Mixed Stack Compounds, Phonons and Solitons

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Abstract
Phase transitions in organic mixed-stack compounds, known as “neutral-ionic” transitions, are reinterpreted as the onset of bond dimerization. The phase diagram is derived and the effects of temperature, pressure and the difference of donor-acceptor potentials are correlated. Electron-multi phonon couplings lead to renormalization effects both above and below the transition. A soft mode at the transition is predicted. Analysis of the phonon spectra is an efficient test for any quantitative theory of these compounds. Solitons are possible excitations below the transition and may account for conductivity and ESR data. The incommensurate situation is described by a soliton lattice.