Pressure Dependence of the Phase Transitions in Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ): Evidence for a Longitudinal Lockin at 20 kbar

We report the pressure dependence of the three phase transitions in TTF-TCNQ as determined by resistivity measurements. There is only one transition above 15 kbar, and we find a 4-kbar regime centered at 20 kbar where the transition temperature peaks (the distortion temperature $T_d=71\mathrm{}$ K at 20 kbar), and the transition is first order. We suggest that there is a commensurate ×3 superlattice in this pressure regime resulting from an increase in charge transfer to $\frac{2}{3}$.