Electronic structure ofBaPb1−xBixO3
Electronic-structure calculations have been carried out for representative members of the $\mathrm{Ba}{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{O}}_{3}$ alloy series with the use of the self-consistent, scalar-relativistic, linear augmented plane-wave (LAPW) method and the virtual-crystal approximation. The study includes LAPW calculations for the high-temperature cubic ($x=0,0.3, 1$) as well as the room-temperature tetragonal ($x=0.3$) and monoclinic ($x=1$) phases. The cubic results exhibit nearly-rigid-band---like behavior and feature a ten-band $\mathrm{O}(2p)\ensuremath{-}[\mathrm{Pb}(6s),\mathrm{Bi}(6s)]$ complex near ${E}_{F}$ containing a pair of broad (\ensuremath{\sim}16 eV) $\ensuremath{\sigma}$-bonding $2p\ensuremath{-}6s$ subbands that are approximately centered on nonbonding O $2p$ states. $6s\ensuremath{-}2p$ band overlap and metallic properties are predicted for all $x$ in the cubic phase. The principal features of the cubic band structure survive the tetragonal distortion, predicting a nearly spherical Fermi surface and a low density of states at ${E}_{F}$ in the concentration range $0.05\ensuremath{\le}x\ensuremath{\le}0.30$ where high-temperature superconductivity (${T}_{c}\ensuremath{\approx}13$ K) is observed. A strong Fermi-surface instability is shown to occur in the terminal ($x=1$) BaBi${\mathrm{O}}_{3}$ compound, explaining both the stability of the monoclinic phase as well as its semiconducting properties.