Order-disorder transition in solid C60 charged with O2 and with N2:: A study with dielectric and 13C NMR spectroscopies

C-60 pellets, produced by uniaxial compaction and then charged with O-2 or N-2 under high pressure, have been studied with dielectric and C-13 nuclear-magnetic-resonance spectroscopies. Oxygen was found to depress their fee-sc transition temperature T-c from the normal value of 260 K, as indicated by the minimum in the temperature-dependent permittivity epsilon', and also enlarge the drop in epsilon' at the transition. The magnitudes of the downward shift in T-c and the drop in s' were roughly proportional to the O-2 occupancy at octahedral lattice sites, as estimated from the resonance heights distribution. We propose that interstitial O-2 molecules not only exert a negative pressure effect on the C-60 lattice, but also enhance order-parameter fluctuations below T-c. In the case of nitrogen, in addition to a mobile (local) minimum in epsilon'(T), another that is fixed at 256 K, was also observed. The second immobile minimum reveals that parts of these pellets were free of N-2, due, no doubt, to its lower diffusivity. [S0163-1829(98)02826-4].