Analysis of Experimental Results of Defect Concentraions in the Surface Layer of Acceptor Doped Ceria

Recent experimental results of the dependence on oxygen partial pressure, P(O2), of the small polarons concentration in the monomolecular surface layer of doped ceria, are interpreted theoretically. Two different, experimental conditions are examined. First, the highly reduced surface of Sm0.2Ce0.8O1.9 – δ (SDC) is treated and the [Ce3+]s--P(O2) relations are evaluated. The experimental data are explained by a metallic surface layer, approximately neutral, i.e. with negligible transfer of charge to the bulk. The surface is metallic as the outer electrons on Ce3+ ions, which are small polarons when under low concentration, lose their localization under high concentration. Second, the surface of Pr0.1Ce0.9O2 – δ (PCO) is examined. The experimental data of [Pr3+]s--P(O2) relations are explained by a semiconducting, negatively charged, monomolecular surface layer forming a double layer with a positive bulk. In both SDC and PCO the surface defect concentration dependence on P(O2) is different from that of the bulk.