Doped Fullerenes

The picture on the left illustrates the structure of the pure C₆₀. In this lattice, there is plenty of space to intercalate other atoms or small molecules in between the bucky balls. (To learn more about the structure take a virtual tour inside the solid fullerides!) C₆₀ has a free electronic band which can hold up to six more electrons. When alkali metal atoms are put into this structure they donate their 1 valence electron to a neighboring C₆₀ molecule.

The first stable structure is A₁C₆₀ - one Alkali atom in each "octahedral" site. Further doping produces A₃C₆₀ - now the C₆₀ molecules have their electronic band exactly half full so this is a metal and in fact it becomes superconducting at 30K for Rubidium, 20K for Potassium, and various mixtures can produce other superconducting transition temperatures. Next is A₄C₆₀ - the lattice must now be expanded to include more than three intercalated atoms, and becomes body-centered-tetragonal. The structure can be expanded once more, to body-centered-cubic, and now 6 atoms will fit in. This is the highest Alkali doped fullerene, A₆C₆₀, which now has a full electronic band and is therefore an insulator.

In our laboratory we have developed a special glass vacuum chamber for performing these doping experiments while measuring resistance and Infrared properties at the same time.

Polymerized Rb₁C₆₀

The A₁C₆₀ compound has turned out to be much more interesting than one would have guessed. This picture shows the structure of the polymeric phase of Rb₁C₆₀, produced by slow cooling the material from the high temperature rocksalt phase described above. The fullerenes are now chemically bonded to form long polymer chains. This phase transition is of first order, with a transition temperature around 400K. The mid IR spectrum shows a characteristic change in the intra molecular vibrational modes at this transition.

Another phase is produced when the material is fast cooled. The IR properties indicate that the quenched sample is an insulator. The structure of this phase is composed of C₆₀ dimers.

Pictures courtesy of J. Lauher's Fullerene Structure Library , where you can find more C₆₀ pictures.

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