Effect of geometric and electronic structures on the finite temperature behavior of Na${}_{58}$, Na${}_{57}$, and Na${}_{55}$ clusters

Abstract

An analysis of the evolutionary trends in the ground state geometries of Na${}_{55}$ to Na${}_{62}$ reveals Na${}_{58}$, an electronic closed--shell system, shows namely an electronically driven spherical shape leading to a disordered but compact structure. This structural change induces a strong {\it connectivity} of short bonds among the surface atoms as well as between core and surface atoms with inhomogeneous strength in the ground state geometry, which affects its finite--temperature behavior. By employing {\it ab initio} density--functional molecular dynamics, we show that this leads to two distinct features in specific heat curve compared to that of Na${}_{55}$: (1) The peak is shifted by about 100 K higher in temperature. (2) The transition region becomes much broader than Na${}_{55}$. The inhomogeneous distribution of bond strengths results in a broad melting transition and the strongly connected network of short bonds leads to the highest melting temperature of 375 K reported among the sodium clusters. Na${}_{57}$, which has one electron less than Na${}_{58}$, also possesses stronger short--bond network compared with Na${}_{55}$, resulting in higher melting temperature (350 K) than observed in Na${}_{55}$. Thus, we conclude that when a cluster has nearly closed shell structure not only geometrically but also electronically, it show a high melting temperature. Our calculations clearly bring out the size--sensitive nature of the specific heat curve in sodium clusters.

7 pages, 11 figures

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Cited by in corpus (1)

• The electronic structures, the equilibrium geometries and finite temperature properties of Na_n (n=39-55)