Transport properties of some derivatives of tetrathiafulvalene-tetracyano-p-quinodimethane (TTF-TCNQ)

Data on dc conductivity $\sigma \mathrm{}\left(T\right)\mathrm{}$ and thermoelectric power $S\left(T\right)\mathrm{}$ for four organic conductors related to tetrathiafulvalene-tetracyano-$p$-quinodimethane (TTF-TCNQ) are presented. They all qualitatively behave as TTF-TCNQ with metal-insulator ($M-I$) transitions at temperatures around 50 K. Tetramethyltetraselenafulvalene-tetracyano-$p$-quinodimethane (TMTSeF-TCNQ) has $\sigma \left(300\mathrm{} \mathrm{K}\right)=1000\mathrm{}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{{\sigma }_{max}}{\sigma \left(300\mathrm{} \mathrm{K}\right)}=7\mathrm{}$, $S\left(300\mathrm{} \mathrm{K}\right)=8\mathrm{}\mu \mathrm{V}/\mathrm{K}$. For tetramethyltetraselenafulvalene-dimethyltetracyano-$p$-quinodimethane (TMTSeF-DMTCNQ) $\sigma \left(300\mathrm{} \mathrm{K}\right)=500\mathrm{}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{{\sigma }_{max}}{\sigma \left(300\mathrm{} \mathrm{K}\right)}=10\mathrm{}$, $S\left(300\mathrm{} \mathrm{K}\right)=11\mathrm{}$ μV/K. The sulphur analogue tetramethyltetrathiafulvalene-dimethyltetracyano-$p$-quinodimethane (TMTTF-DMTCNQ) has $\sigma \left(300\mathrm{} \mathrm{K}\right)=120\mathrm{}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{{\sigma }_{max}}{\sigma \left(300\mathrm{} \mathrm{K}\right)}=3\mathrm{}$, $S\left(300\mathrm{} \mathrm{K}\right)=-30\mathrm{}$ μV/K, while for diethyldimethyltetraselenafulvalene-tetracyano-$p$-quinodimethane (DEDMTSeF-TCNQ) we find $\sigma \left(300\mathrm{} \mathrm{K}\right)=500\mathrm{}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{{\sigma }_{max}}{\sigma \left(300\mathrm{} \mathrm{K}\right)}=9\mathrm{}$, $S\left(300\mathrm{} \mathrm{K}\right)=18\mathrm{}\mu \mathrm{V}/\mathrm{K}$. $S\left(T\right)\mathrm{}$ for the selenium-containing materials is small and metallic above the $M-I$ transitions. $S\left(T\right)\mathrm{}$ for TMTTF-DMTCNQ is high and negative with only slight temperature dependence above 100 K. TMTTF-DMTCNQ and TMTSeF-DMTCNQ are discussed in terms of a simple model of independent stacks. The transport on the DMTCNQ stacks is found to be diffusive while the TMTSeF stacks are in the band regime. Polarized-reflectance data are given for TMTSeF-TCNQ, TMTSeF-DMTCNQ, and TMTTF-DMTCNQ. A plasma edge in the near infrared is found in all cases. The small shifts observed are interpreted as arising from variations in effective bandwidths and degrees of charge transfer. The extrapolated zero-frequency optical conductivity is similar in all materials and greater than the measured dc conductivities, indicating different relaxation mechanisms at dc and at optical frequencies. In the $M-I$ transition region zero crossings of $S\left(T\right)\mathrm{}$ are generally found and at low temperatures both $\sigma \mathrm{}\left(T\right)\mathrm{}$ and $S\left(T\right)\mathrm{}$ show semiconducting behavior. Exceptions are TMTSeF-DMTCNQ doped with methyl-TCNQ (MeTCNQ) and DEDMTSeF-TCNQ, where the transitions are smeared. This smearing is attributed to static disorder.