Thermodynamics of the neutral-to-ionic transition as condensation and crystallization of charge-transfer excitations

The pressure-temperature phase diagram of tetrathiafulvalene-p-chloranil, prototype for the neutral-to-ionic transition, is determined by neutron diffraction and nuclear quadrupole resonance and discussed in relation to thermally induced charge-transfer (CT) excitations. The respective role of ionicity and dimerization is highlighted. Supported by theoretical considerations taking into account the interplay between quantum and thermal effects, the experimental results show that this uncommon electronic-structural transition can be described in terms of condensation and crystallization of CT excitations with a solid-liquid-gas-like phase diagram.