Substituting a copper atom modifies the melting of aluminum clusters

Heat capacities have been measured for Al(n-1)Cu(-) clusters (n=49-62) and compared with results for pure Al(n) (+) clusters. Al(n-1)Cu(-) and Al(n) (+) have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n=55 and 56 for pure aluminum clusters does not occur for the Al(n-1)Cu(-) analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Al(n) (+) isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the Al-Al bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution.