Pressure-dependent optical investigations of Fabre salts in the charge-ordered state.

In a comprehensive infrared study, the molecular vibrational features of (TMTTF)SbF, (TMTTF)AsF and (TMTTF)PF single crystals have been measured down to temperatures as low as 7 K by applying hydrostatic pressure up to 11 kbar. We follow the charge disproportionation below the critical temperatures T as pressure increases, and determine the critical pressure values p at which the charge-ordered phase is suppressed. The coexistence of the spin-Peierls phase with charge order is explored at low temperatures, and the competition of these two phases is observed.