Counterintuitive Anisotropy of Electron Transport Properties in KC₆₀(THF)₅⋅2 THF Fulleride

Abstract

The discovery of unique physical properties possessed by alkali metal fullerides A x C 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. [1] 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 structureproperty 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. [2] Moreover, the strong tendency of C 60 anion radicals to recombine and form diamagnetic single-bonded (C 60 ) 2 2À ions, [3] has further impaired the ability to obtain the requisite close-packed C 60 C À 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. [4] Recently, a 2Dlayered structure in which C 60 anion radicals form a 2D honeycomb sublattice with short fullerene-to-fullerene distances has been reported and investigated. [5] The compound was found to be a fascinating example of the first fullerenebased two-dimensional organic metal to be described. In contrast, a later reported 2D-layered structure of CsC 60 -(THF) 4 comprising layers of an atypical square-shaped configuration of fully ordered fullerene anion radicals with short C 60 -C 60 center-to-center distances has shown no sign of conductivity. [6] These findings clearly point to the importance of the relative orientational configuration of fullerene anions in the crystal for manifestation of conductivity phenomena in the solid state.Herein, we report the combined results of investigations into the electronic and structural properties of KC 60 -(THF) 5 ·2 THF single crystals. The title compound exhibits a unique 2D arrangement of C 60 anion radicals, in which corrugated layers are formed with close fullerene-fullerene contacts. Important findings include the presence of onedimensional electric conductivity and the observation of a counterintuitive anisotropy for electron transport. A detailed crystallographic and spectroscopic study carried out with individual single crystals, together with quantum chemical calculations, reveals the main factors responsible for effective interactions between fullerene anions leading to the manifestation of conductivity in fullerides.The C 60 anion radicals were obtained from sol...

Anorganische Chemie 2013

“…KC60(THF)5·2THF weist eine gewellte Schichtstruktur auf mit kurzen Abständen zwischen den Radikalanionen in der kristallographischen a‐c‐Ebene. Die elektrische Leitfähigkeit der Einkristalle ist stark anisotrop, wie ESR‐spektroskopische Untersuchungen bestätigen 5…”

Festkörperchemie 2013

“…Das gilt auch für Fulleride — nicht aber für ihr jeweiliges Somo, also das halbbesetzte Molekülorbital des [C60]— in KC60‐(THF)5·2 THF. Für diese Verbindungen maßen Jansen und Mitarbeiter4 eine unerwartete anisotrope Leitfähigkeit. Die C60‐Moleküle ordnen sich in der orthorhombischen Struktur (Pca21) unterhalb von 240 K mit längeren Abständen in a‐ als in c‐Richtung an (Abbildung 3a und 3b).…”