Transition metals fullerene doping

The DC arc technique is carried out in a water-cooled chamber with a graphite cathode and a graphite anode that has been doped with the desired metal and a catalyst in the presence of a gas, usually helium. One reason for the lack of transition-metal endohedral fullerenes can be attributed to the difficulty of current DC arc-discharge methods to cause encapsulation of transition metals in fullerenes, for which the reason remains unclear. For this reason, transition metals are... 

Boron nitride fullerenes can be doped with transition metal atoms. Interestingly, the doped BN fullerenes have the same magnetic moment as the isolated dopants (Batista et al., 2007). 

Batista, R.J.C., Mazzoni, M.S.C., and Chacham, H. (2007) Boron nitride fullerene B36N36 doped with transition metal atoms first prindple calculations. Phys. Rev. B, 75 (3), 035417. 

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 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... 

The carbon atoms are in a nearly sp hybridized state which relates these compounds strongly to the conjugated polymers and guarantees the same colorful vibrational spectroscopy as described in the above subsections. Also, the fullerenes can be doped to a metallic state which in some cases even goes superconducting with a rather high transition temperature (Rosseinsky et al., 1991). The state of the art in fullerene research up to 1993 is well described in (Dresselhaus et al., 1993). 

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. 

The synthesis of macroscopic amounts of C o and C70 (fullerenes) has stimuiated a variety of studies on their chemical and physical properties. We recently demonstrated that C o and C70 become conductive when doped with alkali metals. Here we describe iow-temperature studies of potassium-doped both as films and bulk samples, and demonstrate that this material becomes superconducting, Superconductivity is demonstrated by microwave, resistivity and Melssner-effect measurements. Both polycrystalline powders and thin-flim samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor. 

Small amounts of the individual fullerenes ( 0.5mg) were placed in quartz tubes with alkali metals and sealed under vacuum. These samples were subjected to a series of heat treatments and tests for superconductivity by 9-GHz microwave-loss experiments. Preliminary tests indicated that only the K-doped Cfto showed a response consistent with a superconducting transition (Fig. 1). For this reason, together with the fact that K Cgo showed the highest film conductivity, we focused our studies on the K-doped compound. 

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