The influence of OH− impurity on the oxidation of iron impurity in MgO above 1000 °C

In a study of the charge compensation mechanism accompanying the oxidation of iron impurity of MgO, it is found that OH− impurity is removed by the oxidation anneal at temperatures above 1000 °C. Since dissolved OH− is, like Fe3+, compensated by cation vacancies, the oxidation of Fe2+ to Fe3+ can occur without a change of cation vacancy content as long as OH− is being removed to maintain charge balance. This result explains the hitherto puzzling observation that oxidation of iron impurity in MgO at elevated temperatures is not accompanied by an increase in the V-band (2.3 eV) intensity. A mass spectrum analysis on the effluent moisture from a D2O-droped MgO crystal during oxidation demonstrates conclusively that the hydrogen is actually removed from the crystal during prolonged annealing in air. Once OH− is eliminated, annealing in hydrogen causes a decrease in both Fe3+ as revealed by its 4.3 eV charge-transfer band and the V-band intensity at 2.3 eV. Subsequent oxidation causes an increase in both Fe3+ and cation vacancies as expected to occur for cation vacancy compensation of the express positive charge of Fe3+.