The electronic structure of a high pressure monoclinic selenium and the structural phase transition

The electronic structure calculation for a high pressure monoclinic selenium (Se-II) was performed from first-principles with experimental lattice constants. We investigated the band dispersion and partial densities of states associated with atomic orbitals. It is found that the change and the disappearance of the lone-pair orbitals occupied by electrons occur at the transition from the trigonal to the monoclinic structure. We optimized internal parameters for the monoclinic structure and determined atomic positions. In comparison with the atomic positions of both trigonal and monoclinic, the structural transformation is explicitly discussed. From a rough estimation of the energy barrier at the transition, it turns out that the transition is considerably sensitive to pressure.