Theory of the Neutral-Ionic Transition in TTF-Chloranil

Abstract

A new mechanism of the neutral-ionic transition in TTF-chloranil is presented, through which we derive the temperature-induced transition as a statistical mechanically cooperative phenomenon, without supposing the lattice thermal contraction. We take into account the coulomb interaction up to second nearest-neighbors within a chain, the spin degeneracy of the ionic molecular state, as well as the quantum-mechanical charge transfer. In addition, the inter-chain coulomb interaction is considered in the mean field approximation. The free energy approximately calculated as a function of the degree of ionicity shows that at low temperatures the ionic phase becomes lower owing to a large splitting of the spin degeneracy caused by a strong mixing of the neutral and tonic states, whereas at room temperature the neutral phase is lower owing to the electrostatic energy balance.