Nitrogen defects in wide band gap oxides: defect equilibria and electronic structure from first principles calculations

The nitrogen related defect chemistry and electronic structure of wide band gap oxides are investigated by density functional theory defect calculations of N-O(q), NHOx, and (NH2)(O)(center dot) as well as V-O(center dot center dot) and OHO center dot in MgO, CaO, SrO, Al2O3, In2O3, Sc2O3, Y2O3, La2O3, TiO2, SnO2, ZrO2, BaZrO3, and SrZrO3. The N-O(q) acceptor level is found to be deep and the binding energy of NHOx with respect to N'(O) and OHO center dot is found to be significantly negative, i.e. binding, in all of the investigated oxides. The defect structure of the oxides was found to be remarkably similar under reducing and nitriding conditions (1 bar N-2, 1 bar H-2 and 1 x 10(-7) bar H2O): NHOx predominates at low temperatures and [N'(O)] = 2[V-O(center dot center dot)] predominates at higher temperatures (> 900 K for most of the oxides). Furthermore, we evaluate how the defect structure is affected by non-equilibrium conditions such as doping and quenching. In terms of electronic structure, N'(O) is found to introduce isolated N-2p states within the band gap, while the N-2p states of NHOx are shifted towards, or overlap with the VBM. Finally, we assess the effect of nitrogen incorporation on the proton conducting properties of oxides and comment on their corrosion resistance in nitriding atmospheres in light of the calculated defect structures.