OPTICAL CONDUCTIVITY OF BA1-XKXBIO3 THROUGH THE METAL-SEMICONDUCTOR TRANSITION

The dielectric response of Ba1-xKxBiO3 (BKBO) is investigated via room-temperature optical reflectance and spectroellipsometry over the doping range 0 less-than-or-equal-to x less-than-or-equal-to 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 BaPbyBi1-yO3.