Bombardment-induced compositional change in MgAl2O4, MgO and Al2O3

From a thermodynamic point of view MgAl2O4 (spinel), as well as the simple oxides MgO and Al2O3, should strongly resist bombardment-induced compositional changes. We have explored these three oxides in macroscopic form (i.e. not as thin films) by bombarding atomically clean surfaces held at 400-degrees-C with 8 keV Ar0. The surfaces were examined by X-ray photoelectron spectroscopy (XPS). We found that MgO and Al2O3 were stable to loss of O to within our uncertainties, as indeed they should be, but that spinel underwent a major loss of MgO to yield approximately 0.70 MgO.Al2O3. Furthermore, by varying the angle of photoelectron take-off, it could be shown that the virgin surface of spinel had excess Al while the bombarded surface had excess Mg. This suggests strongly that bombardment-induced segregation (BIS) of MgO occurs with spinel.