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

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Abstract
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 (< 100). For the latter samples at 25 kbar there is no saturation of the resistance in the low temperature regime, whereas for the former the resistance saturates below 10 K. These results suggest slightly difrent critical pressures for these salts. Pc are much higher for the ClO4 and PF6 salts (> 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