Properties of tetrathiotetracene nickel<i>bis</i>-(ethylene-1, 2-dithiolene), a new quasi-one-dimensional conductor

Abstract

We report the magnetic susceptibility, thermopower, EPR, dc conductivity, and preliminary structural data for a new quasi-one-dimensional (1D) conductor ${\mathrm{TTT}}_{1.2}$Ni${\mathrm{S}}_{4}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}$, where TTT is tetrathiotetracene, and Ni${\mathrm{S}}_{4}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}$ is nickel bis-(ethylene-1,2-dithiolene). The structure consists of 1D chains of stacked TTT units separated by nonstacked Ni${\mathrm{S}}_{4}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}^{\ensuremath{-}}$ ions. Charge transfer between TTT and Ni${\mathrm{S}}_{4}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}$ creates Ni${\mathrm{S}}_{4}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}^{\ensuremath{-}}$ ions and a formal charge state +5/6 on the TTT units. The dc conductivity along the TTT chains is thermally activated and is \ensuremath{\sim} 30 ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ at 25\ifmmode^\circ\else\textdegree\fi{}C. The magnetic susceptibility appears to be negligible on the TTT chains and Curie-Weiss-like on the unstacked Ni${\mathrm{S}}_{4}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}^{\ensuremath{-}}$ ions. The thermopower at high temperature is quite small (+ 1.5 \ifmmode\pm\else\textpm\fi{} 1 \ensuremath{\mu}V/K) and almost temperature independent. We interpret the data in terms of a quasi-one-dimensional conductor which is distorted at all accessible temperatures. It is noteworthy that both the crystal field and the Peierls instability are expected to produce the required distortion in this system.