Infrared Phonons in Semiconducting Phase of BaPb1-xBixO₃

Infrared reflectivity spectra of BaPb 1- x Bi x O 3 alloys in their semiconducting phase ( x ≥0.4) are presented. Four optical phonon bands are clearly observed in the reflectivity spectrum of BaBiO 3 . It is found that the oscillator strength of one phonon mode remarkably decreases with increase of the Pb composition. This provides a strong evidence for the charge disproportionation which leads to the formation of a superlattice of alternating inequivalent Bi sites in BaBiO 3 . It is suggested that the macroscopic formation of such Bi superlattice is also responsible for the semiconducting properties of alloys. Based on the experimental observations the nature of the metal-semiconductor transition of this system is discussed.