An infra-red absorption band centered at 0.85eV, which was below the large optical absorption at the charge-density-wave gap energy of 1.85eV, was observed for semiconducting single crystalline Ba1-xKxBiO3. With substituting K for Ba, the spectral weight of the new band increased with x, while that of the charge-density-wave gap excitation decreased. Since the impurity state with the K substitution was known to be nonmagnetic at low temperatures, the Bi3+ state with 6s2 electrons surrounded by the six Bi5+ ions formed a small bipolaron by losing a pair of electrons in the Rice-Sneddon model. The new band was assigned to a transition from the lower-Peierls band to a state of the bipolaronic point defect.

Spectroscopic Observation of Bipolaronic Point Defects in Ba1-xKxBiO3. T.Nishio, J.Ahmad, H.Uwe: Physical Review Letters, 2005, 95[17], 176403 (3pp)