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Laura Bocher - Université Paris-Sud Orsay

Probing mixed-valence states in complex metal oxides for the direct visualization of charge ordering mechanisms

Recent technological breakthroughs in electron spectro-microscopy techniques have paved the way toward localizing single atom positions and acquiring 2D elemental maps at subangström resolution over large unit cell. High-energy resolution electron energy-loss spectroscopy (EELS) can probe the local bonding environment and the electronic structure of nanostructured systems.

In transition metal oxides, the L2,3 transition metal and O-K near-edge structures present a strong sensitivity to the valence state modulations. Combining this information with imaging capabilities in an aberration-corrected Scanning Transmission Electron Microscope (STEM), it gives us a direct access to the valence state mapping down to the atomic columns.

Hence we recently revealed direct experimental evidence of Fe2+/Fe3+ charge ordering in hematite-ilmenite Fe1.35Ti0.65O3-d thin films using this real-space technique. The local Fe valence state distributions highlight a strong Fe2+ modulation correlated with a significant presence of oxygen vacancies. The magnetic and transport properties of these films are reviewed in the light of the charge ordering mechanism.