Structural transformation in fullerene solids C60 and C70 at low temperatures—new insights

Abstract

This paper reviews several recent investigations by the authors’ group concerning the structural transitions at low temperatures in the fullerene solids C60 and C70. In the case of C60 it has been shown that the heat capacity variation with temperature in the ordered phase reflects the contribution due to configurational reorientation of the molecules in the solid between two energetically favourable orientations. Evidence is presented for the possible occurrence of an intermediate phase in solid C60 in a narrow temperature range just below the ordering transition. In the case of C70 the nature of transition as well as resulting low-temperature structure is kinetic dependent. The equilibrium structure is obtained by the simultaneous freezing of all orientations of the molecules which brings to the fore the importance of polar cap interaction between the molecules of the basal plane and those in the midplane. It has been shown by a potential energy calculation that the structure obtained by slow cooling in experiment corresponds to the minimum energy configuration.