Kinetics of nanocavity formation based on F-center aggregation in thermochemically reduced MgO single crystals

The dynamics of interacting F centers resulting in F aggregates and nanocavities is modeled in thermochemically reduced MgO single crystals. We have recently shown that thermal annealing of thermochemically reduced MgO with an exceptionally high F-center concentration $(6\mathrm{}\times {10}^{18}{\mathrm{cm}}^{\mathrm{-}3})$ induces a brown coloration in the crystals, which was attributed to scattering from nanosize cavities with their walls plated with magnesium metal [Phys. Rev. B 62, 9299 (2000)]. In the present paper, a theory of the nanocavity formation process is developed based on diffusion-controlled aggregation of elastically interacting F centers and their annihilation at traps. We show that in contrast to the generally accepted viewpoint, the F centers in the bulk are not annealed out at the external sample surface but at internal defects, such as dislocations, subgrain boundaries, and impurities. The mutual attraction of the F centers is a key factor controlling the aggregation process.