Anomalous properties of new organic conductors: Comparison of bromanil, chloranil, and tetracyanoquinodimethane (TCNQ) salts of tetramethyltetrathiafulvalene (TMTTF)

Abstract

Structural, conductivity, magnetic susceptibility, optical, infrared, and Raman data are reported for two new highly conducting organic solids, the bromanil and chloranil salts of TMTTF (tetramethyltetrathiafulvalene). These compounds are shown to have the two basic features essential to an organic metal: segregated stacks of planar, polarizable organic molecules and incomplete charge transfer. Nevertheless, their properties are strikingly different than, for example, those of the TMTTF salt of TCNQ (tetracyanoquinodimethane). The properties of the latter are typical of TCNQ-containing organic metals, with the conductivity and magnetic susceptibility both becoming thermally activated below a (presumably) Peierls transition. In TMTTF-bromanil, on the other hand, the conductivity is already activated at room temperature, well above the transition (∼75K) in the susceptibility. This behavior is not characteristic of a simple quasi-one-dimensional metal. Rather it indicates a magnetic semiconductor with appreciable Coulomb interactions, which undergoes a spin-Peierls-like transition at lower temperatures. The reason why the effects of Coulomb interactions are more dramatically manifested in these compounds is not understood.