Thermochemical coloration and annealing of spinel and magnesium oxide

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Abstract
Heating spinel (MgAl2O4) single crystals in aluminum or magnesiummetal vapor at temperatures in excess of 1800 °C results in the introduction of an optical absorption band at 5.3 eV. The same band can be produced by heating in a strongly reducing atmosphere with no metal vapor at 2100 °C. These results strongly support the suggestion of Bunch that the 5.3‐eV band, created by atomic displacement collisions resulting from energetic particle bombardment, is associated with an F‐type center. Isothermal annealing of the 5.3‐eV band in MgAl2O4 and the 5.0‐eV band in MgO introduced by thermochemical coloration suggests that the activation energy for oxide‐ion vacancy motion is, respectively, 1.8 and 3.4 eV.