Influence of charge order on the ground states of TMTTF molecular salts

(TMTTF) 2 AsF 6 and (TMTTF) 2 SbF 6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using 13 C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (TMTTF) 2 SbF 6 . An extension of the well-known temperature/pressure phase diagram is proposed.