Open questions on the thermal stability of primary electronic centers in heavy swift ion irradiated MgO single crystals

Magnesium oxide (MgO) is the simplest representative of wide-gap oxides (Eg = 7.8 eV) with a face-centered cubic structure and high refractory (melting point ~3073 K). Due to fascinating mechanical and physicochemical properties, MgO single crystals and ceramics are exploited as protective layers and materials for fuel cells, catalysis, scintillation and dosimetric detectors. In addition, magnesia demonstrates high resistance against harsh radiation environment and, therefore, is already widely used in fission-based nuclear energetics and considered to be engaged in projected D-T fusion devices. In general, functionality of many materials exploited in nuclear applications is limited by their increasing radiation damage via creation and accumulation of primary lattice defects – interstitial-vacancy Frenkel pairs. Therefore, radiation-induced processes in MgO exposed to fast fission neutrons or energetic ions and electrons have been studied for many years using different experimental methods. Basic properties of the simplest radiation defect – F+ center (an oxygen vacancy with a trapped electron) in the simplest oxide material (MgO with a cubic lattice structure) remain for a long time a subject of discussions, in the light of highly contradictory literature data. In this experimental-theoretical study, new data on the EPR and optical absorption measurements for the neutron- and energetic-ion-irradiated (70-MeV Ar) MgO single crystals are discussed. Theoretical analysis of the diffusion-controlled F+ center isochronal annealing is performed; the obtained migration energies for oxygen interstitials recombining with F+ centers are discussed in the light of the available calculations on the mobility of both neutral and charged oxygen interstitials in metal oxides.