Atomic-resolution imaging of oxidation states in manganites

Abstract

Aberration corrected electron optics allows 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 procedures to obtain this electronic property in ${\text{La}}_{x}{\text{Ca}}_{1\ensuremath{-}x}{\text{MnO}}_{3}$ from atomic-column-resolved electron energy-loss spectra measured in the aberration corrected scanning transmission electron microscope. In bulk samples, several features of both the average $\text{Mn}\text{ }{L}_{2,3}$ edge and the $\text{O}\text{ }K$ edge fine structure change linearly with Mn nominal valence. These linear correlations are extracted and used as a calibration to quantify oxidation states from atomic resolution spectroscopic 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. 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. Specifically, in ${\text{LaMnO}}_{3}$, the two nonequivalent O sites can be distinguished by these methods.