Dimerization model for the metal-semiconductor transition in ${\mathrm{BaPb}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_3$

A Raman study of the lattice vibrations in the new superconductor ${\mathrm{BaPb}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Bi}}_{\mathrm{x}}$ ${\mathrm{O}}_3$ discloses strong resonant scattering of the infrared-active 560-${\mathrm{cm}}^{\mathrm{-}1}$ mode in the semiconducting region. A new model for semiconductivity is proposed in which Bi(Pb)${\mathrm{O}}_{6/2}$ molecules which have trapped excess electrons are dimerized and the vibronic coupling gives rise to both infrared and Raman activities to the out-of-phase oscillation of the intramolecular breathing mode. The transfer integral, on-site Coulomb repulsion, and electron–molecular-vibration coupling constant are obtained. The results are consistent with other experiments.