Studies of surface composition, structure and oxygen adsorption of Ni3Al(110) by Li+ and He+ ion scattering

Abstract

The surface composition, structure and initial stages of oxygen adsorption of Ni₃Al(110) have been investigated by ISS. The surface composition of the Ni₃Al(110) surface was determined by Li⁺ and He⁺ ion scattering using selected scattering geometries. These results consistently show a surface composition of 50% Ni‐50% Al in the outermost layer with some small percentage of Ni enhancement (perhaps 1–2%), while the second layer is basically Ni. The surface structure has been probed by measuring the intensity of Li⁺ ion scattering as a function of incident angle along the [001], [112], [111], [332] and [110] azimuthal directions. The interpretation of experimental data is based on the calibrated shadow cone and chain model simulations. The results show that there is no displacement of first‐layer Al relative to first‐layer Ni, but there is a contraction in the first‐to‐second layer spacing of 0.11 ± 0.08 Å. The adsorption of oxygen at low exposure (∼8L) and pressures in the range 2 × 10⁻⁸−1 × 10⁻⁶ mbar at a temperature of 700°C shows the disappearance of the Ni ISS signal, and the results suggest the formation of a monolayer of an AlO_x overlayer. Using a model similar to that previously studied for the formation of AlO_x islands on the Ni₃Al(001) surface, this oxide overlayer has been shown to be AlO_x islands with no influence on remaining areas on the crystal. The mechanism for apparent interchange of the Al atoms to the surface to replace the Ni atoms is also discussed.