Ionic Conductivity of Doped Cerium Dioxide

Abstract

The oxides most commonly considered to be good ionic conductors via oxygen-ion migration are ZrO₂ and ThO₂ doped with lower-valent cations, notably ZrO₂:CaO (called calcia stabilized zirconia, or CSZ) and ThO₂:Y₂O₃ (called yttria doped thoria, or YDT). In these materials, which have the fluorite structure, vacancies are introduced on the oxygen sublattice to compensate for the lower-valent cations, one vacancy for each Ca²⁺ or for each two Y³⁺ ions. Another oxide of the fluorite structure that has been well studied is CeO₂, but it differs from the other two in that it undergoes large departures frem stoichioraetry in the direction of oxygen deficiency (CeO_2-x) when heated in a reducing atmosphere. The oxygen vacancies thus formed are accompanied by electrons, and the material then displays n-type semiconducting properties [1]. This behavior of CeO₂ is undoubtedly related to the existence of a stable Ce₂O₃ phase, i.e. to the fact that cerium readily goes into the 3+ state. Because of this tendency to become non-stoichicmetric, doped CeO₂ is not often considered as a possible solid oxide-ion electrolyte when applications to oxygen concentration cells [2] or fuel cells [3] are under consideration.