Optical study of the metal-semiconductor transition in ${\mathrm{BaPb}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_3$

The reflectivity spectra of the ${\mathrm{BaPb}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Bi}}_{\mathrm{x}}$ ${\mathrm{O}}_3$ alloy series are investigated over a wide energy range, 20–30 000 ${\mathrm{cm}}^{\mathrm{-}1}$, and over the whole compositional range. As the Bi composition x is increased, the position of the plasma edge is found to shift to higher energy corresponding to an increase in the electron density contributed by the Bi atoms. With further increase in x we find that the spectrum deviates from the usual Drude behavior. From a Kramers-Kronig analysis we infer that a pseudogap, or a minimum in the density of states, develops with x. The pseudogap arises from the trend towards splitting of the Bi-derived band and transforms to a real gap at the metal-semiconductor transition $x_c$≃0.35. The gap formation is supposed to be due to the presence of the short-range order of the Bi charge disproportionation state. Anomalous electronic transport properties observed in the metallic phase are well explained by assuming this pseudogap near the Fermi level.