An orbital-free molecular dynamics study of melting in K_20, K_55, K_92, K_142, Rb_55 and Cs_55 clusters

摘要

The melting-like transition in potasium clusters K_N, with N=20, 55, 92 and142, is studied by using an orbital-free density-functional constant-energymolecular dynamics simulation method, and compared to previous theoreticalresults on the melting-like transition in sodium clusters of the same sizes.Melting in potasium and sodium clusters proceeds in a similar way: a surfacemelting stage develops upon heating before the homogeneous melting temperatureis reached. Premelting effects are nevertheless more important and more easilyestablished in potasium clusters, and the transition regions spread overtemperature intervals which are wider than in the case of sodium. For all thesizes considered, the percentage melting temperature reduction when passingfrom Na to K clusters is substantially larger than in the bulk. Once those twomaterials have been compared for a number of different cluster sizes, we studythe melting-like transition in Rb_55 and Cs_55 clusters and make a comparisonwith the melting behavior of Na_55 and K_55. As the atomic number increases,the height of the specific heat peaks decreases, their width increases, and themelting temperature decreases as in bulk melting, but in a more pronounced way.