Analysis of point defect concentrations in highly reduced, monomolecular surface layer of doped ceria

Abstract

The concentrations of the point defects, Ce3+ and oxygen vacancy, in the highly reduced, monomolecular surface layer of Sm doped ceria, are analyzed applying a method in which the surface layer and the rest of the bulk are treated as two separate, neutral, phases, then brought into contact and the point defect concentrations modified if charge transfer takes place. The theory yields a very weak dependence of the surface negative charge concentration, [Ce3+], on the oxygen partial pressure, P(O2), being at most logarithmic [Ce3+] ∝ − ln (P(O2)), becoming even weaker for very low P(O2). This is in excellent agreement with experimental results reported by Chueh et al. (2012) for reduced Sm0.2Ce0.8O1.9-x (SDC).The analysis treats the surface with a high density of small polarons as a two-dimensional, neutral, metallic layer in which the small polarons lose their localization. The bulk of the oxide is a semiconductor. The surface defect band generated by the cations Ce4+ and Ce3+ has a density of states gs = 1.4x1015eV−1cm−2 and the effective mass of the electrons is meff = 3.3me.The reduced surface exhibits the f.c.c., α phase, known for bulk ceria and bulk doped ceria. There is no significant difference between the α phase composition range in the surface layer and in the bulk of SDC. There is a significant difference in the oxygen partial pressure under which the α phase in the surface and in the bulk is generated. The concentration of the dopant Sm is predicted to be 40% higher in the surface layer than in the bulk in agreement with experiment.