Bismuth charge disproportionation in semiconducting BaPbxBi1-xO3 studied by infrared reflection spectroscopy

The infrared reflectivity spectra of semiconducting BaPbxBi(1-x)O(3) single crystals are measured. From the oscillator strength of the bismuth charge-disproportionation mode we compute the compositional dependence of the Born and Szigeti effective-charge difference between two inequivalent bismuth sites. The Szigeti effective-charge decreases and is found to become zero as the Pb doping approaches the critical value of the semiconductor - metal transition. This behaviour is associated with the Pb composition induced closing of the indirect energy gap, which takes place prior to the direct gap. It further demonstrates that the long-range order of the bismuth charge disproportionation completely vanishes in the metallic phase. It is found that a strong dynamical charge transfer takes place along the bismuth - oxygen - bismuth bond enhanced by the lattice vibration. The results are compared with those of Ba1-xKxBiO3 system.