The effect of aliovalent dopants upon the defect chemistry of metal oxides was reviewed. Attention was focussed on the stoichiometric solid-solutions which were obtained by combining the stoichiometric dopant oxide with the stoichiometric host oxide, without any gain or loss of O. The charge on the dopant centres was then compensated entirely by ionic defects. As the O activity was increased above that which was in equilibrium with the stoichiometric composition, the lattice defects were gradually replaced by holes. On the other hand, when the O activity was reduced below that in equilibrium with the stoichiometric composition, the lattice defects were gradually replaced by electrons. The near-stoichiometric region of a pure metal oxide was replaced by 2 regions: with charge compensation via reduction (O vacancies or electrons) in the region of lower O activity, and oxidation (cation vacancies or holes) in the region of higher O activity.

The Effects of Dopants on the Properties of Metal Oxides. D.M.Smyth: Solid State Ionics, 2000, 129[1-4], 5-12