ORDER AND DISORDER IN (CO)xC60 CRYSTALS

Sander van Smaalen1, Robert Dinnebier1, I. Holleman2, G. von Helden2, G. Meijer2, and W. Schnelle3

1 Laboratory of Crystallography, University of Bayreuth, D-95440 Bayreuth, Germany
2 Dept. Molecular and Laser Physics, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands.
3 Max-Planck-Institut fuer Festkorperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Keywords: fullerenes, phase transitions, powder diffraction.

C60 exhibits an order-disorder phase transition at Tc ~ 260 K, and a glass-transition at Tc 90 K. The character of these transitions is influenced by the presence of impurities (1). The effect of nitrogen and oxygen molecules in particular has been studied extensively. Most experiments show that the effect of these impurities is to broaden the transition region. However, structural information about doped crystals is presently not available.

(CO)xC60 was synthesized recently, and the dynamical behaviour was studied of CO in the octahedral sites of the C60 sublattice by IR spectroscopy and NMR (2). Here we present the results of synchrotron radiation X-ray powder diffraction studies and of the measurement of the specific heat. The low-temperature crystal structure has been determined in full detail (3). The orientational ordering transition is found as an anomaly in the thermal expansion, at a temperature substantially lower than the corresponding transition in pure C60. The glass transition was not visible in the thermal expansion, but it was clearly revealed by the measurement of Cp (4). The structure and the disorder of the C60 sublattice were found to be the same as in pure C60, but with a larger occupation of the major orientation than in pure C60 (0.93 instead of 0.80 at 25 K). Furthermore, we were able to determine the orientations of the six-fold disordered CO molecule on the octahedral sites of the C60 sublattice. The value of x = 0.67 as determined by X-ray structure refinement could be matched quantitatively with the intensity ratio op the resonances observed in IR. The results indicate a weak attractive interaction between CO and the C60 molecules.

 

  1. G. Pitsi, J. Caerels, and J. Thoen Phys. Rev. B55 (1997) 915.
  2. I. Holleman et. al. Phys. Rev. Lett. 79 (1997) 1139.
  3. S. van Smaalen et al. Phys. Rev. B57 (1998) 6321-6324.
  4. S. van Smaalen et al. Europhys. Lett., submitted (1998).