Optical conductivity of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ through the metal-semiconductor transition

The dielectric response of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ (BKBO) is investigated via room-temperature optical reflectance and spectroellipsometry over the doping range 0≤x≤0.51. We find that with increasing x, (i) a semiconductor-to-metal (MS) transition occurs near x=0.35, (ii) the infrared absorption band decreases monotonically in energy and persists into the metallic regime, and (iii) the integrated optical conductivity between 0 and 4 eV increases below the MS transition, but decreases above the MS transition. The decrease in the infrared absorption band energy suggests a relaxation of the distorted oxygen octahedra toward equilibrium with increasing x, and the doping dependence of the integrated optical conductivity is consistent with a crossover from holelike carriers below the MS transition to electronlike carriers above. A self-consistent interpretation of these results is provided by a general bond-order charge-density-wave model in which deviations of the Bi-O bond length from equilibrium influence many aspects of the normal-state response. We discuss the implications of this interpretation for BKBO and ${\mathrm{BaPb}}_{\mathit{y}}$ ${\mathrm{Bi}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{O}}_3$.