Effect of interstitial Ar, Ne, He, and O2 on the glass and phase transitions in solid C60

Abstract

C 60 pellets charged with He, Ne, Ar, or O2 under high pressure, have been studied with complex impedance and high-resolution C13 nuclear magnetic resonance spectroscopes. Ar and O2 were found to depress both the face-centered-cubic–simple cubic and the glass transitions. Storage at room temperature hardly affected these temperatures shifts in the first case, but the shifts decreased exponentially in the O2 case. The effective barriers for the glass transition are determined as 218±15 and 182±20 meV in Ar0.49C60 and (O2)0.5C60, respectively. Neither He nor Ne had a discernible influence on the temperatures and the barrier energy. These observations may be explained as follows. (1) The intercalation of Ar causes the C60 lattice to expand, and the resulting “free volume” increment suppresses Tg. (2) The reduction in effective barriers (by ∼70 meV) weakens the restorative force of the cage libration mode, which decreases Tc. (3) For (O2)0.5C60, an anharmonic coupling between the cage libration and the tumbling of O2 diatomic molecules further modified the potential surface and reduced the effective barriers, therefore inducing a larger temperature shift. (4) Its anharmonic tumbling motion makes O2 more susceptible to the “paddle wheel” action of rotating C60 so that it diffuses readily at room temperature.