Application of about 0.8 GPa pressure is sufficient to induce the neutral-ionic transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-p-phenylenediamine–2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI). The ionicity increases continuously from $\sim 0.35$ at ambient conditions to $\sim 0.65$ when the pressure is raised up to 2 GPa. Moreover, stack dimerization begins well before the crossing of the neutral-ionic interface. The evolution of the transition is similar to what observed in the temperature-induced phase change in the same compound. A distinguishing feature is represented by the simultaneous presence of domains of molecules with slightly different ionicities across the transition pressure. A comparison of the present example of a pressure-driven neutral-ionic transition with the well-studied cases of tetrathiafulvalene-chloranil and of tetrathiafulvalene–2,5-dichloro-p-benzoquinone puts in evidence the remarkably different evolution of the three transitions.

• Received 8 May 2001

DOI:https://doi.org/10.1103/PhysRevB.64.144102