Ab initiothermodynamic properties of point defects and O-vacancy diffusion in Mg spinels

Abstract

We report ab initio plane wave density functional theory studies of thermodynamic properties of isolated cation substitutions and oxygen vacancies in magnesium spinel, $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$. The formation enthalpy of Ca, Cu, and Zn substitutions of Mg cation indicate that transition metal dopants are energetically stable in the bulk of $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ at low oxygen chemical potential. The electronic and thermodynamic properties of isolated defects in ternary spinel show close similarities with those in binary oxides; MgO and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$. The formation enthalpy of the oxygen vacancies are also similar in pure magnesium spinel and in binary oxides, but presence of impurity cations in $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ significantly lowers formation enthalpy of the oxygen vacancy in their vicinity. Calculated energy barriers for oxygen vacancy hopping are lower in the vicinity of impurity atoms in the spinel structure. Our calculations indicate that the charge state of doped cation is modified by the accompanying oxygen vacancy and the vacancy diffusion is more facile around impurity. The present studies suggest that point defects play an important role in diffusion of oxygen vacancies in $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$.