Bipolaronic point defects and small-polaron excitations in Ba _{1 - X}K_xBiO₃ and BaPb_xBi _{1 - X}O₃

Recent studies on spectroscopic and transport properties concerning the semiconductor-metal (SM) transition in Ba1 - xKxBiO3 (BKBO) and BaPbxBi1 - xO3 (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 created across the indirect gap. The small polaron conductivity has a peak at twice the small-polaron binding energy. Temperature dependence of dc conductivity, thermoelectric power, and magnetic susceptibility is interpreted by excitations of small polarons of electrons and holes. The CDW amplitude, or the Szigeti-effective-charge difference between inequivalent Bi sites, which is estimated from the oscillator strength of the disproportionation mode, tends to go to zero as K or Pb content goes to a critical concentration of the SM transition.