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Science 18 February 1994:
Vol. 263. no. 5149, pp. 950 - 954
DOI: 10.1126/science.263.5149.950
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Articles

Crystal Structure, Bonding, and Phase Transition of the Superconducting Na2CsC60 Fulleride

Kosmas Prassides 1, Christos Christides 1, Ian M. Thomas 1, Junichiro Mizuki 2, Katsumi Tanigaki 2, Ichiro Hirosawa 2, and Thomas W. Ebbesen 2

1 School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, United Kingdom
2 Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305, Japan

The crystal structure of superconducting Na2CsC60 was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic

[See equation in the PDF file],

isostructural with pristine C60. Anticlockwise rotation of the C60 units by 98° about [111] allows simultaneous optimization of C60-C60 and alkali-fulleride interactions. Optimal Na+-C603- coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C60 molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to sim1.43 angstroms). On heating, Na2CsC60 undergoes a phase transition to a face-centered-cubic

[See equation in the PDF file]

phase, best modeled as containing quasi-spherical C603- ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.

Submitted on October 25, 1993
Accepted on December 6, 1993


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Orientational Disorder of C60 in Li2CsC60.
I. Hirosawa, K. Prassides, J. Mizuki, K. Tanigaki, M. Gevaert, A. Lappas, and J. K. Cockcroft (1994)
Science 264, 1294-1297
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Science. ISSN 0036-8075 (print), 1095-9203 (online)