Molecular-Dynamics Study On The Melting Behaviors Of Co, Ni And Pd Clusters

The cluster has become an important field of nanomaterial science. It is also an active field for condensed matter physics. However its'many characters are still unclear. As a hot topic, transitional metal nanoclusters have now shown some fancy properties and attracted extensive concern. Study of these properties is important to understand the process of structural evolution and to make nanomaterial, and some new material can be developed. In this paper, we present an extensive molecular-dynamics simulation on the Co, Ni and Pd clusters with the Gupta potantial for the interaction in the atoms. The method has been proven to be good to study the thermodynamics of transitional metal cluster. Our main simulation are concentrated on the following two expects: (1) the melting behaviors of Con (n=13, 14, 38, 55, 56) clusters. (2) Comparison of melting behaviors of the Nin, Pdn (n=13, 55, 147) clusters and studying of their melting transition properties.Our main results are:(1) The heat capacity of the Co₁₃ or Co₁₄ cluster does not match the Lindemann index, and presents a clear lag phenomenon. The melting point of the Co₁₃ or Co₁₄ can be captured by the peak of the heat capacity curve but not the Lindemann index. Isomerization transitions can be found evidently of Co₃₈ cluster during its melting, and its heat capacity curve has not clearδpeak. Different premelting intervals are shown in all clusters during their melting, and the Co₁₄ and Co56 clusters show two premelting stages: surface melting and overall premelting.(2) As the cluster size increases, the melting points of Ni and Pd clusters with same size vary nonmonotonously and have the same varying trend. Compared to the fact that the melting of Ni crystal is lower than that of Pd crystal, the two clusters display the abnormal melting transition phenomena, and the melting points of the Ni clusters are higher than that of Pd clusters with the same size. The difference of the melting points of Ni clusters and Pd clusters decreases gradually with increasing the cluster size. We attribute the main reason of such abnormal melting transition to that the energy gap between ground state and the first excited state of the Ni clusters is larger than that of Pd clusters.