Density functional theory and global optimization study of Sn(m)Pb(n) clusters (7 ≤ m + n ≤ 12, 0 ≤ m/(m + n) ≤ 1)

The global minima of the neutral binary Sn(m)Pb(n) atomic clusters, 7 ≤ m + n ≤ 12, of all the possible stoichiometric ratios have been found using tabu search in descriptor space and density functional theory. The effects of spin-orbit coupling on optimized geometries and energies are important. All of the binary clusters form substitution alloys. Apart from the 11-atom case, the pure clusters of the same size have the same ground state geometry. The relative energies of the isomers of a cluster depend on, in order of decreasing importance: the overall geometry; the specific sites occupied by the two atom types; and the degree of segregation. The total cohesive energy difference between the lowest energy homotops is typically on the order of 0.02 eV. The mixing/segregation trends are found to be very different depending on the size of the basis set. Calculations generally overestimate the dipole moments. The trends in calculated dipole moments agree with experiment for the lead-rich clusters, and to a lesser degree, the tin-rich clusters.