First Principle Investigation of the Structural Evolution of Nan (n=20-55)Clusters and Its Influence on the Melting Characteristics

Density-functional theory has been applied to investigate systematics of sodium clusters nan in the size range ofn= 20-55. a clear evolutionary trend in the growth of their ground-state geometries emerges. the clusters at the beginningof the series (n=20-34) are disordered. for (n=35-43) clusters are symmetric and have partial icosahedral(two-shell) structure. the growth then goes through a series of disordered clusters (n=44-52) where the icosahedralcore is lost. however, for n .53 a three shell icosahedral structure emerges. this change in the nature of the geometryis abrupt. in addition, density-functional molecular dynamics has been used to calculate the specific heat curvesfor the representative sizes n= 43, 45, 48 and 52. these results along with already available thermodynamic calculationsfor n= 20, 25, 40, and 55 enable us to carry out a detailed analysis of the heat capacity curves and their relationshipwith respective geometries for the entire series. our results clearly bring out strong correlation between theevolution of the geometries and the nature of the shape of the heat capacities. the results also firmly establish thesize-sensitive nature of the heat capacities in sodium clusters.