(BAl12)Cs: a cluster-assembled solid

First-principles calculations on the geometry and stability of AlnBm clusters have been carried out to examine the effect of size, composition, and electronic-shell filling on their relative stability. It is shown that although Al and B are both trivalent, a BAl12 cluster is more stable than an Al13 by 3.4 eV. The enhanced stability is shown to arise due to the relaxation of surface strain in the Al cage when the central Al is replaced by a smaller B atom. Replacement of an additional Al by B to produce B2Al11 results in deformation of the icosahedral BAl12 cage and reduces the stability. The possibility of forming crystals using BAl12 and Cs is examined via total-energy calculations. It is shown that a solid with icosahedral or cuboctahedral BAl12 and Cs and having the CsCl structure is metastable and could be synthesized.