Tetrahedral Site

Figure 5.7. Crystal lattice unit cell of Ceo molecules (large spheres) doped with alkali atoms Mdark circles). [From F. J-. Owens and C. P. Poole, Jr., The New Superconductors, Plenum, ¿1998.]

molecules. When C«o is doped with potassium to form K3C«o> the potassium atoms become ionized to form K+ and their electrons become associated with the C^o, which becomes a C^3- triply negative ion. Thus each CM has three extra electrons that are loosely bonded to the and can move through the lattice making Cm electrically conducting, hi this case the CM is said to be electron-doped.

5.3.5. Superconductivity in Ceo

Superconductivity is a state of matter in which the resistance of a sample becomes zero, and in which no magnetic field is allowed to penetrate the sample. The latter manifests itself as a reduction of the magnetic susceptibility x of the sample to X = —1 (in the MKS system). In 1991, when A. F. Hebard and his co-workers at Bell Telephone Laboratories doped C«o crystals with potassium by the methods described above and tested them for superconductivity, to the surprise of all, evidence was found for a superconducting transition at 18 K. Figure S.8 shows the drop in the magnetization indicative of the presence of superconductivity. A new class of superconducting materials had been found having a simple cubic structure and containing only two elements. Not long after the initial report it was found that many alkali atoms could be doped into file lattice, and the transition temperature increased to as high as 33 K in Cs2RbC60- As the radius of the dopant alkali atom increases, the cubic Cm lattice expands, and the superconducting transition temperature goes up. Figure 5.9 is a plot of the transition temperature versus the lattice parameter.

It was mentioned above that graphite consists of parallel planar graphitic sheets of carbon atoms. It is possible to put other atoms between the planes of these sheets, a procedure called intercalation. When intercalated with potassium atoms, crystalline graphite becomes superconducting at file extremely low temperature of a few tenths of a kelvin.

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