Atomic order, electronic structure and thermodynamic stability of nickel aluminate

Abstract

The atomic order, electronic structure and thermodynamic stability of nickel aluminate, NiAl₂O₄, have been analyzed using periodic density functional theory and cluster expansion. NiAl₂O₄ forms a tetragonal structure with P4₁22 space group. At temperatures below 800 K, it is an inverse spinel, with Ni occupying the octahedral sites and Al occupying both the octahedral and the tetrahedral sites. Some Ni^octa + Al^tetra ⇌ Ni^tetra + Al^octa exchange occurs above 800 K, but the structure remains largely inverse at high temperatures, with about 95% Ni^octa at 1500 K. Various functionals, with and without van der Waals corrections, were used to predict the experimental formation energy, lattice parameters and electronic structure. In all cases, the NiAl₂O₄ is found to be ferromagnetic and a semiconductor with an indirect band gap along the Γ → M symmetry points. NiAl₂O₄ is found to be thermodynamically stable at operating conditions of 900–1000 K and 1 atm relative to its competing oxide phases, NiO and Al₂O₃.