Density functional studies of aluminum phosphide cluster structures

Abstract

We have carried out a systematic search for the lowest energy structures of (AlP)N (N⩽6) clusters using density functional theory within the local density approximation. We employ a novel search strategy based on genetic algorithm to find the global minima of these clusters. This study reveals that (AlP)N alloy cluster geometries are significantly different from the Si2N cluster structures, even though AlP and Si2 are isoelectronic and the corresponding bulk materials have similar lattice and band structures. We also observed that phosphorous–phosphorous bonding is favored in small clusters (N⩽3), while in larger clusters phosphorous atoms cap the triangular faces consisting of aluminum atoms. The ground state structures are determined primarily by the need to minimize the electrostatic repulsion between diffuse lone pair electrons on phosphorous atoms. Finally, we observed that large clusters undergo buckling type relaxation dynamics similar to the way the ideal (110) plane of bulk AlP surface reconstructs. The cluster structures are also similar to the reconstructed (110) surface of bulk AlP. These findings lead to the conclusion that AlP clusters with N⩾4 are nanoscale fragments of the bulk surface.