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.