Electron Energy Loss Near-Edge Structures in Complex Perovskites

Abstract

High-resolution electron energy loss spectroscopy (EELS) provides better sensitivity to fine structures related to bonding in complex materials [1,2]. Using monochromators and improved spectrometers it is possible to achieve 0.1eV resolution with symmetric zero-loss peak profiles leading to improved detection of both core-loss and low-loss features [3] in EELS spectra. This capability makes it possible to study systematically changes in bonding in materials that present interesting physical properties. Oxides with the perovskite structure provide an interesting playing field to study many fundamental changes in structure and physical properties when systematic substitutions of atoms in the crystals are made in a controlled fashion. Substitutions of Ti atoms in BaTiO3 with other transition metals provide ingenious ways to change the stability of the basic structure and induce phase transformations. For example, Mn substitutions for Ti in BaTiO3 stabilize the hexagonal phase at room temperature and it has been proposed that this is due to increased metal-metal bonding between adjacent face-sharing octahedra [4]. EELS measurements in Ba(Ti,Mn)O3, however, have shown that, with respect to the tetragonal structure, there are only subtle changes in the oxygen K edge fine structures of the hexagonal compound due to overlaps of the Mn 3d electrons and O 2p electrons while the Ti environment is not altered significantly [5]. Ru substitutions for Ti also lead to the stabilization of the hexagonal phase but there are no reports on the effect of the changes in the near-edge structures related to the structural changes. For this particular case, it is of great interest, to study the closely related hexagonal perovskite BaRuO3. Probing electronic structure changes in these two materials with respect to BaTiO3 makes it possible to gain insight on both the electronic structure of these closely related compounds and the origin of the structural changes. We have therefore used EELS with high energy resolution to probe the near edge structures in BaTiO3, Ba(Ti,Ru)O3 and BaRuO3.