The phase diagram of the organic charge transfer salts (TMTTF)$_2$X

We report on NMR spectroscopy and relaxation measurements probing the temperature/pressure phase diagram of the quasi-one-dimensional organic conductor (TMTTF)$_2$SbF$_6$. This material undergoes a charge ordering (CO) phase transition at $T=156K$, and the ground state is antiferromagnetic (AF) at ambient pressure. Our experiments show that the AF ground state is supressed sharply by applying hydrostatic pressure and a new phase appears at higher pressure. At intermediate pressures, magnetic ordering is suppressed to temperatures below the minimum measured so far, $T=2K$. The nature of the high pressure phase is not yet established; there are features consistent with both spin-Peierls (SP), and antiferromagnetic ordering. We discuss the possible role of counterion disorder in producing an inhomogenous system consistent with our observations, and the implications for a general phase diagram for the TMTTF/TMTSF family of organic conductors.