Polymeric fullerene chains in RbC60 and KC60

• Published in: Nature (London) 1994-08, Vol.370 (6491), p.636-639
• Main Authors: Stephens, Peter W., Bortel, G., Faigel, G., Tegze, M., Jánossy, A., Pekker, S., Oszlanyi, G., Forró, L.
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Humanities and Social Sciences; letter; multidisciplinary; Organic polymers; Physicochemistry of polymers; Properties and characterization; Science; Science (multidisciplinary); Structure, morphology and analysis
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/370636a0

NEARLY all of the molecular crystals containing C 60 , formed at ambient pressure 1,2 have inter-fullerene separations of the order of 10 Å — the expected distance based on the molecular van der Waals radii. The sole exceptions are the room-temperature phases of AC 60 (where A denotes K, Rb or Cs), which are formed by reversible solid-state transformation from high-temperature (>150 °C) phases 3 . These phases have lattice parameters about 9% shorter in one direction, and in addition RbC 60 has magnetic properties suggestive of a one-dimensional metal 4 . We suggested in ref. 4 that this short distance may be due to covalent bonding between neighbouring C 60 molecules. Here we provide direct evidence for such bonding from powder X-ray diffraction studies of RbC 60 and KC 60 . The linkage is through a [2+2] cycloaddition, which has been hypothesized to take place during photopolymerization of solid C 60 (ref. 5), and which has also been proposed 6 for RbC 60 . Such inter-fullerene linkages are calculated 7,8 to be the preferred mode of dimerization of C 60 . The AC 60 phases thus provide an example of a thermal phase transition driven by the reversible formation and breaking of covalent bonds.