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Oct. 5, 2007 Research Highlight Chemistry

Just add barium

Squashed carbon balls show promising electronic properties

Image of clathrate–C60 Figure 1: A graphical representation of clathrate–C60. © Angew. Chem. Int. Ed./Wiley/46/6275 (2007)

Theoretical calculations have shown that an exciting new material—made by squashing together latticework balls of carbon atoms—could be a superconductor.

Such superconductors are widely sought, since they carry electricity with no resistance and could be used to make extremely efficient energy transfer and storage systems.

Materials containing a cage-like structure of atoms, known as clathrates, have shown promise as superconductors. Scientists led by Shoji Yamanaka from Hiroshima University recently made a new clathrate1 based on buckminsterfullerene (C60)—a hollow molecule made of sixty carbon atoms arranged into a soccer ball shape. High temperatures and pressures were used to squeeze the balls together until they became interconnected cubes of carbon, dubbed clathrate-C60 (Fig. 1).

Toshiaki Iitaka of RIKEN’s Discovery Research Institute in Wako and colleagues from the University of Saskatchewan in Saskatoon, Canada, led by John Sak Tse, have now calculated the structural and electronic properties of this material, and report their results in the international edition of Angewandte Chemie2.

They found that pure clathrate-C60 is metallic, and suggest that it could be a superconductor. Recent experiments have confirmed that clathrate-C60 is weakly metallic, but found no superconductivity even at the low temperature of –269 ˚C: just four degrees above the coldest temperature possible, known as absolute zero. Colder temperatures even closer to absolute zero may be needed to uncover the predicted superconductivity, the team suggests.

The structure of clathrate-C60 allows individual metal atoms to sit either inside, or between, each of the carbon cages. The team looked at how adding barium—just the right size to fit in either space—would alter the material’s properties.

The team also found that in both cases, adding barium would release heat energy from the material, making the overall structure more stable. Although the compounds have not been made, this result means that the compounds should be achievable experimentally, the team says.

Further calculations revealed that if the barium atoms sit inside the carbon cages, they lose an electron—this does not change the overall structure by much, but it turns the material into a semiconductor. Conversely, adding barium atoms into the spaces between the carbon cages distorts the overall structure. The metal atoms also lose less charge, and the material remains metallic.

These changes in electronic properties suggest that clathrate-C60 “may potentially be an important electronic material for technology applications,” the scientists predict.

References

  • 1. Yamanaka, S., Kubo, A., Inumaru, K., Komaguchi, K., Kini, N.S., Inoue, T. & Irifune, T. Electron conductive three-dimensional polymer of cuboidal C60. Physical Review Letters 96, 076602 (2006). doi: 10.1103/PhysRevLett.96.076602
  • 2. Yang, J., Tse, J. S., Yao, Y. & Iitaka, T. Structural and electronic properties of pristine and Ba-doped clathrate-like carbon fullerenes. Angewandte Chemie International Edition 46, 6275–6277 (2007). doi: 10.1002/anie.200701555

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