Icosahedral growth, magnetic behavior, and adsorbate-induced metal-nonmetal transition in palladium clusters

Studies of the atomic and electronic structures of Pd clusters with 2-23, 55, and 147 atoms have been carried out using the ultrasoft pseudopotential plane wave method and the spin-polarized generalized gradient approximation for the exchange-correlation energy. We find predominantly an icosahedral growth in these clusters, and size dependent oscillatory magnetic moments that are not confined just to the surface atoms. Atomically closed shell 13-, 55-, and 147-atom clusters have large moments of 0.61μB/atom, 0.47μB/atom, and 0.41μB/atom, respectively. But cubic Pd₅₅ isomer has a low moment of 0.18μB/atom only, indicating the importance of the icosahedral structure in the development of magnetism in Pd clusters. The evolutions of the sp-d hybridization, the magnetic moment and the electronic structure are discussed as a function of the cluster size. The magnetic energy is found to be small. Further studies on the adsorption of H and O show that though both an increase as well as a decrease of the moment are possible, often there is a reduction in the magnetic moments of Pd clusters. A hydrogen induced metal to nonmetal transition is predicted in Pd₆H₈ and Pd₁₃H₈ clusters.