Possible intermediate phase in the orientational ordering in solid C₆₀ - a quasi adiabatic calorimetric study

Abstract

We report quasi adiabatic calorimetric (QAC) investigations about the well known orientational ordering transition, over a close temperature interval of 259 to 264 K on well characterized single crystals of C₆₀. Our work confirms the only available report, by Fischer et al., of the presence of an additional low temperature peak at a temperature, very close to (less than 0.6 K) the main peak which appears at 262.2 K. From a comparative DSC investigations on high quality crystals and those having defects it is shown that defects are not the cause of the additional peak. Our work establishes that it is intrinsic to solid C₆₀ and is also associate with the orientational ordering. From the consideration of excess entropies, associated with the main and the additional peak, it is conjectured that the ordering proceeds from the disordered f.c.c.:Fm3̅m phase to an ordered s.c.:Pa3̅ single domain phase via an intermediate multidomain s.c.:Pa3̅ phase. Alternatively, based on the entropy associated with the librons of HF and PF configurations, one could have the sequence f.c.c. →Pa3̅(HF) →Pa3̅(PF), where HF(PF) correspond to the electron-rich double bond on one molecule facing the electron deficient centre of the hexagon (pentagon) of the neighbouring molecule.