A HIGH-TEMPERATURE STUDY OF DEFECT-INDUCED OPTICAL-ABSORPTION IN NICKEL-OXIDE, NI1-DELTA-O

UV-VIS-NIR absorption spectra of single crystalline nickel oxide, Ni(1-delta)O, have been measured at high temperatures (up to 1200-degrees-C) as a function of oxygen partial pressure, p(O2). The observed p(O2)-dependence of the spectra is discussed in the context of the point defect chemistry of the compound. From its dependence on oxygen partial pressure and temperature, and from its spectral shape it is concluded that the p(O2)-dependent contribution to absorption is due to singly charged cation vacancies, V(Ni)'. The microscopic model of the optical process is discussed in the framework of light-induced charge transfer processes involving ions in the neighbourhood of the cation vacancy.