RESUMO
The infrared reflectivity spectra of semiconducting BaPb(x)Bi(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 Ba(1-x)K(x)BiO(3) system.
RESUMO
An infrared-absorption band centered at 0.85 eV, which is below the big optical absorption at the charge-density-wave (CDW) gap energy of 1.85 eV, has been observed for semiconducting single crystalline Ba(1-x)KxBiO3. With substituting K for Ba, the spectral weight of the new band increases with x, while that of the CDW-gap excitation decreases. Since the impurity state with the K substitution is known to be nonmagnetic at low temperatures, Bi3+ the state with 6s2 electrons surrounded by the six Bi5+ ions forms a small bipolaron by losing a pair of electrons in the Rice-Sneddon model. The new band is assigned to a transition from the lower-Peierls band to a state of the bipolaronic point defect.
