Quantum Chemical Design of Doped Ca₂MnAlO_5+δ as Oxygen Storage Media

Brownmillerite Ca₂MnAlO₅ has an exceptional capability to robustly adsorb half-molecules of oxygen and form Ca₂MnAlO_5.5. To utilize this unique property to regulate oxygen-involved reactions, it is crucial to match the oxygen release–intake equilibrium with targeted reaction conditions. Here we perform a comprehensive investigation of the strategy of tuning the oxygen storage property of Ca₂MnAlO₅ through chemical doping. For undoped Ca₂MnAlO_5+δ, our first-principles calculation predicts that the equilibrium temperature at a pressure of 1 atm of O₂ is 848 K, which is in excellent agreement with experimental results. Furthermore, the doping of alkaline earth ions at the Ca site, trivalent ions at the Al site, and 3d transition metal ions at the Mn site is analyzed. By the doping of 12.5% of Ga, V, and Ti, the equilibrium temperature shifts to high values by approximately 110–270 K, while by the doping of 12.5% of Fe, Sr, and Ba, the equilibrium temperature is lowered by approximately 20–210 K. The doping of these elements is thermodynamically stable, and doping other elements including Mg, Sc, Y, Cr, Co, and Ni generates metastable compounds. The doping of a higher content of Fe, however, lowers the oxygen storage capacity. Finally, on the basis of our calculated data, we prove that the formation energetics of nondilute interacting oxygen vacancy in doped Ca₂MnAlO_5.5 scale linearly with a simple descriptor, the oxygen p-band position relative to the Fermi level. The higher-oxygen p-band position leads to a lower vacancy formation energy and thus a lower oxygen release temperature. Understanding such a relationship between fundamental quantum chemical properties and macroscopic properties paves the road to the design and optimization of novel functional oxides.