Estimation of Off-Site Coulomb Integrals and Phase Diagrams of Charge Ordered States in the θ-Phase Organic Conductors

Intermolecular Coulomb repulsion, V, of the highest occupied molecular orbitals (HOMO) of BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) is calculated for various molecular geometries. The bare V is a quantity that is easily estimated under the point charge approximation. As far as the screened V in actual crystals is proportional to the calculated bare V, the usual θ-phase prefers the horizontal or diagonal stripe, whereas the vertical stripe becomes comparatively stable in the limit of the small dihedral angle (in the metallic limit). The phase diagrams of the θ-phase are discussed under the combination of the static charge distribution (the atomic limit) and the Stoner model (the extended Stoner model). The model contains two order parameters: the spin polarization, Sz, and the charge order, n-1/2. This model explains why the insulating state of the Rb salt below 190 K is a paramagnetic charge-ordered state, while the Cs salt has a different insulating phase below 20 K. The lattice dimerization of the Rb salt can be explained only from V.