Electron-lattice coupling and broken symmetries of the molecular salt (TMTTF)2SbF6

The temperature/pressure phase diagram for ${(\mathrm{TMTTF})}_{2}{\mathrm{SbF}}_{6}$ is determined using $^{13}\mathrm{C}$ NMR spectroscopy. At ambient pressure, a transition to a charge-ordered (CO) state occurs at ${T}_{\mathrm{CO}}=156\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and antiferromagnetic (AF) order is observed below ${T}_{N}=8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Both are suppressed with pressure: when $Pg0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, 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.