EXCEPTIONAL SOLID-STATE PROPERTIES OF ORGANIC 2-1 DONOR-ACCEPTOR METALS WITH INTEGRAL CHARGE-TRANSFER

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 D2+ A- and A2-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 D2+ A- and A2+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 which 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 D2+A- and A2-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 D2+A- and A2-D+ compounds in their ability to stabilize a low-temperature superconducting ground state is suggested.