C_(60)晶体在有序相的分子取向结构

Molecular Orientational Structure of C_ (60) Crystal in Ordered Phase

以C60 晶体中分子两种取向排列形成的双能级系统为基础 ,通过探讨C60 晶体在有序相的热力学性质 ,得到了 38o 和 98o 两种取向排列的分子在晶体中均匀分布的结构稳定性结论 .根据已报道的C60 晶体在有序相两端点温度 85K和 2 6 0K取向分布的实验结果 ,将较高的分子取向 (38o)能级的概率转化成分数值 1/ 6和 3/ 8,得到了有序相两种取向分子在两温度端点的分布规律 .当概率的分数值为 1/ 4和 1/ 3时 ,C60 晶体将具有较大的结构稳定性和较小的介电损耗及内耗值 ,对应的温度分别为 12 2 .6和 194 .3K .因而解释了介电实验结果出现异常的现象是由 38o 取向分子均匀分布的“规则 无规 规则”变化所造成的 .

Conclusion of structural stability requesting uniform distributions of molecular 38degrees and 98degrees orientational states is obtained by investigating the thermodynamic properties of C-60 crystal 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 38degrees orientational. molecules should distribute among the 38degrees orientational molecules equably and could not accumulate in the space to form a 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 C-60 crystal, 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 38degrees orientational molecules is not simply the change from 98degrees orientational molecules to 38degrees orientational molecules, but a change in the whole structure to ensure the uniformity of 38degrees orientational molecules among the majority of 98degrees orientational molecules. In a single C-60-molecule, there exist electron-rich regions and electron-pool regions. Two kinds of regions influence the interaction of two molecules and produce electric dipoles. In sc phase, the magnitude of electric dipole will increase with the rise of B(T), and will express as the increase of dielectric constants, i.e. the more 38degrees orientational 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 38degrees orientational molecules in the whole crystal over the 90 similar to 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 C-60 crystal 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'.