It was recalled that this mixed oxide was of interest because of its marked resistance to displacement damage. This abnormal behavior was attributed to a high concentration of intrinsic defects which was due to the up to 30% of cation antisite disorder which occurred in synthetic crystals. This introduced a very high concentration of electron traps (Al3+ in tetrahedral symmetry sites) and hole traps (Mg2+ in octahedral symmetry sites). The crystals also exhibited some deviation from stoichiometry. Thus, cation vacancies were formed in order to compensate the extra charge of Al3+ ions. From optical absorption data, it was clear that neutron irradiation introduced new O vacancy defects (mainly F centers). The height of the band corresponded to a concentration of 1017/cm3. Annealing of these defects occurred between 200 and 500C, and, because there was no significant difference between annealing with and without X-irradiation, it was concluded that it was due to the recombination of interstitials and vacancies. It was noted that the annealing out of F centers occurred at temperatures which were higher than those previously observed. It also did not begin until optical bands in the visible region disappeared. The origin of optical absorption bands in the visible region was less clear. They had previously been observed at both low and high doses, and were suggested to be related to small aggregates of vacancy defects or impurities that were perturbed by O vacancies. This could not be confirmed on the basis of the available data, but the lower thermal stability of these bands when compared with the thermal stability of F-centers, suggested that the first alternative was more probable. On the other hand, the results supported a model which had been proposed for the 2 electron paramagnetic resonance bands. The present results could be explained if it were assumed that the F-centers which were produced by neutron irradiation produced a distortion of the defect structure, which was responsible for the electron paramagnetic resonance spectrum, in such a manner that it erased the hyperfine structure. Thermal annealing annihilated the neutron irradiation-induced defects, and subsequent X-irradiation recovered the hyperfine structure of the spectrum.

A.Ibarra, D.Bravo, F.J.Lopez, I.Sildos: Materials Science Forum, 1997, 239-241, 595-8