Electrical, magnetic, and optical properties of the tetrathiafulvalene (TTF) pseudohalides,(TTF)12(SCN)7and(TTF)12(SeCN)7

Abstract

The electrical, magnetic, and optical properties of charge-transfer salts containing tetrathiafulvalene (TTF) and the pseudohalides, thiocyanate (SCN) and selenocyanate (SeCN), have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to a cation-radical-anion-radical system, such as tetrathiafulvalene tetracyanoquinodimethane (TTF) (TCNQ). Measurements of electrical conductivity, thermoelectric power, and optical reflectivity of single crystals of the nonstoichiometric salts ${(\mathrm{TTF})}_{12}$${(\mathrm{SCN})}_{7}$ and ${(\mathrm{TTF})}_{12}$${(\mathrm{SeCN})}_{7}$ show metal-like characteristics above 200\ifmmode^\circ\else\textdegree\fi{}K (high-temperature region). The conductivities at room temperature are \ensuremath{\sim} 750 ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$${\mathrm{cm}}^{\ensuremath{-}1}$, comparable to those found in (TTF) (TCNQ), and increase with decreasing temperature down to \ensuremath{\sim} 200\ifmmode^\circ\else\textdegree\fi{}K. The thermoelectric power at room temperature is small and positive (\ensuremath{\sim} 9 \ensuremath{\mu}V/\ifmmode^\circ\else\textdegree\fi{}K), and decreases linearly with decreasing temperature in this region (as expected for metal-like hole conduction along the TTF chains). The ESR intensity, however, decreases with decreasing temperature above 200\ifmmode^\circ\else\textdegree\fi{}K. At 170\ifmmode^\circ\else\textdegree\fi{}K a metal-nonmetal transition occurs, and the transport and magnetic properties below this temperature are characteristic of a semiconducting state.