Pressure-induced neutral-to-ionic phase transition in organic charge-transfer crystals of tetrathiafulvalene-p-benzoquinone derivatives

Infrared and visible absorption spectra have been measured on three kinds of mixed-stack organic charge-transfer crystals of tetrathiafulvalene (TTF) -p-benzoquinone derivatives under hydrostatic pressures up to about 50 kbar. In TTF-tetrachloro-p-benzoquinone and TTF-trichloro-p-benzoquinone crystals, the regularly stacked quasineutral phase at ambient pressure undergoes a change to the dimerized quasi-ionic phase under the increasing pressure, passing through an inhomogeneous mixed state where the quasineutral molecular region and the quasi-ionic molecular region are spatially separated. On the other hand, in TTF-2,5-dichloro-p-benzoquinone, a continuous increase in the degree of charge transfer is observed under increasing pressure up to about 50 kbar. The nature of these pressure-induced neutral-to-ionic transitions in the respective TTF-complex crystals is discussed.