The Orbital Nature of Electron Holes in BaFeO<sub>3</sub> and Implications for Defect Chemistry
Maximilian F. Hoedl, Christina Ertural, Rotraut Merkle, Richard Dronskowski, Joachim Maier
Abstract
Understanding the nature of electron holes in mixed protonic–electronic conducting perovskites is key to the successful development of novel electrode materials for protonic ceramic fuel and electrolyzer cells (PCFCs). Here, we use density functional theory to investigate the orbital interactions and defect chemistry of BaFeO3−δ─a model material for more complex cation stoichiometries used in PCFCs. The calculations revealed BaFeO3−δ to be a negative charge-transfer material with a dominating d5L ( L = ligand hole) configuration. Chemical bonding analysis using the projected Crystal Orbital Hamilton Population (COHP) further showed that the ligand holes are partially delocalized in pdσ* bonds with an approximately 80% share at the oxygen ions. The defect chemistry of BaFeO3 was explored with respect to oxygen vacancies and protonic defects. The oxygen vacancy formation energy increases with decreasing ligand hole concentration due to a rising Fermi level at which electrons from the removed oxygen are accommodated. This effectively outweighs a concomitant decrease in the Fe–O bond strength evidenced by energy-integrated COHP analysis. The dissociative H2O absorption was studied, where OH– fills an oxygen vacancy and H+ is attached to a regular oxygen ion. The energy of this reaction becomes more negative with decreasing ligand hole concentration. This trend largely reflects a more favorable OH– affinity with decreasing ligand hole concentration.