Neutral-ionic phase transition of (BEDT-TTF)(ClMeTCNQ) under pressure

Observation and characterization are reported on a neutral-ionic (NI) phase transition under pressure in (BEDT-TTF)(ClMeTCNQ), which is a mixed-stack charge-transfer compound with BEDT-TTF-based intercolumnar networks. Optical and electrical experiments revealed that the compound undergoes a pressure-induced NI transition at 1 GPa at room temperature. The temperature-pressure phase diagram of this phase transition was established by the electrical resistivity measurements at low temperature under pressure. The observed symmetry breaking in the infrared spectra and dramatic decrease of resistivity indicate that the NI transition in (BEDT-TTF)(ClMeTCNQ) at 1 GPa at room temperature is qualitatively similar in nature to that in the standard material (TTF)(CA). In contrast, the NI transition in (BEDT-TTF)(ClMeTCNQ) at low temperature under lower pressure considerably differs from that in (TTF)(CA) as to the resistivity behaviors around the phase boundary: The former becomes almost continuous without showing any anomaly, while the latter is significantly discontinuous. We discuss the origin of the observed electrical behaviors in (BEDT-TTF) (ClMeTCNQ) in terms of the effect of intercolumnar interaction associated with BEDT-TTF moieties.