1999 Volume 72 Issue 2 Pages 179-197
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° (parallel) to 90° (perpendicular), the overlap integral increases and attains a maximum around 90°. This accounts for the “universal phase diagram” of the θ-phase; θ-salts vary from an insulator to a metal with decreasing the dihedral angle. The ratio of the lattice constants in the conducting plane, c/a, 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 α and α″, and also to the corresponding phases of other donors. The properties of κ-phase salts are similarly scaled by c/a. As c/a increases, the intradimer overlap integral decreases owing to the increase of the intradimer spacing, and correlated insulator, superconductor, and simple-metal phases appear in succession. When c/a increases further, another insulating phase emerges due to the decrease of the interdimer overlaps. Chemical pressure in both θ- and κ-phases reduces c/a, and stabilizes the insulating state. Hydrostatic physical pressure gives the same influence in the θ-phase, but enhances the interdimer interaction in the κ-phase to result in the opposite effect. A diagram is proposed to illustrate which structures of β, β″, θ, and κ are favored by BEDT-TTF and other donors.
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