Optical-Phonon Precursory Behavior towards Semiconductor–Metal Transition in Ba1-xKxBiO3

Abstract

Optical reflectivity measurements in a frequency range of 50–1000 cm -1 have been carried out for semiconducting Ba 1- x K x BiO 3 (BKBO) single crystals. Optical-phonon spectra exhibit the single-mode behavior in contrast to the two-mode one of BaPb x Bi 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 Δ e 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 Δ e 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.