Clusters of aluminium, a density functional study

Density functional calculations have been performed to study the structure and energetics of mainly neutral Al-n with n up to 153. Clusters up to n = 15 have been investigated systematically, in part by simulated annealing techniques. No pronounced islands of stability are found in this range although Al-7 and Al-13 are of special importance. The treatment of selected larger clusters with different packings and cluster shapes leads to the following conclusions. Icosahedral packing is favoured only for n = 13; around n = 55, decahedral packing is most stable, whereas fee is definitely preferred, energetically, for n > 80. Among clusters of comparable size and packing those with dominantly (111) surfaces are found to be most stable. These trends are rationalized by considering the average number of next neighbours and the internal strain arising from a mismatch of bond distances. Extrapolation of the computed cluster binding energies yields a cohesive energy of bulk Al of 3.3-3.4 eV, in close agreement with experiment (3.36 eV).