Instability and thermal conductivity of pressure-densified and elastically altered orientational glass of Buckminsterfullerene.

We report on the temperature, pressure, and time (T, p, and t)-dependent features of thermal conductivity, κ, of partially ordered, non-equilibrium state of C-OG, the orientational glass of Buckminsterfullerene (at T below the orientational freezing temperature T) made more unstable (i) by partially depressurizing its high-p formed state to elastically expand it and (ii) by further pressurizing that state to elastically contract it. The sub-T effects observed on heating of C-OG differ from those of glasses because phonon propagation depends on the ratio of two well-defined orientational states of C molecules and the density of the solid. A broad peak-like feature appears at T near T in the κ-T plots of C-OG formed at 0.7 GPa, depressurized to 0.2 GPa and heated at 0.2 GPa, which we attribute to partial overlap of the sub-T and T features. A sub-T local minimum appears in the κ-T plots at T well below T of C-OG formed at 0.1 GPa, pressurized to 0.5 GPa and heated at 0.5 GPa and it corresponds to the state of maximum disorder. Although Buckminsterfullerene is regarded as an orientationally disordered crystal, variation of its properties with T and p is qualitatively different from other such crystals. We discuss the findings in terms of the nature of its disorder, sensitivity of its rotational dynamics to temperature, and the absence of the Johari-Goldstein relaxation. All seem to affect the phenomenology of its glass-like transition.