RECENT ADVANCES IN X-RAY PHOTOELECTRON-SPECTROSCOPY STUDIES OF OXIDES

X-ray photelectron spectroscopy (XPS) has often played the major role in the chemical characterization of select surface species, but the extension of that role to larger classes of compounds has generally been limited by such problems as cleanliness, charging, and relaxation shifts. In this article we suggest that adequate command of these difficulties has now permitted the collective chemical description of quite diverse surfaces. In this regard the progressive bonding differences of representative, simple group A oxides (e.g., BaO and SiO2) are analyzed employing various features in the XPS core level and valence band results. Regular progressions in covalency/ionicity are demonstrated. Extensions of these studies to complex (A(z)M(s)O(t)) oxides (e.g., Na2Al2O4) are also described in which progressive alterations of the XPS spectra suggest that during this complex formation the A-O bond is generally made more ionic, whereas the M-O bond becomes more covalent. These results also indicate that for many of these systems the concept of a uniform, spherical oxide ion is inappropriate, and should often be replaced with a polarized model.