Molecular Dynamical Simulation Of Melting Behaviors Of Metal Clusters

Clusters have been widely applied in lots of fields due to their peculiar physical and chemical characteristics. As a very interesting characteristic of clusters, melting behavior has become an important research field. Dynamical behaviors of clusters include melting point, dynamical stability of structures, structural change during melting etc. Recent year there have been a lot of studies on the melting behaviors of clusters, especially metal clusters.In this work, applying molecular dynamics simulation methods, melting behaviors of metal clusters are compared in a wide range of cluster sizes and elements. Results are as below:(1) Melting behaviors of closed shell structured Aln(n<10000) clusters are investigated. For each chosen size, three structures, Icosahedron(Ih), truncated Octahedron(Oh) and annealing structure, are considered as the initial structures of the molecular dynamical simulation. In the studied size range, Ih structure is the dynamically most stable structure. Dynamical stability of Oh structure is related to the cluster size: for small clusters(n<1000), dynamical stability of Oh cluster is low(clusters change into Ih structure before melting). For medium sized(1000<n<2000) clusters, dynamical stability of Oh structure has changed. Oh structures can be stable in the whole melting process in the case of big clusters(n>2000); When the cluster size is larger than 50, melting points of clusters change monotonically with the changing cluster size. When the cluster size is larger than 200, melting points of clusters have linear relationship with n-1/3.(2) The melting behaviors of different Mn(M=Mg, Al, Ti, Co, Ni, Cu, Zr, Rh, Pd, Ag, Ir, Pt, Au and Pb; n=13,55,147) clusters are compared by applying molecular dynamical simulations. For some of the clusters, there are fluctuations in their heat capacity curves due to structural competition. And for other clusters, because of the high dynamical stability of their ground state structures, heat capacity curves show clear peaks. For each cluster size, there is no regularity in melting points of clusters as a function of atomic numbers, but the trends are almost the same for three sizes(13,55,147) and bulk materials; For small clusters, dynamical stability of Oh structure is very low. They transit into more stable Ih structures before melting or even at the initial temperature. Dynamical stability of Oh structure enhances as cluster size increases. When the cluster size is big enough, Oh structure can remain in the whole pre-melting process. And between these “big sizes” and “small sizes” there is an interesting size range for each metal. In this size range that called crossover size range, dynamical stability of Oh structures show randomness. Crossover size ranges of different elements are all different. These crossover size ranges can be predicted by Gupta parameters for each metal.