Bipolaronic Point Defects and Small-Polaron Excitations in Ba1-xKxBiO3 and BaPbxBi1-xO3

Abstract

Recent studies on spectroscopic and transport properties concerning the semiconductor-metal (SM) transition in Ba 1 − x K x BiO 3 (BKBO) and BaPb x Bi 1 − x O 3 (BPBO) are reviewed. For a three-dimensional Peierls semiconductor, BaBiO3(BBO), direct-exciton excitations correspond to the big optical response at 1.85 eV, while indirect exciton excitations reproduce a weak absorption at infrared region. When holes are doped for semiconducting single crystalline BKBO at room temperature, an infrared-absorption band has been observed centered at 0.85 eV below the big optical absorption at 1.85 eV. The new band is assigned to a transition from the lower-Peierls band to a state of the small bipolaronic point defect. With substituting K for Ba, doped holes form small bipolaron defects so that the semiconducting region extends in a wide region. At high temperatures up to 400 K, furthermore, a new excitation appears due to intraband optical excitations of small polarons for electrons and holes, which are thermally c...