Complex TCNQ salts with asymmetric donors. I. Transport properties

Abstract

The temperature dependence of transport properties (DC conductivity, 9.1 GHz conductivity and dielectric constant, thermoelectric power) measured on single crystals of a series of well-conducting, complex, charge-transfer salts of the form D(TCNQ)₂ is presented. In all except one case the donors are asymmetric and the corresponding dipole moments were calculated by a molecular orbital method. It is demonstrated that donors with large dipole moments are arranged regularly leading to a charge-ordered state with low conductivity and a large energy gap. Smaller donor dipoles are stacked randomly. When intrachain and interchain Coulomb forces are much larger than the random donor-acceptor interactions the material is a band semiconductor with a small but well-defined energy gap. For large random site energies random localization is important; the implication of this on the transport properties is discussed in view of different theories.