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 ${\mathrm{}\left(\mathrm{TTF}\right)\mathrm{}}_{12}$ ${\mathrm{}\left(\mathrm{SCN}\right)\mathrm{}}_7$ and ${\mathrm{}\left(\mathrm{TTF}\right)\mathrm{}}_{12}$ ${\mathrm{}\left(\mathrm{SeCN}\right)\mathrm{}}_7$ show metal-like characteristics above 200°K (high-temperature region). The conductivities at room temperature are ∼ 750 ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, comparable to those found in (TTF) (TCNQ), and increase with decreasing temperature down to ∼ 200°K. The thermoelectric power at room temperature is small and positive (∼ 9 μV/°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°K. At 170°K a metal-nonmetal transition occurs, and the transport and magnetic properties below this temperature are characteristic of a semiconducting state.