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An electronic band structure study of TTF-TCNQ and (SN)x Friesen, Waldemar Isebrand

Abstract

The electronic energy band structures of two highly conducting, anisotropic solids have been calculated using the extended Hückel method. One-, two-, and three-dimensional models of the organic charge transfer salt tetrathiofulvalinium tetracyanoquinodimethan (TTF-TCNQ) and of the inorganic polymer polysulphur nitride (SN)x have been studied. The results indicate that the band structure of TTF-TCNQ is well described by a tight-binding, one-dimensional model in which interactions between stacks of molecules are neglected. The Fermi surface is seen to consist of extremely flat electron and hole surfaces, the nature of which is inconclusive in predicting a Fermi-surface-related instability leading to a Peierls distortion. A one-dimensional model of (SN)x predicts metallic behaviour as the Fermi energy is found to lie at a symmetry-induced point of degeneracy where two bands cross. The single chain is highly unstable against a symmetry-reducing distortion; however, three-dimensional interchain interactions appear to stabilize the structure. Consequently, the Fermi surface is that of a semimetal with electron and hole pockets. The essential features of the band structure can be explained by a simple tight-binding model involving SN molecular anti-bonding π orbitals. Differences in the reported crystal structures used in the calculation are seen to have no qualitative effect.

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