Optical conductivity of nonmetallic Ba0.69K0.31BiO3 single crystals: Evidence for bipolaron formation

We report the far-infrared to near-ultraviolet reflectivity and ellipsometric measurements on an insulating Ba1-xKxBiO3 single crystal, with a doping level x=0.31 just below the insulator-metal transition boundary. The optical conductivity was calculated using Kramers-Kronig analysis. We examine the experimental data in terms of a small bipolaron model. This model, with no free parameters, allows us to obtain a satisfactory agreement with our experimental results, including the overall shape and the peculiar temperature dependence of the optical conductivity. The same model provides a qualitative explanation for some aspects of the transport-measurement results reported previously.