Magnetic susceptibility, electrical resistivity, and thermoelectric power measurements ofbis(tetrathiotetracene)-triiodide

The magnetic susceptibility ($\chi$), electrical resistivity ($\rho$), and thermoelectric power ($S$) of bis(tetrathiotetracene)-triiodide were measured for $9<T<\mathrm{}300\mathrm{}$ K. Bis(tetrathiotetracene)-triiodide crystals were synthesized by vapor-phase cosublimation. X-ray crystal-structure determination on crystals prepared by this technique indicates a potential range of stoichiometries, i.e., TTT-${\left({\mathrm{I}}_3\right)}_x$ where $0.5\le x\le 0.52$. TTT-${\left({\mathrm{I}}_3\right)}_{0.5}$ is highly conducting at room temperature ($\rho \sim {10}^{-3\mathrm{}}$ Ωcm). The properties of TTT-${\left({\mathrm{I}}_3\right)}_{0.5}$ are consistent with a metallic state: (i) resistivity $\rho ={\rho }_0+{\rho }_1{T}^2$; ${\rho }_0\sim 4\times {10}^{-4\mathrm{}}$ Ωcm, ${\rho }_1\sim 8\times {10}^{-9\mathrm{}}$ Ωcm/${\mathrm{K}}^2$ for $T>\mathrm{}100\mathrm{}$ K, and (ii) thermopower $S=\frac{S_0}{T}+S_{1}T$ where $S_0=1124\mathrm{}$ μV and $S_1=1.07\mathrm{}\times {10}^{-1\mathrm{}}$ μV/${\mathrm{K}}^2$. The magnetic susceptibility, corrected for 0.25% spin-½ impurities, is + 2.2 × ${10}^{-4\mathrm{}}$ emu/mol at room temperature. The term in the thermopower data linear in temperature is interpreted using a one-dimensional tight-binding model to yield a value for the transfer integral ($t$) of 0.16 eV. The high-temperature value of $\chi$ is enhanced by Coulomb correlations. Using the antiferromagnetic Heisenberg-chain model appropriate to this limit we find that the on-site electron-electron repulsion ($U$) is ∼ 2.0 eV. Phase transitions have been found at 50 K in $\chi$ and at 21 K in $\frac{d\ln \rho }{\mathrm{dT}}$. The conductivity of TTT-${\left({\mathrm{I}}_3\right)}_{0.5}$ remains large at low temperatures [