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

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.