Bipolaronic point defects and small-polaron excitations in Ba 1 - XKxBiO3 and BaPbxBi 1 - XO3

Hiromoto Uwe, Hidetoshi Minami, Taichiro Nishio, Javed Ahmad, Mitsuyuki Shizuya

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1 Citation (Scopus)


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.

Original languageEnglish
Pages (from-to)1-9
Number of pages9
Publication statusPublished - 2007
Event8th Russia/CIS/Baltic/Japan Symposium on Ferroelectricity, RCBJSF-8 - Tsukuba, Japan
Duration: 15 May 200619 May 2006


  • BaKBiO
  • BaPbBi O
  • Charge-density-wave (CDW)
  • Small bipolaron
  • Small polaron


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