Packing Transitions in Nanosized Li Clusters

Packing transitions in the lowest energy structures of Li clusters as a function of size have been identified via simulated annealing. For $N\le 21$, the large $p$ character of Li leads to unexpected ionic structures. At $N\approx 25$, a packing pattern based on interpenetrating 13-atom icosahedra and similar to that of Na and K appears. This pattern persists until at $N=55\mathrm{}$, where another transition to a structure based on a Mackay icosahedron (MIC) occurs. For clusters of size 55 and 147, the optimized fcc structure representative of the bulk is still slightly higher in energy than the optimal MIC.