Abstract
The dissolution of excess O 2 or traces of H 2O and CO 2 in non-transition metal oxides such as MgO leads to the formation of positive holes b ., chemically O −, which tend to undergo spin pairing: O − + O − = O 2 2−. The peroxy anions thus formed and mostly associated with cation vacancies are thermally stable up to about 800–900 K. Above this temperature they release unbound positive holes which interact with transition metal impurities, for instance Fe 2+, leading to their oxidation: Fe 2+ + h . = Fe 3+. Upon slow cooling the process reverses. The oxidation states of Cr, Mn and Fe in arc-fused MgO powder and single crystals have been used to monitor, by EPR, these internal redox reactions.
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