Abstract
Aberration corrected electron optics allow routine acquisition of high-spatial resolution spectroscopic images in the scanning transmission electron microscope, which is important when trying to understand the physics of transition metal oxides such as manganites. The physical properties of these perovskites are intimately related to the occupancies of the partially filled 3d bands, which define their oxidation state. In this work, we review the procedures to measure this electronic property in LaxCa1-xMnO3 from electron energy loss spectra obtained in the aberration corrected scanning transmission electron microscope. In bulk samples, several features of both the average Mn L2,3 edge and the O K edge fine structures change linearly with Mn nominal valence. These linear correlations are extracted and used as a calibration to quantify oxidation states from atomic resolution images. In such images, the same fine structure features exhibit further changes, commensurate with the underlying atomic lattice. Mn valence values calculated from those images show unexpected oscillations with the period of the atomic lattice. The combination of experiment with density-functional theory and dynamical scattering simulations allows detailed interpretation of these maps, distinguishing dynamical scattering effects from actual changes in electronic properties related to the local atomic structure.
