Au_(19)团簇热力学特性的微观分析

The Micro-analysis of the Thermodynamical Properties of the Amorphous Gold Cluster

利用经典微正则系综分子动力学,采用Gupta原子间作用势,对非晶结构的19个金原子团簇的热力学特性进行了研究.通过计算团簇内原子时间平均势能的特性以及对团簇瞬时结构的分析发现,团簇的融化过程较对称团簇具有一些新的特征,团簇的融化过程可以分为多个过程.第1阶段的融化涉及团簇表面多个原子(14个原子),其余(4个)原子则处于准固态;第2阶段涉及表面全部原子,但此时有2个位置原子逗留时间较长;第3阶段表面的原子全部开始了快速的换位;第4阶段产生了包括团簇内部原子在内的结构异构化过程.由此,导致了团簇的卡路里曲线、相对均方根键长涨落和热容量的一些新的变化.

Using classical microcanonical molecular dynamics, the thermodynamical properties of the amorphous cluster of 19 gold atoms are studied by Gupta potential. Through calculating the values of the atomic time average potential in the cluster and the analysis of the instantaneous structures,some new features of the melting processes are found compared to the high-symmetry cluster. The melting processes can be divided into several steps. The first stage involves the majority of the surface atoms （14 atoms）. The residual 4 atoms remain in a quasi-solid. Then all the atoms in the surface of the cluster take part in the melting process, but there are two sites where the atoms stay for a relative time. The third stage corresponds to the frequent isomerizations involv- ing all the surface atoms. Finally,all the atoms,including the inner atom of the cluster, begin an isomerization. Thus,the caloric curve,the root-mean-square bond-length fluctuation and the heat capacity exhibit some unique features.