The Origin of Radiation Resistance of Magnesium Aluminate Spinel
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The Origin of Radiation Resistance of Magnesium Aluminate Spinel Vasyl T. Gritsyna, Yurij G. Kazarinov, Volodymyr A. Kobyakov, Kurt E. Sickafus1 [email protected] Kharkiv National University, Kharkiv 61077, Ukraine 1 Los Alamos National Laboratory, Los Alamos, NM 87545, USA ABSTRACT We propose here a new mechanism to explain the observed high radiation tolerance of magnesium aluminate compounds with crystal structures known as spinel. By using optical methods, we found that the kinetics of accumulation of optical absorption centers under different types of irradiation, as well as the kinetics of absorption decay after termination of irradiation, along with radio-luminescence processes, are consistent with a new model regarding defects and radiation damage in spinel. This model assumes the existence of spatially-correlated antisite defects in the form of dipoles: (Al3+tet)+-(Mg2+oct)-. These spatially-correlated point defect complexes serve as centers for annihilation of radiation-induced cation Frenkel pairs. In addition to the spatially-correlated defects, the high concentration of cation structural vacancies inherent to the spinel lattice also serves to promote high mobility of both Mg and Al interstitial species. This enhanced mobility leads to increased probability of annihilation at the dipole centers proposed in this model. Such annihilation then diminishes the probability for formation of defect clusters, dislocation loops, or amorphization of the irrradiated spinel. INTRODUCTION Magnesium aluminate spinel, MgAl2O4 (or more generally, MgO·nAl2O3), has been proposed as a potential optical and insulation material for use in nuclear fusion reactors, since it possesses excellent radiation resistance properties. The high radiation tolerance of spinel may relate to several spinel properties, such as the inherent high concentration of structural vacancies, difficulties in forming clusters of point defects, or easy accommodation of cationic disorder [1]. Therefore, the nature and concentration of pristine defects in spinel crystals can play an important role in the behavior of this material under irradiation. The unit cell of MgAl2O4 spinel consists of a face-centered cubic lattice of 32 oxygen ions and 64 tetrahedral and 32 octahedral interstices between these anions. In normal spinel crystals, Mg2+ ions occupy 1/8 of the tetrahedral interstices, while Al3+ ions occupy 1/2 of the octahedral positions. It is known that spinel crystals grown under laboratory conditions are partially inverse, i.e., up to 0.3 Al3+ ions per unit cell occupy tetrahedral sites and an equal fraction of Mg2+ revert to octahedral positions. This cationic disorder results in so-called antisite defects: (Al3+Mg)+ tetrahedral (tet) ions, and (Mg2+Al)- octahedral (oct) ions, with excess of positive and negative charge, respectively. Non-stoichiometric spinel crystals (n>1.0) with excess Al2O3 contain additional cationic vacancies, required in order to charge compensate additional Al3+ ions in tetrahedral (Mg2+) positions. The
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