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

Abstract

The conductivity $\ensuremath{\sigma}$ and the thermoelectric power $S$ have been studied experimentally on the organic conductors bis-tetramethyltetraselenafulvalenium hexafluorophosphide [${(\mathrm{TMTSF})}_{2}$P${\mathrm{F}}_{6}$] and bis-tetramethyltetrathiafulvalenium hexafluorophosphide [${(\mathrm{TMTTF})}_{2}$P${\mathrm{F}}_{6}$] and their solid solutions: ${(\mathrm{TMTS}{\mathrm{F}}_{1\ensuremath{-}x}\mathrm{TMTT}{\mathrm{F}}_{x})}_{2}\mathrm{P}{\mathrm{F}}_{6}$. Dramatic effects are seen in $\ensuremath{\sigma}$ already when dilute concentrations of TMTTF molecules are introduced in the TMTSF chains, and for $x=0.1$, $\ensuremath{\sigma}$ 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 ${t}_{b}$ associated with the TMTTF molecule, which leads to large on-site Coulomb repulsion. The parallel transfer integrals ${t}_{a}$ 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.