Intermediate regime in Tetrathiafulvalene-Chloranil (TTF-CA) pressure-induced neutral-ionic transition

We report a detailed spectroscopic study of the pressure induced neutral-ionic phase transition (NIT) of the mixed-stack charge-transfer (CT) crystal tetrathiafulvalene-chloranil (TTF-CA). We show that the pressure induced phase transition is still first-order and involves the presence of an intermediate disordered phase, defined by the coexistence of two species of different ionicity. Further application of pressure gradually converts this phase into an homogeneous ferroelectric phase with a single ionicity. In addition, we detect strong pretransitional phenomena which anticipate the intermediate phase and are indicative of a precursor dynamic regime dominated by fluctuations.