Color-center formation and thermal recovery in X-ray and electron-irradiated magnesium aluminate spinel

We have studied the formation of color centers in magnesium aluminate spinel (MgAl ₂ O ₄ ) by X-ray and electron irradiations near room temperature (RT). For this purpose, Cu Kα radiation and three electron energies (1.0, 1.4, and 2.5 MeV) were used for variable fluences (up to 4.2 × 10 ¹⁸ cm ^-2 ). Off-line UV-visible absorption spectra were recorded at RT as well as at low temperature down to 27 K after electron irradiation. The dependence of the production rate of F centers (oxygen vacancies) on the electron energy yields a threshold displacement energy of 190 ± 10 eV for oxygen atoms at RT, which is much higher than the determinations by molecular-dynamics simulations. Such a discrepancy is discussed on the basis of available migration data of point defects in spinel. Equipartition of F ⁰ and F ⁺ centers, i.e., the neutral (V _O ^x ) and singly ionized (V _O ^. ) oxygen vacancies, is reached for high electron fluences. Moreover, the evolution of the width of color-center absorption bands versus temperature is interpreted with the classical theory for F centers (neutral halogen vacancies) in alkali halides. The Stokes shifts are deduced from the temperature dependence of the absorption bandwidths of color centers like for alkali halides and alkaline-earth oxides. Finally, isothermal annealing data for long annealing time show a non-zero asymptotic behavior for both F ⁰ and F ⁺ centers. This uncommon behavior is interpreted by charge exchange processes leading to an equilibrium state between those two color centers.