THE ROLE OF LONG-RANGE COULOMB INTERACTION IN THE NEUTRAL-TO-IONIC TRANSITION OF QUASI-ONE-DIMENSIONAL CHARGE-TRANSFER COMPOUNDS

The neutral (N)-to-ionic (I) transition with dimerization, found in or ganic charge transfer compounds with mixed stacking, has been studied using a self-consistent theory which takes exact account of long-range Coulomb interactions but neglects interchain coupling. The unrestrict ed Hartree-Fock approximation gives a set of optimally mixed donor-acc eptor bands with optical gap which bifurcates in the I phase. The dime rization is ascribed to the electrostatic instability, rather than to the distance dependence of the intermolecular electronic transfer ener gy as is usually done. The phase diagram for the first-order N-I trans ition with dimerization and discontinuous change of charge transfer is presented on the parameter plane of the redox potential and the trans fer energy. The results of numerical calculations are consistent with the experimental data for tetrathiafulvalene-chloranil compound.