Perturbation theory for the formation energy of solitons and polarons in mixed-valence linear-chain complexes

A perturbation calculation shows that the relative formation energies of solitons and polarons in halogen-bridged linear-chain complexes depend strongly on the transfer term and Coulomb interactions even if the electron-lattice coupling is dominant. As the values of the transfer term and Coulomb interactions increase, various soliton states tend to have energies close to one another. It is suggested that the neutral soliton of two-site type is the lowest-energy defect state in a certain range of the strengths of those perturbations. This situation is consistent with the results of the ultraviolet-induced EPR experiment in [Pt(en${)}_2$][Pt(en${)}_2$ ${\mathrm{Cl}}_2$](${\mathrm{ClO}}_4$ ${)}_4$ (with en=ethylenediamine). The present theory provides an explanation for a highly mobile character of the neutral soliton observed in this material.