Metal-doped fullerene

Since alkali metal doped fullerenes are conductors, and low-temperature superconductors, one wonders if the curved polynuclear hydrocarbons might exhibit a similar behavior. 

The discovery of superconductivity [105] in alkali metal doped attracted the attention of a broad cross-section of the scientific community. The composition of the first alkali metal doped fullerene was determined to be [106,107]. In this compoimd, the transition to a superconducting state occurs at = 19.3 K [106]. In a short time, a large variety of alkali metal fullerides M Cgo have been synthesized and studied with respect to their solid-state properties [1, 106-114]. Thereby, the stoichiometries of as well as the alkali metal with M = Na, K, Rb, Cs have... 

Other subjects of possible future interest include high-pressure studies on en-dohedral fullerene material and on alkali-metal-doped fullerenes under pressure, subjects not dealt with in this review, and polymerization studies on heavy fullerenes. The fact that C70 has recently been shown to polymerize into a well-defined chain phase [165] under high pressure gives hope that similar methods can also be used to orient and polymerize pure isomers of even heavier fullerenes. 

The sensitivity to air of alkali-metal-doped fullerenes (A C ) limits the choice of sample preparation and characterization techniques. To avoid sample degradation, we carried out reactions with the alkali metal vapour and Ceo in sealed tubes either in high vacuum or under a partial pressure of helium. The Cso was purified by chromatography of fullerite and was heated at 160 °C under vacuum to remove solvents. 

We note in passing that the recent transmission electron microscopy pictures of Cgo films by Wang and Buseck show evidence of Cjo-Cgo coalescence to form cylindrical bucky tubes in the solid Film, presumably triggered by the 400-keV electron beam. We wonder if metal atom encapsulation events would occur under similar circumstances with metal-doped fullerene films. 

New carbon modifications, which consist of discrete polyhedral molecules, have been intensively investigated since the early 1990 s. The structures of these fullerenes range from the highly symmetrical buckminsterfullerenes, C50, to the chiral fullerenes, 7, to larger tubular or carbon particles built up of onionscale-shaped molecules. Possible applications, such as the manufacture of superconducting alkali metal-doped fullerenes or their utilization as a support for catalysts, are being developed, but are not yet market ripe. 

Theoretical as well experimental studies show that although there are some differences in the electron structure of pure and alkali metal-doped fullerenes, the band structures of both type of compounds are fundamentally similar. Band width as well as the energy gap between the bands remain qualitatively unchanged after the fullerene doping. In other words, the molecular features of Ceo dominates the electronic structure of the solid phases. 

In contrast to carbon, which forms structures derived from both sp2 and sp3 bonds, silicon is unable to form sp2 related structures. Since one out of four sp3 bonds of a given atom is pointing out of the cage, the most stable fullerene-like structure in this case is a network of connected cages. This kind of network is realized in alkali metal doped silicon clathrate (19), which were identified to have a connected fullerene-like structure (20). In these compounds, Si polyhe-dra of 12 five-fold rings and 2 or 4 more six-fold rings share faces, and form a network of hollow cage structures, which can accommodate endohedral metal atoms. Recently, the clathrate compound (Na,Ba), has been synthesized and demonstrated a transition into a superconductor at 4 K (21). The electronic structure of these compounds is drastically different from that of sp3 Si solid (22). 

The compound Ceo is not itself a superconductor, but when alkali metals are added it becomes superconducting. The doped compound forms a face-centered cubic lattice with a lattice constant of 10.04 A, and this structure has two tetrahedral holes and one octahedral hole per Ceo molecule. If all of these holes are occupied by alkali metals A, the resulting compound is A Ceo- An example of such a compound is K2RbC6o with potassium in the smaller tetrahedral holes and rubidium in the larger octahedral holes. The transition temperatures of these doped fullerenes range from 19 to 47 K. The compound RbsCeo was found to have an isotope effect exponent a = 0.37, somewhat less than the BCS value 0.5. 

Fullerenes show quite different electronic properties to other carbon al-lotropes. The carbon in diamond has a nonconductive sp hybrid orbital, while that in graphite is conductive since it has an sp hybrid orbital. Fullerene carbons have orbitals that are intermediate between sp and sp, and so the fullerenes behave hke semiconductors. Graphite can be oxidized and reduced. In contrast, fullerenes are easily reduced but difficult to oxidize. Fullerenes can also be doped with metal ions (Fig. 3.2). In doped fullerenes, the metal ion is... 

Graphite was known from the 60 s to become metallic under suitable doping with potassium. It was then normal to try and dope with alkali metals the fullerenes Qq molecules discovered in 1985 by H.W. Kroto and R.E. Smalley, and to extend this treatment to balloons or tubes-like C molecules later developed in great quantities. The superconductivities observed here, often at fairly high temperatures, seem of a normal BCS type, with notable electron-phonon coupling and also with electron-electron repulsions weakened by the electrical polarisability of these big and easily excited molecules. Here again, more remains to be done on the detailed characteristics of conduction and superconductivity. 

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