Small-polaron excitations in Ba1-xKxBiO3 studied by optical reflectivity measurements -: art. no. 125103

We present optical reflectivity of single crystals of Ba1-xKxBiO3 in the semiconducting range from 20 K to 400 K in the infrared-visible frequency range. Above a certain temperature T-*=300 K(200 K) for x=0.15 (x=0.21) we observe a new excitation below the peak across the direct gap and also the transition to the bipolaronic point defects in the dielectric spectral function epsilon(2). The spectral weight of the new excitation increases while that of other excitations decreases with temperature. We assign the new excitation to intraband optical excitations of small polarons for electrons and holes, which are thermally created across the indirect gap. A function representing the small polaron conductivity is fitted to the conductivity spectra with a peak at twice the small-polaron binding energy.