Structural genealogy of BEDT-TTF-based organic conductors -: II.: Inclined molecules:: θ, α, and χ phases

Overlap integrals between HOMO's of two non-parallel BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) molecules have been calculated. As the dihedral angle between the molecular planes decreases from 180 degrees (parallel) to 90 degrees (perpendicular), the overlap integral increases and attains a maximum around 90 degrees. This accounts for the "universal phase diagram" of the theta-phase; theta-salts vary from an insulator to a metal with decreasing the dihedral angle. The ratio of the lattice constants in the conducting plane, cia, changes in proportion to the dihedral angle. Thus c/a can be used instead of the dihedral angle. A similar universal phase diagram is applicable to analogous phases like alpha and alpha", and also to the corresponding phases of other donors. The properties of kappa-phase salts are similarly scaled by cia. As cia increases, the intradimer overlap integral decreases owing to the increase of the intradimer spacing, and correlated insulator, super conductor, and simple-metal phases appear in succession. When cia increases further, another insulating phase emerges due to the decrease of the interdimer overlaps. Chemical pressure in both theta- and kappa-phases reduces cia, and stabilizes the insulating state. Hydrostatic physical pressure gives the same influence in the theta-phase, but enhances the interdimer interaction in the kappa-phase to result in the opposite effect. A diagram is proposed to illustrate which structures of beta, beta" theta, and kappa are favored by BEDT-TTF and other donors.