Segregated stack stability of conducting organic charge transferred compounds

It is shown that the Madelung repulsion along the stacking direction of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ) makes a relatively small contribution to the observed compressibility along this direction. One therefore suspects that it is not particularly important in destabilizing or expanding the segregated stacks of conducting donor-acceptor charge-transferred compounds. In the present paper, a simple phenomenological theory of lattice stability along the stacking direction of such compounds is developed. It is shown that if the Madelung repulsion was switched off one might expect a stacking-axis lattice contraction of approximately 1%. Such small contraction results from the rapid exponential band shifts used to simulate the increase in short-range intermolecular repulsion with lattice contraction. © 1978 The American Physical Society.