Pd-Pt团簇升温过程中Pd原子偏析的分子动力学模拟研究

On Atomic Segregation Study of Pd in Pd-Pt Clusters with Molecular Dynamics Method

采用分子动力学模拟初始结构为二十面体Pd13Pt134,Pd55Pt92升温过程,研究发现团簇升温过程中内层Pd原子向外偏析,且原子分布位置和数量对升温过程中的热力学性质有影响.Pd55Pt92团簇的势能曲线在720-880K之间向下跃变,而Pd13Pt134团簇的势能曲线异常波动出现在980K附近.对Pd13Pt134的进一步研究表明Pd原子的位置是导致势能曲线异常的主要原因,Pt原子掺在核心层时势能曲线在升温过程中上升,而Pt原子掺在第二层时势能曲线出现异常变化,在900K处出现向下跃变.分析发现Pt原子在不同位置层的势能差别是导致势能曲线异常变化的主要原因.

Segregation occurs in bimetallic clusters during the special condition because of the surface energies between two elements.The Pd atomic segregation has been investigated by means of the molecular dynamics simulation during the melting processes of Pd13Pt134 and Pd55Pt92clusters.The atomic radial distribution shows that the Pd atoms are segregated from inner to outer in clusters.Meanwhile,the curve of atomic energies in the cluster show an irregular phenomenon,i.e.,the atomic energy decreases with the increasing of temperature in Pd55Pt92 cluster between 720 Kand 880K.But atomic energies of Pd13Pt134 cluster increases when temperatures are lower than 980 Kand decreases when temperatures are higher than980 K.The different potential energy plays a key role for the atomic energies irregular phenomena.It is found that the atomic energies are different for Pd embedding in different positions of Pt cluster.The most stable position is surface layer,then the core layer,and finally the subsurface layer,whereas it is contrast for Pt atoms.This leads to that the atomic energy with temperature sharply increases when the Pd atoms segregate from the core layer to the subsurface layer and sharply decreases when the Pd atoms segregate from the subsurface layer to the surface layer.