Coordination and dispersion of Co2+ ions in CoO—MgO solid solutions

Abstract

CoO–MgO solid solutions of CoO concentrations 0.25–15.3 mol % have been investigated by e.s.r. spectroscopy in the temperature range 4–77 K. The distribution of isolated Co2+ ions between octahedral and tetrahedral sites has been estimated quantitatively. In the most dilute solid solutions (up to 3.00 mol % CoO) Co2+ ions occupy predominantly trigonally distorted tetrahedral sites. The exchange interactions extend up to the 3rd coordination sphere in CoO–MgO. The dependence of the number of paramagnetic O–2 radicals formed in the course of oxygen adsorption on CoO–MgO at room temperature is correlated with the analogous dependence of the number of isolated Co2+ ions of tetrahedral symmetry with trigonal distortion. Two kinds of adsorption centres are postulated in CoO–MgO solid solutions: tetrahedrally coordinated Co2+ ions with trigonal distortion isolated or coupled to the paramagnetic neighbours.