Ground State Phase Diagram of Mixed-Stack Compounds with Intermolecular Electron Transfer
Tadeusz Luty
Institute of Physical and Theoretical Chemistry, Technical University of Wrocław, Wyb. Wyspiańskiego 29, 50-370 Wrocław, Poland
and Groupe Matiere Condensée et Materiaux, URA au CNRS 804, Université de Rennes I, 35042 Rennes, Cedex, France
Received: January 12, 1995
Dedicated to Professors Krzysztof Pigoń, Józef W. Rohleder and Zdzisław Ruziewicz on the occasion of their 70th birthday
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The ground state energy for a chain of donor and acceptor molecules (mixed-stack architecture) is calculated within the three-state model. The model describes the intermolecular electron transfer and, in particular, stres­ses the role of the diagonal coupling of the electron to symmetry breaking molecular displacements and the local electric field. The modulation of the in­termolecular Coulomb interaction is shown to have important consequences for the ground state and its dynamics. In particular, the ground state en­ergy as a function of the displacement may show one, two or three minima with varied molecular ionicity. An analysis of the function gives a phase diagram which indicates a possibility for the coexistence of neutral (undis­torted) and ionic (distorted) chains of molecules in the ground state. The function is illustrated by numerical calculations with parameters appropriate for the tetrathiafulvalene-chloranil crystal which undergoes a neutral-to-ionic phase transition induced by either temperature or pressure. The effect of the electron transfer on the lattice dynamics of the mixed-stack system is briefly considered. It is suggested that the thermodynamical phase diagram for tetrathiafulvalene-chloranil system can be understood as a result of two effects: pressure induced quantum mixing between diabatic states which de­termine a nature of components and temperature stimulated classical mixing of the components.
DOI: 10.12693/APhysPolA.87.1009
PACS numbers: 71.35.+z, 71.38.+i