Electronic structure and defect chemistry in iron perovskites

Abstract

This thesis systematically investigates the electronic structure and defect chemistry of BaxSr1-xFeO3-d through first-principles density functional theory (DFT) calculations. First, the electronic structure of defect-free, cubic BaFeO3 was calculated using DFT and analyzed in terms of local atomic orbitals. The calculations revealed BaFeO3 to be a negative charge transfer material with a dominating d5L (L = ligand hole) configuration. A detailed chemical bonding analysis further showed that the Fe-O bond has a mixed ionic-covalent character, and that the frontier orbitals at the Fermi level (and ligand holes) have an anti-bonding pdsigma* character. The susceptibility of the ideal cubic perovskite structure towards phase transformations was evaluated on the basis of first-principles phonon calculations. The phonon dispersion revealed distinct dynamically unstable modes which are isostructural to Jahn-Teller type distortions. The distortion is able to lift the orbital degeneracy of O 2p dominated ligand holes inherent to the cubic phase, thereby alleviating stresses in the electronic structure. The defect chemistry of BaxSr1-xFeO3-d was explored with respect to two different types of point defects: oxygen vacancies and protonic defects. The energy of oxygen vacancy formation, i.e. the release of neutral oxygen at the expense of electron holes, increases with increasing Sr-content and increasing oxygen vacancy concentration. Both compositional variations correlate with an increasing Fermi level at which electrons from the removed oxygen have to be accommodated. With increasing oxygen vacancy concentration, the Fe-O bond is weakened which facilitates oxygen excorporation and should decrease the vacancy formation energy. However, this contribution is effectively outweighed by the concomitant increase in Fermi level, rendering the vacancy formation energy to experience a net increase. In solid oxides containing oxygen vacancies, protons can be incorporated via the hydration reaction, i.e. the absorption of water vapor in dissociated form (H+, OH-), with the proton being attached to a regular oxygen ion and the hydroxide ion filling an oxygen vacancy. A thermodynamic formalism was developed that allows quantifying the energy changes during the two partial reactions - the proton- and hydroxide affinities - from first-principles DFT calculations. The new formalism was applied to a wide range of solid oxides, ranging from binary oxides such as MgO to various perovskite oxides, including BaZrO3 and BaFeO3. The study revealed an intriguing correlation between proton- and hydroxide affinities and the ionization potential (IP, position of O 2p band relative to the vacuum level) of the materials across the various structure families investigated. In the series of compositions BaxSr1-xFeO3-d, the hydration energy becomes more negative with increasing Ba-content and increasing concentration of oxygen vacancies. Evaluation of the proton and hydroxide affinities in oxygen non-stoichiometric BaFeO3-d showed that the trend with oxygen vacancy concentration largely reflects an underlying trend of increasingly more negative hydroxide affinities. This is suggested to stem from the annihilation of delocalized ligand holes during oxygen vacancy formation; lattice oxygen ions (and incorporated OH-) become subsequently more negatively charged, and thus experience a stronger electrostatic interaction with their ionic environment.