Ab initio plane wave density functional theory studies were made of the thermodynamic properties of isolated cation substitutions and O vacancies in this Mg spinel. The formation enthalpy of Ca, Cu and Zn replacements of Mg cations indicated that transition metal dopants were energetically stable in the bulk at low O chemical potentials. The electronic and thermodynamic properties of isolated defects in the ternary spinel exhibited close similarities to those in binary oxides such as MgO and α-Al2O3. The formation enthalpy of the O vacancies were also similar in pure Mg spinel and in binary oxides, but the presence of impurity cations in MgAl2O4 significantly lowered the formation enthalpy of the O vacancy in their vicinity. The calculated energy barriers for O vacancy hopping were lower in the vicinity of impurity atoms in the spinel structure. The calculations indicated that the charge state of the doped cation was modified by the accompanying O vacancy and that vacancy diffusion was easier around the impurity. The results suggested that point defects played an important role in the diffusion of O vacancies in MgAl2O4.

Ab initio Thermodynamic Properties of Point Defects and O-Vacancy Diffusion in Mg Spinels. Z.Lodziana, J.Piechota: Physical Review B, 2006, 74[18], 184117 (11pp)