Effect of Impurities on the Defects in Oxides and Their Relationship to Oxidation of Metal^* Available to Purchase

The defect structure of the oxygen-deficient oxides of Nb₂O₅, Ta₂O₅, and ZrO₂ are discussed. It is suggested that the oxygen vacancies are probably the important defects, except possibly at the most oxygen deficient part of the homogeneity ranges where interstitial cations may predominate. At high oxygen pressures, (small deviations from stoichiometry) effects of impurities become important and, in reported studies, probably determine the point defect concentration. Oxidation of Nb, Ta, Zr, resulting in growth of compact scales of Nb₂O₅, Ta₂O₅, and ZrO₂, is discussed on the basis of these considerations. It is shown that effects of impurities may explain apparent deviations from the simple behavior predicted by the Wagner theory. It is emphasized that analysis and control of impurities is essential in interpretation of defect structures of oxides and defect-controlled properties and reactions.