The Crystal Chemistry of Organic Metals. Composition, Structure, and Stability in the Tetrathiafulvalinium-Halide Systems

The results of crystal chemical studies of systems containing the organic π donor tetrathiafulvalene (TTF) with halogens (Z) are reported. In addition to the expected isovalence salts of the TTF mono- and dication, these systems also exhibit a series of segregated stack mixed valence salts of the type (TTF)ZP, where p < 1. These compounds are comprised of separate TTF and Z sublattices where the ratio TTF is nonintegral and defines the charge transfer p from the TTF stacks to the halide ion (which is fully charged). The mixed valence salts can be further classified into ordered or disordered halide sub-lattice types, with 0.7 ≤ p(ordered) ≤ 0.8, and p(disordered) < 0.7. The ordered phases have small homogeneity ranges which were determined both by x-ray diffraction and chemical analysis techniques. The composition shift to lower halide content for the disordered phases suggests that the disorder is of the intrachain type. Using a simplified model structure, the unusual stoichiometries of the mixed valence phases are shown to be determined by the electrostatic Coulomb energies, which are maximized for p considerably less than one. Excellent agreement is obtained between calculated and observed mixed valence compositions. The importance of mixed valency to the general class of π-donor/acceptor salts, such as (TTF)(TCNQ), is also discussed within the context of this ionic model. Finally, from optical absorption and reflectivity studies of mixed valence (p=0.59, 0.76) and fully (p=1) charge transferred (TTF)Brp phases, an upper limit to the on-site Coulomb correlation energy, U ≈ 1.5 eV at optical frequencies, is determined. © 1977, American Chemical Society. All rights reserved.