΔE) and (W/U) decrease and the transition is from metal to non-metal. We also discuss the distribution of electrons in the metallic phase.