Lattice vibrations in the strong electron-phonon-interaction system ${\mathrm{BaPb}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_3$ studied by Raman scattering

Lattice vibrations of the high-${\mathrm{T}}_{\mathrm{c}}$ superconductor ${\mathrm{BaPb}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Bi}}_{\mathrm{x}}$ ${\mathrm{O}}_3$ are investigated by Raman scattering. The breathing mode of ${\mathrm{BiO}}_6$ octahedra is assigned to the 569 ${\mathrm{cm}}^{\mathrm{-}1}$ peak which has large scattering intensity in ${\mathrm{BaBiO}}_3$. The temperature-dependent soft modes at 121 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{BaPb}}_{0.7}$ ${\mathrm{Bi}}_{0.3}$ ${\mathrm{O}}_3$ and 48 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{BaBiO}}_3$ are assigned to the rotational modes of Pb(Bi)${\mathrm{O}}_6$. A remarkable correlation is found between ${\mathrm{T}}_{\mathrm{c}}$ and the scattering intensity of the 43- and 100-${\mathrm{cm}}^{\mathrm{-}1}$ peaks. The origin of these modes is discussed.