Thermoelectric power of 1:2 tetracyanoquinodimethanide (TCNQ) salts

Abstract

A distinctive feature of a number of 1:2 tetracyanoquinodimethanide (TCNQ) salts, such as triethylammonium-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$ [TEA-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$], quinolinium-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$, and acridinium-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$ is a constant thermopower $Q\simeq -60\mathrm{}$ μV/K over a wide temperature range, ≳ 100 K, along the highly conducting or TCNQ chain direction. These salts are characterized by $\rho$, the fraction of filled sites on the TCNQ chain ≃ ½. Past calculations of Beni et al. have shown that the extended Hubbard model with very strong on-site Coulomb repulsion and zero bandwidth can account for the magnitude and temperature variation of $Q$ for quinolinium-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$ and acridinium-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$. I discuss the approximation of zero bandwidth and conclude that it is not justifiable. I then show that a near-constant value close to -60 μV/K for temperatures ≳ 100 K can be obtained using a model (although not the extended Hubbard model) with two bands having nearly identical widths and scattering times and retaining the feature of very strong on-site repulsion. Near identity of the two bands is reasonable for $\rho \simeq \frac{1}{2}$. This model can also account for the magnitude and temperature variation of the conductivity of these salts over the same large temperature range.