2.5.1 Oxygen Deficient Oxides

• Formation of oxygen vacancies or metal interstitials or both are possible.

• Formation occurs only at the surface.

       2.5.1.1 If oxygen vacancies are the dominating defects

• Depicted by MO_2-x (x is the extent of non-stoichiometry) and overall reaction as

  MO2⇔MO2-x+ x/2 O2↑

• Due to loss of oxygen, possible defect reactions would be

  • Electronic compensation leading to creation oxygen vacancies and of electrons

    O0 VO••+ ½ O2+ 2e'

  • Ionic compensation leads to formation of oxygen vacancies and reduction of metal ions on their sites.

  O0 VO••+ ½ O2+ 2M'M

• Depicted as  (M_1+yO₂ is the extent of non-stoichiometry)

• Possible defect reactions are

  • Ionic compensation leading to the formation of metal interstitials and reduction of metal ions on their sites

    MM Mi•••• + 4 M'M

    OR

  • Electronic compensation leading to the formation of metal interstitials and free electrons

  M Mi••••+ 4e'

• Creation of quasi-free electrons (extra charge is represented as M’)

• Conduction occurs due to transport of electrons

• Typically n-type conductors.

• Example: TiO₂, ZrO₂, CeO₂, Nb₂O₅