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Molecular Orientational Structure of C60 Crystal in Ordered Phase
Cao Wanqiang1, Liu Jundiao2, Li Jingde3
1.Physics and Electronic Technology School, Hubei University, Wuhan 430062;2.Physics Department, Huizhou Institute, Huizhou 516015;3.Physics Department, Zhongshan University, Guangzhou 510275
Abstract:
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”.
Key words:  C60, Two-level system, Molecular orientational structure
FundProject:广东省高教厅自然科学基金和湖北大学人才基金资助的课题.
C60晶体在有序相的分子取向结构
曹万强*1, 刘俊刁2, 李景德3
1.湖北大学物理学与电子技术学院,武汉,430062;2.惠州学院物理系,惠州,516015;3.中山大学物理系,广州,510275
摘要:
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85 K 和260 K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122.6和194.3 K.因而解释了介电
关键词:  C60  双能级系统  分子取向结构
DOI:10.1088/1674-0068/15/5/387-392
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