Localised polaronic states in mixed-valence linear chain complexes

The authors describe a model of halogen-bridged mixed-valence linear chain complexes (transition-metal ions bridged by halogen ions) incorporating electrons coupled to on-site and inter-site phonons. They show that the charge-transfer situation appropriate to many members of the class corresponds to a charge-density wave with strong electron-phonon coupling. In this regime, they calculate intrinsic defect states, including kinks, polarons, bipolarons and excitons. These are strongly localised and are important in view of the growing attention given to local modes in solids. Available structural and optical data can be explained if an on-site Coulomb interaction is also included. The resulting parameter values suggest that, despite the large distortions observed, the mixed-valence state in many of these compounds is only marginally stable.