Core level absorption spectroscopy of perovskites and high temperature superconductors

We present K-edge, or Is, absorption spectra of oxygen in different oxides, and relate the fine structure near the absorption edge to different degrees of charge transfer between the cations and anions, and to features of the electronic structure. We use the experimental technique of electron energy loss spectroscopy (EELS) where incident fast electrons, rather than incident X-rays, excite the core electrons of the atoms. The spectrometer is attached to a transmission electron microscope enabling us to analyze nanometer sized volumes and study bonding and local electronic structure near defects and interfaces. Comparisons are made with the closely related technique of X-ray absorption spectroscopy. By examples we demonstrate that increase in the oxygen content above, or the reduction of the cation content below, the equilibrium level, as well as the interchange of a cation with one of lower formal valency result in a pre-edge peak a few eV below the threshold of the oxygen K-edge. We attribute this feature to transfer ofs valence electrons away from the oxygen anions, thus creating empty states below the conduction band of the originally stoichiometric compound.