An auger study of the adsorption of oxygen on TiNi, TiFe and Ti₄Fe₂O

The room temperature adsorption of oxygen on clean TiNi, TiFe and Ti₄Fe₂O was studied using Auger spectroscopy. On initial exposure the Ti(M₂₃VV) signal rapidly decreases and is replaced by a peak at 23 eV which arises from the bonding of incorporated oxygen to titanium. At ~ 2 L an autoionization emission feature characteristic of TiO₂ emerges, and the 23 eV feature begins to decrease. We attribute this behaviour to the initial incorporation of oxygen to a critical concentration followed by the nucleation of a stoichiometric titanium oxide. In all three compounds only the titanium component is oxidized, the Auger features of the partner elements are simply attenuated. At ~ 70 L exposure the attenuation of M₂₃VV and LVV peaks of Ni in TiNi indicates that effectively three monolayers of TiO₂ are formed, and that a nickel rich region precipitates beneath the oxide. Ti(LMV) lineshape changes at high exposure are related mainly to the removal of defects in the oxide layer. Heating an oxide covered TiNi sample to 800 K leads to the complete dissolution of the oxide. At intermediate annealing temperatures, ~ 650 K, the data are consistent with the formation of a surface ternary phase, such as Ti₄Ni₂O.