First-principles calculations were made of NaN (N = 55, 147 or 309) clusters that showed icosahedral growth to be most favorable; in agreement with photo-electron spectroscopic data. The icosahedral structures were found to be significantly compressed in the central region. However, a vacancy at the center or the first shell was found to cost much higher energy compared to other sites in the clusters. The vacancy formation energy was the lowest at the vertex sites followed by the edge sites on the cluster surface. These results showed that disordering was easier to happen at the surface region of the clusters. Recent experiments on Na clusters also suggested surface pre-melting. A higher value of the vacancy formation energy was obtained for N = 55, as compared to the cases of N = 147 or 309. This was in agreement with the experimental finding of a higher melting temperature for Na55. The perturbation due to a central vacancy was found to be confined mainly to 2 atomic shells; similarly to the phenomena observed at the surfaces of bulk metals.

Ab initio Calculations of the Stability of a Vacancy in Na Clusters and Correlation with Melting. M.Itoh, V.Kumar, Y.Kawazoe: Physical Review B, 2006, 73[3], 035425 (6pp)