Electron Spin Resonance Studies of Impurity Ions in Magnesium Oxide

The univalent ions Fe 1+ , Co 1+ and Ni 1+ have been produced by ultra-violet or X-irradiation of impure MgO crystals. The electron spin resonance spectra of these ions are compared with those of the isoelectronic ions Co 2+ , N1 2+ and Cu 2+ which they resemble closely. The spectra of Fe 1+ , Co 1+ and Ni 2+ show line-width effects which may be interpreted as being due to the presence of small distortions in the cubic crystal lattice. A detailed report of the 3d 9 configuration in a cubic field is given. There is a transition at low temperature from an isotropic to an anisotropic spectrum, presumably due to the `freezing in' of Jahn-Teller distortions. Observation of hyperfine structure from 61 Ni has made it possible to estimate the nuclear moment by comparison with the observed hyperfine structure from Co 1+ . The ease of formation and stability of these univalent ions is shown to be related to the concentration of positive ion vacancies and to the concentration of trapped hole centres.