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

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 C-60 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 C-60 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.:Fm (3) over bar m phase to an ordered s.c.:Pa (3) over bar single domain phase via an intermediate multidomain s.c.:Pa (3) over bar phase. Alternatively, based on the entropy associated with the librons of HF and PF configurations, one could have the sequence f.c.c. -->Pa (3) over bar(HF)-->Pa (3) over bar(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. (C) 1998 Elsevier Science Ltd. All rights reserved.