Optical-phonon precursory behavior towards semiconductor-metal transition in Ba1-xKxBiO3

Optical reflectivity measurements in a frequency range of 50-1000 cm(-1) have been carried out for semiconducting Ba1-xKBiO3 (BKBO) single crystals. Optical-phonon spectra exhibit the single-mode behavior in contrast to the two-mode one of BaPbxBi1-xO3 (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 BaBiO3. The Szigeti-effective-charge difference Deltae(s)* between inequivalent Bi sites is estimated from the oscillator strength of the thirds highest-frequency TO-phonon mode for each concentration. The results show that Deltae(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.