Pressure-temperature phase diagrams of several (TMTTF)2X compounds : stabilisation of a highly conducting metallic state under pressure in (TMTTF)2Br

Needle axis resistivity measurements on several TMTTF (tetramethyltetrathiafulvalene) salts of the form (TMTTF)2X, with X = Br, ClO4, PF 6, are presented for pressures up to 32 kbar. The pressure-temperature phase diagrams of all three materials are similar with the stabilization under pressure of a metallic state to low temperatures (∼ 20 K), below which there is a well defined metal-insulator transition. For (TMTTF) 2Br the M-I transition is completely suppressed for pressures above some critical pressure Pc ∼ 25 kbar and the compound is metallic to the lowest temperatures considered in this study, ∼ 1 K. Significant differences are found between samples grown from different chemical solvents : in one case the samples have resistance ratios exceeding 400 at 25 kbar with a conductivity greater than 106 (Ωcm) -1 at 4 K, the highest values yet reported for any organic conductor. In the other case the samples show a possible superconducting transition near 3.5 K at 25 kbar, but have smaller resistance ratios ( 35 kbar). Magnetoresistance measurements have been made on samples of (TMTTF)2 Br as a function of pressure, for field applied along the c* axis. The magnetoresistance progressively increases with increasing pressure obtaining substantial values ((ρ(80 kG)-ρ(H = 0))/ρ( H = 0) ∼ 2-4) near Pc. These results establish that under pressure the physical properties of the (TMTTF)2X salts become increasingly to ressemble those of their selenium counterparts, (TMTSeF) 2X.