Theory of Neutral-Ionic Transition in Organic Crystals. IV. Phenomenological Viewpoint

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Abstract
The phenomenological theory of the neutral-ionic transition in TTF-Chloranil is developed taking into account the thermal excitations of the domain walls on a chain and the three dimensional interchain interactions. The changes in various physical quantities around the transition point reflect the competition between these two effects. We mainly discuss the optical spectra which give the microscopic probes of the degree of charge transfer ρ at each molecule. The effect of a single impurity detected in the optical spectra is also discussed. Comparing with the experimental data in TTF-Chloranil, the following two conclusions are derived for this material. (1) The energy of the domain wall is small (of the order of room temperature). (2) The interchain interaction is weak effectively.