Conductivity and structure of bilinear organic addition compounds

Abstract

The relationship between crystal structures and metallic conductivities of linear organic materials such as tetrathiafulvalene-tetracyanoquinodimethane is explained in terms of strong lateral e astic interactions between chains. A microdomain model is presented in which at high temperatures there are, in general, two coexisting phases on each stacked molecular chain. The ordering of (in general, four) phases on two interacting chains leads to a variety of phase transitions at low temperatures. By examining temperature-dependent electrical conductivities and magnetic susceptibilities one can establish the character of each such transition.For several years solid-state chemists have synthesized and measured the properties of highly anisotropic organic crystals consisting of cations and anions stacked in segregated molecular chains (1, 2). Charge transfer in these systems is incomplete, giving rise to partially filled valence bands and very high axial electrical conductivities [-103 (ohm-cm)-l at room temperature in favorable materials]. As the temperature is lowered, these conductivities increase rapidly until a phase transition to some kind of ordered state takes place, when the conductivity decreases rapidly with further temperature reduction, in an activated manner similar to that found in semiconductors.The behavior just described is typical of many more isotropic (three-dimensional or two-dimensional) inorganic crystals that undergo metal-semiconductor transitions. Moreover, the metal-semiconductor transition in the bilinear organic crystals has been identified in x-ray, electron, and neutron diffraction studies by the formation of charge density waves, whose period may be distinct from that of the lattice, as originally suggested by Peierls (3). However, there is a very serious theoretical dilemma posed by the wealth of transport and structural data now available. The charge density wave (insulating phase) forms at high temperatures, where the conductivity is metallic, and it grows in intensity as the conductivity increases with decreasing temperature, down to the temperature at which the phase transitions begin. This is highly contradictory behavior, and this article proposes a theoretical model that resolves the paradox. Several other models have been proposed to deal with this problem; they are discussed at the conclusion of this article. Bilinear structures Under the influence of constraints, a system undergoing a first-order phase transition separates into coexisting phases until the transition is complete; the fraction of the system in a given phase in, e.g., a gas-liquid transition, depends on temperature and volume for a given mass. In strictly one-dimensional systems, however, it is well known (4) that coexistence of phases is impossible because of logarithmic divergence of the entropy with the creation of more domains of smaller length. In a statistical treatment the domains are separated by domain walls, but in a linear system that is free to vibrate perpendicular to the Abbreviations: TT...