Conductivity and thermopower studies of bis-tetramethyltetraselenafulvalenium hexafluorophosphide, bis-tetramethyltetrathiafulvalenium hexafluorophosphide, and their solid solutions, ${\left(\mathrm{TMTS}{\mathrm{F}}_{1-x}\mathrm{TMTT}{\mathrm{F}}_x\right)}_2\mathrm{P}{\mathrm{F}}_6$

The conductivity $\sigma$ and the thermoelectric power $S$ have been studied experimentally on the organic conductors bis-tetramethyltetraselenafulvalenium hexafluorophosphide [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$P${\mathrm{F}}_6$] and bis-tetramethyltetrathiafulvalenium hexafluorophosphide [${\mathrm{}\left(\mathrm{TMTTF}\right)\mathrm{}}_2$P${\mathrm{F}}_6$] and their solid solutions: ${\left(\mathrm{TMTS}{\mathrm{F}}_{1-x}\mathrm{TMTT}{\mathrm{F}}_x\right)}_2\mathrm{P}{\mathrm{F}}_6$. Dramatic effects are seen in $\sigma$ already when dilute concentrations of TMTTF molecules are introduced in the TMTSF chains, and for $x=0.1\mathrm{}$, $\sigma$ 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 $x=0.85\mathrm{}$ salt behaves generally similar to the pristine TMTTF material, though dominated by additional impurity levels.