Role of the Madelung Energy in the Neutral-Ionic Phase Transition of Tetrathiafulvalene Chloranil

In order to elucidate what determines the degree of charge transfer from donor to acceptor in charge-transfer solids, we have calculated the crystalline electrostatic Madelung energy, EM, for tetrathiafulvalene (TTF) chloranil as a function of pressure and temperature. At 300 K between 1 bar and 11 kbar, where there is a transition between Neutral and Ionic ground states, EM becomes more negative by ~0.10 eV. This increase in binding energy suggests that the pressure dependence of the Madelung energy may be sufficiently large to drive the Neutral-Ionic phase transition at high pressures. The magnitude of EM = -3.8 eV at 11 kbar is used to show that the calculated energies of the Neutral and Ionic states become approximately equal at this pressure. On the other hand, at 1 bar between 300 and 50 K, where another transition between Neutral and Ionic ground states has been found, EM becomes more negative by only ~0.05 eV, or half as much as for the transition at high pressure. This suggests that there is an additional driving force for the Neutral-Ionic transition at low temperature (which we believe is the dimerization of the stacks). Better quantitative agreement for the transition and for the charge-transfer band energy is obtained by formally including partial charge transfer in the ground state. © 1985, American Chemical Society. All rights reserved.