Exceptional solid-state properties of organic 2:1 donor–acceptor metals with integral charge transfer

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Abstract
The material properties of low‐dimensional 2:1 donor (D)–acceptor (A) salts with integral mutual charge transfer(CT) are exceptional in the class of the synthetic metals. The electronic charge fluctuations in the majority component of D + 2 A − and A − 2 D +compounds are of extremal character in the presence of strong electronic correlations. In the minority component the fluctuations are extensively suppressed. The corresponding solids are one‐chain conductors. The charge fluctuations are investigated by a simple analytic model formulated on the basis of the local approach for the many‐particle problem and the bond‐orbital approximation for the independent‐particle wave function. The Fermi surfaces of the D + 2 A − and A + 2 D −CT salts are effectively flat also for smaller anisotropy ratios. Structural phase transitions of the Peierls type leading to one‐dimensional as well as two‐ and/or three‐dimensional ordering are analyzed in a tight‐binding framework. There exists a large range, where the one‐dimensional dimerization is suppressed, but where n‐dimensional (n=2, 3) ordering can still exist. Fermi‐surface nesting in materials with smaller anisotropy ratios allows for the conservation of metastable solid‐state configurations, i.e., configurations where the electron–lattice interaction is enhanced, but where a metal–insulator transition is suppressed. For the quasi‐one‐dimensional metals with D + 2 A − and A − 2 D + stoichiometry it is shown that electronic correlations lead to a remarkable enhancement of the metal–insulator transition temperature T P . Experimental data available for the respective 2:1 CT salts can be rationalized by simple analytic models. A possible violation of particle–hole symmetry between D + 2 A − and A − 2 D +compounds in their ability to stabilize a low‐temperature superconducting ground state is suggested.