Pressure-driven neutral-ionic phase transition in tetrathiafulvalene-2,5-dichloro-p-benzoquinone: Example of continuous ionicity change

The infrared spectra of tetrathiafulvalene-2,5-dichloro-p-benzoquinone charge-transfer complex have been measured as a function of pressure up to a maximum of 11.9 GPa. A careful interpretation of the data shows that the ionicity of the complex increases steadily with pressure, going from values inherent to a quasineutral (N) ground state to values inherent to a quasi-ionic (I) one. The N-I borderline is crossed without jumps in ionicity in correspondence with a Peierls phase transition starting at 3.8 GPa (midpoint at {~}5 GPa). This experiment shows that a N-I transition can occur also through a continuous ionicity change.