Anomalous dielectric response in tetrathiafulvalene-p-chloranil as observed in temperature- and pressure-induced neutral-to-ionic phase transition

The temperature and pressure dependences of the dielectric constants have been measured on the mixed-stack organic charge-transfer crystal, tetrathiafulvalene-p-chloranil, in a frequency range from 100 Hz to 10 MHz. The dielectric response increases remarkably with decreasing temperature down to ${\mathit{T}}_{\mathit{c}}$=81 K, where the neutral-to-ionic (NI) phase transition occurs. On the other hand, with applied pressure at room temperature, the change in the dielectric response is less remarkable, whereas the dc conductivity ${\mathrm{\sigma }}_{\mathrm{dc}}$ increases considerably. Such dynamical responses are explained by a Debye-type relaxation model. The strength of the dielectric response is closely correlated with both the relaxation time τ and ${\mathrm{\sigma }}_{\mathrm{dc}}$. These anomalous dielectric responses are attributed to the dynamics of the NI domain-wall pairs and ionic domains in the neutral lattice.