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Structural and superconducting transition in selenium at high pressure

First-principles calculations are performed for electronic structures of two high-pressure phases of solid selenium, b-Po, and bcc. Our calculation reproduces well the pressure-induced phase transition from b-Po to bcc observed in selenium. The calculated transition pressure is 30 GPa lower than the observed one, but the calculated pressure dependence of the lattice parameters agrees fairly well with the observations in a wide range of pressure. We estimate the superconducting transition temperature Tc of both the b-Po and the bcc phases by calculating the phonon dispersion and the electron-phonon interaction on the basis of densityfunctional perturbation theory. The calculated Tc shows a characteristic pressure dependence. In bcc Se, Tc increases considerably with decreasing pressure and its maximum may exceed 10 K. In b-Po Se, Tc depends sensitively on the lattice constants we have used. For the lattice constants estimated by calculation, the values of Tc is as high as that in bcc phase and decreases with increasing pressure. For the lattice constants determined by measurements, on the other hand, Tc is less pressure dependent and there is a large jump in Tc at the transition from b-Po to bcc.