Optical-Phonon Precursory Behavior towards Semiconductor-Metal Transition in Ba_<1-x>K_xBiO_3 (Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties)

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Optical reflectivity measurements in a frequency range of 50-1000 cm^<-1> have been carried out for semiconducting Ba_<1-x>K_xBiO_3 (BKBO) single crystals. Optical-phonon spectra exhibit the single-mode behavior in contrast to the two-mode one of BaPb_xBi_<1-x>O_3 (BPBO). Frequencies plot for transverse-optical (TO) and longitudinal-optical (LO) phonons in BKBO almost coincides with the upper-mode frequencies for the two-mode BPBO derived from the terminal BaBiO_3. The Szigeti-effective-charge difference Δe^*_s between inequivalent Bi sites is estimated from the oscillator strength of the third-highest-frequency TO-phonon mode for each concentration. The results show that Δe^*_s is anomalously large as compared with the nominal one and tends to go to zero as x goes to a critical concentration of the semiconductor-metal transition. We conclude that the charge difference arises from a strong dynamical charge transfer during the anti-parallel vibrational motion of Bi ions.
社団法人日本物理学会の論文
2003-05-15