Counterintuitive Anisotropy of Electron Transport Properties in KC_(60)(THF)5·2THF Fulleride

摘要

The discovery of unique physical properties possessed by alkali metal fullerides A_xC_(60) (x = 1-3, A = K, Rb, Na, Cs) such as conductivity and superconductivity has prompted intensive investigation in recent years with regards to these materials. Nevertheless, the exact nature of the electronic properties exhibited by fulleride-based solids is not yet fully clarified and still remains a much discussed subject in many research groups worldwide. As a basis for the deeper understanding of fulleride properties and the further elaboration of a structure-property relationship, detailed knowledge of their structural features is indispensable. Single-crystal X-ray structure determination is the most powerful experimental technique for detailed structural analysis; unfortunately, progress in this field with respect to fulleride characterization has been slow because of difficulties in the synthesis of high-quality single crystals containing fullerene anions. As a rule, the fulleride crystals are twinned and frequently suffer orientation disorder of the sphere-like C_(60) anions. Moreover, the strong tendency of C_(60) anion radicals to recombine and form diamagnetic single-bonded (C_(60))2~(2-) ions, has further impaired the ability to obtain the requisite close-packed C_(60)~(·-) structures essential for manifestation of conductivity. As a matter of fact, only a few high-quality structures have been reported to date, where individual C_(60) anion radicals have remained separated at low temperatures. Recently, a 2D-layered structure in which C_(60) anion radicals form a 2D honeycomb sublattice with short fullerene-to-fullerene distances has been reported and investigated. The compound was found to be a fascinating example of the first fullerene-based two-dimensional organic metal to be described.