Electron binding energies and oxidation behaviour in UNiAl, YNiAl and ZrNiAl

The electron binding energies of the metal components on the surface layers of ANiAl (A ≡ U, Y, Zr) were examined by X-ray photoelectron spectroscopy (XPS). The samples were initially exposed to air and then a depth profile was performed utilizing argon ions of energy 1 kV. The constituents were in oxidized states before the sputtering procedure and their bulk electron binding energies were significantly different from those of the pure metals. In the usual experimental XPS set-up the binding energies are related to the Fermi level of the sample. To obtain the total electron binding energy (i.e. related to the vacuum level of the sample), a knowledge of the relevant work functions is required. The work function of UNiAl was estimated by operating the Auger electron spectroscopy (AES) pulse counting mode at low electron kinetic energies. The electron binding energies obtained in this way showed an improved correlation with the electronegativity values of the metal components. The metal electron binding energies demonstrate a complete surface oxidation only on UNiAl after exposures of up to 80 langmuirs of O₂. The corresponding oxygen to metals (U + Ni + Al) atomic ratio is significantly lower than the ratio expected from the relevant binary oxides. This behaviour may be accounted for by the formation of ternary oxides or if oxygen is absent from the first atomic layer of the oxidized UNiAl. The latter possibility is considered in view of the absence of oxygen from the first atomic layer of oxidized UFe₂ surface as reported by Erbudak and Stucki.