CONDENSATION OF SELF-TRAPPED CHARGE-TRANSFER EXCITATIONS AND INTERPLAY BETWEEN QUANTUM AND THERMAL EFFECTS AT THE NEUTRAL-TO-IONIC TRANSITION

The cooperativity between self-trapped electronic excitations is carri ed to extreme in the case of the neutral-to-ionic phase transition whi ch is observed in some organic mixed-stack charge-transfer crystals. T his electronic-structural phase transition manifests itself by a chang e of the degree of charge-transfer and a dimerization distorsion along the stacking axis in the ionic phase. Thermal charge-transfer excitat ions associated with the formation of structurally related ionic strin gs along neutral chains are at the heart of mechanism of this uncommon phase transition. Symmetry and thermodynamics analysis of the neutral -to-ionic transition in the prototype compound, tetrathiafulvalene-p-c hloranil, in particular the recent determination of the pressure-tempe rature phase diagram, male possible to present a consistent picture of this phase transition. Supported by a phenomenological approach, taki ng into account the quasi-one-dimensional nature of the system and the interplay between quantum and thermal effects, the experimental resul ts show that the neutral-to-ionic transition results from the condensa tion and the ordering (crystallization) of charge-transfer excitations , following a phase diagram analogous to the solid-liquid-gas one.