The temperature/pressure phase diagram for ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{SbF}}_6$ is determined using ${}^{13}\mathrm{C}$ NMR spectroscopy. At ambient pressure, a transition to a charge-ordered (CO) state occurs at $T_{\mathrm{CO}}=156\mathrm{K}$, and antiferromagnetic (AF) order is observed below $T_N=8\mathrm{K}$. Both are suppressed with pressure: when $P>0.5\mathrm{GPa}$, there is no evidence for CO, and the low-temperature ${}^{13}\mathrm{C}$ NMR spectrum is consistent with a singlet [spin-Peierls (SP)] ground state. At a given pressure, the temperature dependence of the CO order parameter is not monotonic, and provides an opportunity to identify what processes could be controlling the CO amplitude. ${}^{19}\mathrm{F}$ NMR spectroscopy provides empirical evidence that electron-counterion coupling is crucial to stabilizing the CO and AF phases.

• Received 2 April 2004

DOI:https://doi.org/10.1103/PhysRevB.70.121101