ABSTRACT
In recent years important information concerning the chemical composition of the surface layer of mixed oxides was gathered mainly due to the photoelectron (XPS and UPS) and Auger electron (AES) spectroscopic investigations. It appears that frequently the surface composition of such oxides is different from that of the bulk. This may be the effect of the tendency of surface energy to reach a minimum, and in such a case enrichment of the surface in one of the components would correspond to an equilibrium state. However, the accumulation of one component at the surface may also be a metastable state reached as a result of some kinetic factors.
