Using one-and two-dimensional NMR spectroscopy applied to 13 C spin-labeled (TMTTF)2AsF6 and (TMTTF)2PF6, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent environments, or molecules. Below a continuous charge-ordering transition temperature Tco, the spectra are explained by assuming there are two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge/lattice coupling is too weak to drive the transition.

(TMTTF)2AsF6 undergoes two phase transitions upon cooling from 300 K. At TCO=103 K a charge-ordering (CO) occurs, and at TSP (B=9 T)=11 K the material undergoes a spin-Peierls (SP) transition. Within the intermediate, CO phase, the charge disproportionation ratio is found to be at least 3:1 from 13 C NMR T −1 1 measurements on spin-labeled samples. Above TSP up to about 3TSP T −1 1 is independent of temperature, indicative of low-dimensional magnetic correlations. With the application of about 0.15 GPa pressure, TSP increases substantially, while TCO is rapidly suppressed, demonstrating that the two orders are competing. The experiments are compared to results obtained from calculations on the 1D extended Peierls-Hubbard model. PACS numbers: 71.20.Rv, 71.30.+h, 71.45.Lr, Inhomogenous charge and spin structures are a consequence of competing interactions and therefore of general interest in correlated electron systems. Examples include the high-T c cuprates 1 and manganites 2 as well as the quasi-2D organic conductors 3 . The quasi-1D salts made from TMTTF or TMTSF molecules are also susceptible to charge-ordered states. Independent of that, they are well-known for the sequence of ground states accessible by applying pressure or selecting different counterions. For example, the material (TMTTF) 2 PF 6 undergoes transitions from spin-Peierls, antiferromagnetic (AF), spin-density wave (SDW), and finally to superconducting (SC) ground states as the pressure is increased to 4-5 GPa 4,5 . For a long time, it was known that another phase transition occurs in a number of TMTTF salts with both centrosymmetric (e.g., AsF 6 , SbF 6 ) and non-centrosymmetric (e.g., ReO 4 ) counterions. Only recently 6,7 was the broken symmetry associated with this transition identified as a charge disproportionation.In TMTTF salts the characteristic temperature of the onset of the charge-ordered (CO) phase is high, on the order of 100 K. It indicates that the interactions driving the CO are relatively strong, and therefore potentially impact the electronic and magnetic properties of the disordered phase. Issues associated with CO correlations in these systems take particular relevance when considering that the nature of the metallic phase of TMTSF salts remains controversial 8,9,10 . Below we report the results of a number of NMR measurements on 13 C spin-labeled samples of (TMTTF) 2 AsF 6 in the CO phase. Our principle result is a mapping of the temperature/pressure phase diagram of the SP and CO phases that includes a tetracritical point with a region of coexistence of the two forms of order. There is good agreement between the experiments and the results of calculations on the 1D extended Hubbard 11 and Peierls-Hubbard models 12,13 .A review of the characteristics of the CO phase and the phase transition is in order. With counterions PF 6 , AsF 6 , and SbF 6 , the ordering temperature is 62 K, 103 K, and 154 K, respectively. Upon cooling, the salts made with the first two are already well into a region of thermally activated resistivitie...

TMTTF)2SbF6 is known to undergo a charge ordering (CO) phase transition at TCO ≈ 156K and another transition to an antiferromagnetic (AF) state at TN ≈ 8K. Applied pressure P causes a decrease in both TCO and TN . When P > 0.5GP a, the CO is largely supressed, and there is no remaining signature of AF order. Instead, the ground state is a singlet. In addition to establishing an expanded, general phase diagram for the physics of TMTTF salts, we establish the role of electron-lattice coupling in determining how the system evolves with pressure.