PHYSICS OF NEUTRAL-TO-IONIC PHASE-TRANSITION IN ORGANIC CHARGE-TRANSFER SEMICONDUCTING COMPOUNDS

An uncommon excitonic instability takes place in some exotic semicondu cting compounds. Indeed, the equilibrium neutral-to-ionic (N-I) phase transition, as well as the non-equilibrium photo-induced phase transfo rmation, observed in some organic charge-transfer complexes, originate from intra-and inter-chain cooperative effects between structurally r elaxed charge-transfer excitations. This electronic-structural phase t ransition manifests itself by a change of the degree of charge-transfe r and a dimerization distortion with the formation of donor-acceptor p airs along the stacking axis in the I phase. Thermal charge-transfer e xcitations associated with the formation of I strings along N chains a re at the heart of the mechanism of this phase transition. These relax ed electronic excitations, which are an intrinsic feature of low-dimen sional systems with strong electron-phonon coupling, can be described in terms of self-trapping and self-multiplication of charge-transfer e xcitons. Precise structural studies on the prototype compound, tetrath iafulvalene-p-chloranil allow to highlight the respective role taken b y the ionicity and the dimerization. Symmetry and thermodynamics analy sis of the N-I transition, based on recent determination of the pressu re-temperature phase diagram, make possible to present a consistent pi cture of this phase transition. Supported by theoretical consideration s taking into account the interplay between quantum and thermal effect s, the experimental observations show that the N-I transition results from the condensation and the ordering (crystallization) of charge-tra nsfer excitations, following a phase diagram analogous to the solid-li quid-gas one.