ELECTRICAL-RESISTIVITY AND THERMOELECTRIC-POWER OF (TMTSF)2M(TDS)2, M=PT, CU AND NI - EVIDENCE FOR THE EXISTENCE OF 2 DIFFERENT PHASES

The electrical resistivity and thermoelectric power of (TMTSF)2M(tds)2 single crystals, M = Ni, Pt and Cu, were investigated in the range 20 - 300 K. These measurements show two distinct behaviours and provide evidence for the existence of two different phases in the Pt and Ni co mpounds. One, observed in plate shaped crystals, referred to as alpha- phase, presents a 1st order transition, at ca. 275 and 245 K for the N i and Pt compounds respectively, seen as an increase of conductivity a nd decrease of thermopower. The other one, observed in thin needle cry stals, referred to as beta-phase, has a clear metallic regime at high temperature without such a 1st order transition. M = Cu crystals appea r only as thin plates, with metallic properties at high temperature an d without any 1st order transition. They present a different 2nd order M-I transition at ca. 60 K with hysteresis in thermopower in the rang e 60-120 K.