Al6H18: a baby crystal of γ-AlH3

Using global-minima search methods based on the density functional theory calculations of (AlH(3))(n) (n = 1-8) clusters, we show that the growth pattern of alanes for n ≥ 4 is dominated by structures containing hexa-coordinated Al atoms. This is in contrast to the earlier studies where either linear or ring structures of AlH(3) were predicted to be the preferred structures in which the Al atoms can have a maximum of five-fold coordination. Our calculations also reveal that the Al(6)H(18) cluster, with its hexa-coordination of the Al atoms, resembles the unit-cell of γ-AlH(3), thus Al(6)H(18) is designated as the "baby crystal." The fragmentation energies of the (AlH(3))(n) (n = 2-8) along with the dimerization energies for even n clusters indicate an enhanced stability of the Al(6)H(18) cluster. Both covalent (hybridization) and ionic (charge) contribution to the bonding are the driving factors in stabilizing the isomers containing hexa-coordinated Al atoms.