Molecular Orientational Structure of C60 Crystal in Ordered Phase

Conclusion of structural stability requesting uniform distributions of molecular 38oand 98oorientational states is obtained by investigating the thermodynamic properties of C60crystal in ordered phase, based on a two-level energetic system formed by molecular rotations. At any temperature, the structural equilibrium is restricted by the minimum of free energy, which requests the 38oorientational molecules should distribute among the 38oorientational molecules equably and could not accumulate in the space to forma single sub-system in thermo-equilibrium. According to the reported experimental results of orientational occupancies at two edges of ordered phase 85 and 260 K of C60crystal, which can be accurately expressed as 1/6 and 3/8, the cubic molecular orientational distributions at two temperatures are acquired. When temperature rises, the increase of 38oorientational molecules is not simply the change from 98oorientational molecules to 38oorientational molecules, but a change in the whole structure to ensure the uniformity of 38oorientational molecules among the majority of 98oorientational molecules. In a single C60-molecule, there exist electron-rich regions and electron-pool regions. Two kinds of regions influence the interaction of twomolecules and produce electric dipoles. In sc phase, the magnitude of electric dipole will increase with the rise ofB(T), and will express as the increase of dielectric constants, i.e. the more 38oorientational molecules, the larger the dielectric constant. The uniform distribution of local electric charges varies with the orientational change among molecules. So, there are several regular systematic uniform distributions for 38oorientational molecules in the whole crystal over the 90~260 K temperature range. Orientational occupancies of 1/4 and 1/3 are the special points like two transition points, orientational occupancies of 1/6 and 3/8. Orientational occupancies of 1/4 and 1/3 in C60crystal have more structural stability and less dielectric dissipation at correspondent temperatures of 122.6 and 194.3 K, which can explain the anomalous behavior in dielectric dissipation and can be described as the uniform distribution of ”symmetry-asymmetry-symmetry”.