Thermopower studies of a series of salts of tetramethyltetrathiafulvalene [(TMTTF)2X, X=Br, ClO4, NO3, SCN, BF4, AsF6, and PF6]

Abstract

Thermoelectric power measurements on a group of tetramethyltetrathiafulvalene ${\left(\mathrm{TMTTF}\right)}_{2}X$ salts show a wide variety of behavior. In the range of temperatures $300\ge T\gtrsim 100$ K several of the salts ($X=\mathrm{B}{\mathrm{F}}_4, \mathrm{Cl}{\mathrm{O}}_4, \mathrm{N}{\mathrm{O}}_3, \mathrm{SCN}$) have a constant thermopower $S\simeq 35$ μV/K, while the P${\mathrm{F}}_6$ and As${\mathrm{F}}_6$ salts have $S\propto \frac{1}{T}$, and the Br salt has $S$ increasing with $T$. Below ∼100 K, $S$ shows a steep increase or decrease with decreasing temperature for each salt which, with the exception of the SCN salt, is not associated with a phase transition. These materials had been identified as semiconductors below the maximum in conductivity $\sigma$ vs $T$. We give arguments that the gap at the Fermi energy exists at temperatures above the maximum as well. The existence of a gap is good evidence that the material is characterized by Coulomb repulsion $U$ for a second electron on a site larger than the bandwidth $4t$. However, the constant finite $S<\mathrm{}60\mathrm{}$ μV/K found for several of the compounds above ∼ 100 K suggests that $U$ is not much larger than $4t$. The different behavior of $S$ for the P${\mathrm{F}}_6$ and As${\mathrm{F}}_6$ salts in the same temperature range and the sharp changes in $S$ vs $T$ at low temperatures (except for the SCN salt) are attributed to defects rather than, e.g., localization. In the SCN, Cl${\mathrm{O}}_4$, and Br salts, features are seen at low temperatures that reflect known phase transitions in these materials.