Study of Pure and Doped Cobaltous and Nickelous Oxide

We give results for a number of "critical" tests intended to eliminate some of the structural models which have been proposed for nickelous and cobaltous oxide prepared at temperatures of order 300 to 400°C. For the case of cobaltous oxide we are able to eliminate all of the proposed models, leaving us with no completely satisfactory explanation of the striking properties of this material. The importance of the semiconducting properties of these oxides is demonstrated by measurements of the ratio of ${\mathrm{Fe}}^{2+}$/${\mathrm{Fe}}^{3+}$ observed in the Mössbauer patterns as a function of the $p$-type dopant lithium and the $n$-type dopants chromium and gallium. When the Li doping is raised sufficiently, it is possible to completely suppress the ${\mathrm{Fe}}^{2+}$ resonance at room temperature.