RARE-GAS INTERCALATED C60.* B. Morosin, Sandia Natl Lab., Albuquerque, NM 87185, G.H. Kwei, Lawrence Livermore Natl Lab., Livermore, CA 94550, J.D. Jorgensen and S. Short, Argonne Natl Lab., Argonne, IL 60439 and J.E. Schirber, Sandia Natl Lab.
The intercalation of the rare gases into interstices of C60 at 0-6 kbar pressures has been studied by neutron diffraction using a gas pressure cell at the Intense Pulsed Neutron Source (Special Environment Powder Diffractometer). He intercalates easily while Ar does not; for Ne, intercalation kinetics may be monitored using lattice parameter changes as a function of time following sudden changes in Ne pressure. Intercalation rates decrease with increasing Ne pressure, with time constants on the order of several hours. Upon pressure release, diffusion out of the C60 occurs at rates too fast to measure by this technique (i.e., a few minutes).
The fcc-sc (Fm3m to Pa3) orientational ordering transition occurs at a pressure which is highest for He, intermediate for Ne and smallest for Ar, the latter with a value in agreement with literature values for several nonpenetrating media.1 For each intercalated compound in either the fcc or sc phases, these data yield the room-temperature compressibilities.2 Rietveld refinement of the time of flight data for the more extensive room temperature Ne data sets show that only the octahedral interstices are occupied with Ne, that there is a pressure dependence on the octahedral site occupancy, and that the cubic lattice parameter increases linearly with the amount of intercalated Ne. The observed lattice parameter behavior is similar to the problem of packing of hard spheres with large size dissimilarity, previously addressed with molecular dynamics calculations.3 We believe that this is the first observation of this effect in a realistic physical system.
1. W.I.F. David et al., Proc. Roy. Soc. Lond. A442 129 (1993); J. Phys. Condens. Matter 5 7923 (1993); G.A. Samara et al., Phys. Rev. B47 4756(1993).
2. J.E. Schirber et al., Phys Rev B51 12014 (1995).
3. D.L. Ermak et al., J. Phys. Chem. 85 3221 (1981).
* This work was supported by DOE Contracts DE-AC04-94AL85000 at SNL, W-7405-ENG-48 at LLNL, and W-31-109-ENG-38 at ANL.