Site symmetry approach applied to the supercell model of MgAl₂O₄ spinel with oxygen interstitials: Ab initio calculations

In this study we simulate structural, electronic and phonon properties of MgAl₂O₄ spinel containing a single neutral oxygen interstitial (O_i) per crystalline L4 and L8 supercells, e.g., its dumbbell formed with one of the nearest regular oxygen atoms of the lattice (O_i-O_reg). Due to the splitting of the Wyckoff positions in supercell models of a perfect crystal, five possible O_i positions with different site symmetry have been identified and studied (C₁, C_s, C_{3 v} D_{2 d} and T_d). First principles hybrid HSE06 DFT functional calculations on perfect and defective spinel structures have been accompanied by geometry optimization. The calculated properties of spinel crystal (lattice constants, bulk modulus, band gap as well as frequencies of infrared- and Raman-active vibrational modes) are in a good qualitative agreement with the corresponding experimental data. The formation energy of O_i is found to be minimal for the interstitial site of the lowest symmetry (C₁). The results obtained are important, in particular, for understanding the radiation and chemical stability as well as other key properties of MgAl₂O₄ spinel-type oxide crystals.