THERMODYNAMICS OF THE NEUTRAL-TO-IONIC TRANSITION AS CONDENSATION ANDCRYSTALLIZATION OF CHARGE-TRANSFER EXCITATIONS

The pressure-temperature phase diagram of tetrathiafulvalene-p-chloran il, prototype for the neutral-to-ionic transition, is determined by ne utron diffraction and nuclear quadrupole resonance and discussed in re lation to thermally induced charge-transfer (CT) excitations. The resp ective role of ionicity and dimerization is highlighted. Supported by theoretical considerations taking into account the interplay between q uantum and thermal effects, the experimental results show that this un common electronic-structural transition can be described in terms of c ondensation and crystallization of CT excitations with a solid-liquid- gas-like phase diagram.