Native defects as sources of optical transitions in MgAl₂O₄ spinel

The outstanding physical and chemical properties of the magnesium aluminate (MgAl₂O₄) spinel makes it an important material for novel technological applications. Considering that a presence of native defects can promote important changes in those properties, in this work we present a study of the structural, electronic and thermodynamic properties of the MgAl₂O₄ spinel. The calculated formation energy for isolated defects, such as the vacancies of magnesium (V_Mg), aluminum (V_Al) and oxygen (V_O), oxygen interstitial (O_i), magnesium and aluminum antisites (MgAl, AlMg), as well as some complex defects (VO+Oi,VO+Al_Mg, VO+Mg_Al, Mg_Al+Al_Mg) in the most stable charge states are shown. Through experimental data, we obtained that complex defects centers, such asVO, VO+Oi, VO+Al_Mg and VO+Mg_Al at different charge states are good candidates for the observed optical transitions at 4.75, 5.3, and 6.4 eV. Our findings were obtained from ab initio electronic structure calculations performed by using density functional theory. The Perdew-Burke-Ernzerhof generalized gradient approximation was used for the exchange-correlation potential. Furthermore, a modified Becke-Johnson exchange potential (GGA-mBJ) correction to the exchange potential were used to obtain a suitable value for the band gap energy, 7.40 eV, in accordance with the experimental one of 7.8 eV.