Radiation damage in magnesium oxide. II. Effect of pre-irradiation annealing in hydrogen

When MgO crystals are subjected to reactor irradiation after annealing in hydrogen, large differences in the colour centre concentration are observed compared with unannealed crystals. These are believed to be due to vacancies diffusing to large lenticular voids produced in MgO crystals by the annealing treatment. Thus for doses less than 10 19 nvt there is an excess of interstitial defects over vacancies which leads to an inequality between the growth of the macroscopic and unit-cell dimensions of the crystals. Detailed analysis of the changes in defect concentration, lattice parameter and density gives an estimated volume contraction per single vacancy of the order of 1 atomic volume. The formation of F + centre/cation vacancy pairs (F c + centres) on annealing below 550 degrees C proceeds with an activation energy of only 0.96+or-0.2 eV. Since this is much lower than the activation energy for cation vacancies to migrate, it is suggested that this corresponds to the activation energy for the aggregation of close pairs of defects.