Morphology and structural phase transitions of Pd monolayers on Ta(110)

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Abstract
The morphology of Pd overlayers on Ta(110) was studied for various Pd coverages before and after heat treatment. As was reported for Pd on Nb(110), a structural phase transition from a commensurate epitaxial overlayer to an incommensurate overlayer was found at approximately one monolayer (ML) coverage (1 ML=1.89 Å). Valence-band photoemission and Auger-electron energy distribution curves show modifications of the Pd-derived valence-band states attributable to Pd—Ta bonding and reduced Pd nearest-neighbor coordination. Little change is seen in the Pd density of states during the commensurate-incommensurate transition. Thicker Pd layers (theta>1 ML) evolve toward the structure and photoemission properties characteristics of Pd(111). Thermal treatment of the thick Pd overlayer triggered the interdiffusion of Ta and Pd and produced an extended Pd-Ta surface compound. Further heat treatments caused additional Pd and Ta interdiffusion and left a surface-retained Pd ‘‘monolayer’’ with the same structure as the Ta(110) substrate.