SULFUR AT NICKEL ALUMINA INTERFACES - MOLECULAR-ORBITAL THEORY

An atom superposition and electron delocalization molecular orbital study has been made of surface atomic layer relaxations in Ni(110) and Ni(111), the binding of p(2 × 2)S to Ni(111), S on Ni(100), and the binding of A12O3 to clean Ni(111) and p(2 × 2)S covered Ni(111). Surface Ni atom relaxations for three-layer thick cluster models are calculated to be close to the experimental results and S heights of 1.68 Å on Ni(111) and 1.40 Å on Ni(100) are also in close agreement with experimental determinations, which indicates cluster models are suitable for determining surface properties. Al3+ in the basal plane of Al2O3 are predicted to support a low-lying surface state band in the O2pA13s3p band gap. These orbitals are found to participate in strong AlNi bonds and AlS bonds of intermediate strength at the interfaces with Ni(111) and p(2 × 2) S-covered Ni(111). Should such bonds form, they are expected to be too few in number to lead to strong adhesion. The strongest Al2O3Ni bonding is predicted to occur when the Ni surface is oxidized. In this case, though NiO bonds are relatively weak, their number is high. It is concluded from the structure models that are studied that the segregation of impurity S in Ni to the interface will markedly decrease the adhesion of protective Al2O3 films that grow on NiAl based alloys. © 1990.