2.16 Extent of Non-Stoichiometry

• In highly stoichiomteric pure oxides such as MgO, Al₂O₃, ZrO₂, the extent of  oxidation  or reduction is very small. These are often characterized by large energy for oxidation or reduction. Changes in oxygen pressure have very little effect on the defect concentration. When cations are of fixed valence, the tendency for retaining the stoichiometry is even larger.

• Oxides containing multivalent cations, such as transition elements, are much more prone to be non-stoichiometric. Examples are TiO_2+x, BaTiO_3-x and SrTiO_3-x where Ti⁴⁺ ions can be easily reduced to Ti³⁺ creating oxygen deficiency of order 1% within the limits of the stability of oxide i.e. before decomposition and phase change.

• Transition metal mono-oxide series Ni_1-xO, Co_1-xO, Mn_1-xO and Fe_1-xO are the oxides in which a fraction of the divalent cations is easily oxidized to the divalent state resulting in cation deficiency, x . The deficiency is ~5x10^-4 % for Ni_1-xO, ~1% for Co_1-xO, ~0.1% for Mn_1-xO and ~0.15% for Fe_1-xO. FeO is seldom stoichiometric and it has a minimum non-stoichiometry of 0.05%.