Defect structure model for nickel oxide

Abstract

Nickel oxide is regarded as a model compound for semiconductors of low current carrier mobility. Owing to this fact and due to the practical applications of this oxide, its t ransport properties have been the subject of numerous studies. The knowledge of defect structure in nickel oxide is also necessary for a better understanding of the mechanism of nickel oxidation. In spite of this, the structure of native defects in NiO has not been as yet satisfactorily elucidated. Taking into account the fact that the self-diffusion coefficient of nickel in NiO 1-5 is significantly larger than the oxygen self-diffusion coefficient, 6 it may be assumed that in this compound only the cation sublattice is substantially defected. On the basis of theoretical consideration, Kr6ger v concludes that nickel oxide should exhibit only an excess of oxygen. Furthermore, because NiO is a p-type semiconductor, it is assumed that in this oxide the nickel vacancies and holes are the predominant defects. However, the degree of ionization of the vacancies in nickel oxide has been a matter of discussion. On the basis of studies on the dependence of electrical conductivity of single crystals of NiO on oxygen pressure, Mitoff 8 assumes that nickel vacancies are doubly ionized at high temperatures. Pizzini and Morlott i 9 interpret their results obtained f rom polycrystalline material in a similar way. On the other hand, the results of experiments of Baumbach and Wagner, 1~ of Duquesnoy and Marion, 11 of Eror, 12 of Uno, 13 and recently of Bransky and Tallan 14 and of Cox and Quinn a5 indicate that the defects in NiO are singly ionized. This hypothesis is based on the confirmed fact that in the equation = const p l /~ (1) ~ 0 ~