Accumulation of radiation damage and disordering in MgAl₂O ₄ under swift heavy ion irradiation

Accumulation and recovery of radiation-induced damage with swift heavy ions in stoichiometric magnesium aluminate spinel, MgAl₂O₄, has been investigated. Microstructural change and atomic disordering was examined through transmission electron microscopy (TEM) techniques, with bright-field (BF) and high-resolution (HR) TEM images, and high angular resolution electron channelling X-ray spectroscopy (HARECXS), for single crystal MgAl₂O₄ irradiated with 200 MeV Xe, and 340 or 350 MeV Au ions. The density of core damage region, detected by BFTEM with Fresnel-contrast, increased proportionally with ion fluence at the early stage of accumulation and saturated at a fluence higher than 10¹⁶ ions m^-2. This result is discussed with a balance between the formation and recovery of the core damage region under irradiation, and the influence region to induce the recovery was evaluated to be 7-9 nm in radius. HARECXS and electron diffraction analysis revealed that cations at tetrahedral sites preferentially occupy octahedral sites to transform to defective rock-salt structure. The structure of the core damage region is found from HR and BFTEM images to be a columnar vacancy-rich region with a low atomic density.