The surface layer, only one molecule thick, in reduced ceria has, in recent year, been the subject of experimental investigation applying XPS. Two compositions were examined (SDC) [1] and (PCO) [2]. In SDC the surface is highly reduced and the concentration of Ce³⁺ ions, [Ce³⁺], reaches about 30 cation%. In PCO, reduction is done at a higher oxygen pressure, P(O­₂), under which Pr⁴⁺ is reduced to Pr³⁺ but Ce⁴⁺ stays intact. The concentration of Pr⁴⁺ ions, [Pr³⁺], reaches at most 10 cation%. Theoretical analysis of the experimental [Ce³⁺] - P(O₂) relations in SDC [3] and those of [Pr³⁺] – P(O₂) [4] require that the surface of SDC becomes metallic while the bulks stays semiconducting. On the other hand, the surface of PCO is semiconducting. The latter difference between SDC and PCO surfaces is due to the difference in the charged defect concentration. The electrons on single Ce³⁺ ions and single Pr³⁺ ions are small polarons, i.e. localized and can propagate to nearby ions by hopping. However, under high defect concentration, as is the case in SDC, the electrons become delocalized and the surface becomes metallic. This is reminiscent of the Mott transition where in the bulk localized electrons become delocalized when their concentration exceeds a critical value.

     The theoretical interpretation of the SDC experimental results shows that the surface is a two-dimensional metallic layer, one molecule thick on top of a poorly conducting semiconductor bulk. The interpretation provides the density of state of the metallic conduction band and the electron effective mass.

     In PCO the theoretical interpretation yields that the defect concentration, [Pr³⁺], in the semiconducting surface is different from that in bulk and a double layer between the surface and bulk exists with the surface being negatively charged. This is in contrast to the case of SDC where there is no (or negligible) charge transfer between the metallic surface and the bulk and the surface is neutral.

     The phase diagram (which reflects the ion and oxygen vacancy arrangement) of the metallic surface in SDC is examined based on the assumption that the ion arrangement in surface is forced to comply with the arrangement of ions in the bulk, as is the case in epitaxial growth. This forces the surface to maintain the a phase of the reduced SDC bulk [3] while a difference is allowed in the concentration of Ce³⁺ and Sm³⁺ between the surface and the bulk.

     In the metallic surface of SDC the concentration [Ce³⁺] and hence that of oxygen vacancies is higher than in the bulk for a given temperature and P(O₂). Similarly, in the semiconducting surface of PCO, the concentration [Pr³⁺] and that of oxygen vacancies is higher than in the bulk.