Radiation damage in MgAl2O4

Exposure of single crystals of Mg${\mathrm{Al}}_2$ ${\mathrm{O}}_4$ to fast neutrons and to Van de Graaff electrons with energies in excess of 0.35 MeV introduces an optical-absorption band at 5.3 eV with a 1-eV half-width. This band can be partially bleached at temperatures as low as 40 K and a shoulder at 4.75 eV develops concurrently. This bleaching treatment also partially destroys a previously reported $V$-type absorption centered at 3.2 eV. Subsequent exposure to ionizing radiation destroys the 4.75-eV band and restores both the 5.3- and 3.2-eV bands to their original intensities. Since this behavior is analogous to the interconversion of $F$ to $F^{+}$ centers in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$, it is concluded that the 5.3-eV band is the principal optical transition of the $F$ center (two electrons trapped at an oxide-ion vacancy) and the 4.75-eV band is attributed to absorption by the $F^{+}$ center (one electron trapped at an oxide-ion vacancy). In electron-irradiated crystals the 5.3-eV absorption begins to anneal near 110°C and is about 90% destroyed upon isochronal annealing (10-min pulses) up to 355°C. Neutron-irradiated crystals behave similarly. Measurement of the threshold energy for damage by electrons at 77 K yields a displacement energy for the creation of ${\mathrm{O}}^{2-}$ interstitial-vacancy pairs of 59 eV. The defect yield drops off substantially with increasing temperature, and at room temperature the apparent ${\mathrm{O}}^{2-}$ displacement energy is 130 eV. Possible reasons for this strong temperature effect are discussed.