Ground-state properties and neutral-to-ionic transformation of organic mixed-stack compounds: Mean-field approximation.

Abstract

A one-dimensional chain of alternating donor and acceptor molecules has been modeled by a generalized Hubbard Hamiltonian which includes the long-range Coulomb interaction and a coupling of the electronic system to the lattice. The ground-state properties such as charge-density waves centered on the molecules and lattice dimerization have been analyzed. We show that the isostructural neutral-to-ionic phase transition will be discontinuous according to our model. In a system characterized by a large transfer integral a dimerized chain can be stable in a narrow range of balance between the on-site potential and the intersite Coulomb interaction; it suggests the possibility of a reentrant structural phase transition. A qualitative analysis of the model indicates that the traditional Peierls mechanism of dimerization becomes uneffective for a system characterized by a small transfer integral. An alternative approach to the neutral-to-ionic transformation is briefly outlined.