What can be learned from observed charged transfers in quasi-one-dimensional salts?

Abstract

The charge transfer rho which results from a minimisation of the total energy in one-chain or two-chain quasi-one-dimensional compounds is evaluated as a function of the relative values of the ionisation. Madelung, delocalisation and van der Waals energies. The author emphasises that fractional values of rho occur only if the delocalisation term is at least of the order of the ionisation and Madelung ones, i.e. if the screening and polarisation processes are more efficient than what is generally believed. He finds that rho >1 is suppressed when the second ionisation potential is too large, and that rho <0.4 is made unlikely by the double effect of the ionisation and van der Waals terms. The role of second-order terms in Coulomb interactions (i.e. of screening and polarisation effects) is discussed and it is shown that, in all compounds built with molecules largely compared with the lattice parameters, they contribute significantly to lattice sums of the Madelung type. Finally, he compares the charge transfer observed in the TTF-TCNQ family with their selenium analogue from this standpoint.