A VALENCE-BOND APPROACH TO THE ELECTRONIC LOCALIZATION IN 3/4 FILLED SYSTEMS

The Diagrammatic Valence Bond (DVB) method has been used to investigate the electronic properties of one-dimensionnal (1D) clusters, for which exact solutions were obtained within the framework of an (extended) Hubbard hamiltonian. The correlation parameters, U, V and V2 are tentatively calculated by means of SCF semi-empirical methods and compared to the data issued from experiment. From the crystal structures and the conductivity data, the M2X salts (M: organic molecule, X- anion) may be classified in two groups: on one hand, the compounds with a dimerized structure which are semi-conductor at 300 K and, on another hand, the systems with a regular or quasi-regular 1D stacking which are metallic at 300 K and which exhibit a change in the electronic localization around 100-200 K. For the first class, the DVB calculations lead to a strong bond order wave picture for the localized ground state. The effect of the next nearest neighbor V2 is small although it may affect long distance charge correlations and the magnetic behavior. The magnetic susceptibility chi calculations on a 8 sites ring lead to a good agreement with 300 K experimental data and the temperature dependence of chi is best reproduced with V2 close to V/2. For the second class, it is shown that, the electronic dimerization might be modified by the anion potential, as indicated by model ab initio calculations. The behavior of the resistivity as a function of temperature is qualitatively described by a change in the intra and interdimer transfer integrals. The DVB results on 12 sites rings indicate that this difference leads to an enhanced localization when the correlations are turned on compared to the high temperature regular system.