Glass and phase transitions in solid C60 charged with Ar, Ne, He, and O2

Abstract

C60 pellets, charged respectively with He, Ne, Ar, and O2 under high pressure, were studied with complex impedance spectroscopy. The latter two were found to depress the temperatures for both the phase change (Tc) and glass transition (Tg), although in time shifts, they stayed almost constant in the Ar, but decreased in the O2, case. The effective barriers for the glass transition were determined as 218±15 meV in Ar0.49C60 and 182±20 meV in (O2)0.5C60. In contrast, neither He nor Ne had a discernible influence on these material parameters. These observations could be explained as follows. (1) Interstitial Ar causes a reduction in the energy barrier and a weakening in the restorative force of the cage libration mode, thereby depressing Tc. (2) For (O2)0.5C60, coupling between the cage libration and the tumbling of an O2 diatomic molecule further weakens the restorative force, so that a larger temperature shift results. (3) Its tumbling motion makes O2 more susceptible to the “paddle wheel” action of rotating C60 and more easy to diffuse even under ambient conditions.