Conductivity and thermopower studies of bis-tetramethyltetraselenafulvalenium hexafluorophosphide, bis-tetramethyltetrathiafulvalenium hexafluorophosphide, and their solid solutions, (TMTSF1-xTMTTFx)2PF6

The conductivity σ and the thermoelectric power S have been studied experimentally on the organic conductors bis-tetramethyltetraselenafulvalenium hexafluorophosphide [(TMTSF)2PF6] and bis-tetramethyltetrathiafulvalenium hexafluorophosphide [(TMTTF)2PF6] and their solid solutions: (TMTSF1-xTMTTFx)2PF6. Dramatic effects are seen in σ already when dilute concentrations of TMTTF molecules are introduced in the TMTSF chains, and for x=0.1, σ shows generally activated behavior. The thermopower, on the contrary, remains basically unaffected for x at least as large as 0.25. These unusual findings are attributed to the small transverse transfer integral tb associated with the TMTTF molecule, which leads to large on-site Coulomb repulsion. The parallel transfer integrals ta appear to be very similar for the two constituent molecules. A slight x dependence of S, appearing below approximately 100 K, is attributed to enhanced one-dimensionality when the amount of TMTTF is increased. Close to and below the spin-density-wave ordering temperature, more marked x dependences appear, indicative of impurities. The x=0.85 salt behaves generally similar to the pristine TMTTF material, though dominated by additional impurity levels.