Dimerization of Defect Fullerenes and the Orientational Phase Transition in Oxidized C₆₀ Fullerite

Oxidized fullerite was obtained by heating a fullerite sample intercalated with oxygen, (O₂)_0.44C₆₀, up to 300 °C. Orientational phase transitions in the oxidized fullerite are studied using differential scanning calorimetry (DSC) and have been found to possess a specific enthalpy whose value is lower by 25% than in the initial (O₂)_0.44C₆₀ sample. In order to find possible reasons for hindrance to the buckyball rotations, we performed optimizations of defect buckyball fullerenes C_60−n with different distributions of vacancies along with the dimers C_60−n –C_60−n and C₆₀–C_60−n for n = 1–4 using density functional theory with generalized gradient approximation. We found that the dimerization energy ranges from 1.07 eV (C₅₈–C₅₈) to 6.56 eV (C₅₆–C₅₆) and from 1.81 eV (C₆₀–C₅₈) to 4.29 eV (C₆₀–C₅₆), respectively. The formation of such dimers, which could in addition interact with defect buckyball cages and form larger aggregates, is to be related to the lowering of the orientational transition enthalpy.