The impact of crystal symmetry on the electronic structure and functional properties of complex lanthanum chromium oxides

Abstract

Complex oxides exhibit a wide range of crystal structures, chemical compositions and physical properties. The underlying drivers determining the complicated structure–composition–property phase diagrams are the relative positions and orbital overlaps between the metal cations and the oxygen anions. Here we report a combined experimental and theoretical investigation of the structure and bonding in a series of lanthanum chromium oxides prepared by molecular beam epitaxy. Of particular interest is the charge state and local coordination of the Cr. We have stabilized LaCrO₃, LaCrO₄ and La₂CrO₆ films by controlling the three elemental fluxes during deposition, and have carried out X-ray diffraction, X-ray photoemission, and X-ray absorption spectroscopy, as well as first-principles calculations, to determine structure, charge state, chemical bonding, and electronic structure. Significant changes in bonding character and orbital interaction are revealed with decreasing ligand symmetry from octahedral to tetrahedral Cr coordination. Both LaCrO₄ and La₂CrO₆ with tetrahedrally coordinated Cr show strong pre-edge features in the Cr K-edge near-edge structure whereas LaCrO₃ with octahedrally coordinated Cr exhibits very weak pre-edge features. The origin of these pre-edge features is discussed based on various selection rules and ligand symmetry. We also demonstrate an increase in cation–anion orbital hybridization and a decrease in long-range ligand coupling induced by this symmetry reduction. These in turn result in dramatic modifications of the macroscopic optical and magnetic properties.