Adsorbed layer and multilayer materials: The energetics and bonding of Pd and Ag on Nb(001) and Nb(110)

The energetics of transition-metal–transition-metal adlayer adhesion and the heats of formation for competing ordered compounds have been investigated using total-energy calculations in order to disentangle the factors controlling adlayer behavior. The calculated energies are used to estimate interface energies, distortion energies associated with requiring an adlayer to be commensurate with its substrate, the surface energies of the bare metal surfaces of Nb(110), Nb(001), Ag(110), Ag(001), Pd(110), and Pd(001), and the surface energies associated with the presence of an adlayer. The results provide a picture consistent with the experimentally known adlayer behavior for Pd-Nb(001), Pd-Nb(110), and Ag-Nb(110) and have implications for the growth of artificial multilayer (superlattice) materials. While Pd adsorption on Nb can already be understood in terms of bulk interface energetics, for Ag on Nb the surface contributions are essential. Densities of states obtained for bulk multilayers display markedly different bonding at the Pd-Nb and Ag-Nb interfaces, although all show a pronounced interface state ∼1.5 eV above the Fermi level. The intensity of this peak, which also exists at the surface, reflects the strength of bonding at the interface.