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Probing the Fe<SUP>2+</SUP>/Fe<SUP>3+</SUP> mixed-valence state in magnetic semiconductor Fe oxides at the atomic scale... and evidencing an expected charge ordering


We reveal the direct experimental evidence of Fe2+/Fe3+ charge ordering at room temperature in hematite-ilmenite Fe1.35Ti0.65O3-δ (FTO) thin films using the NION UltraSTEM 200 aberration-corrected scanning transmission electron microscopy (STEM) coupled to high-resolution energy electron-loss spectroscopy (EELS).

Atomically-resolved STEM-EELS experiments allow us to demonstrate first the presence of a cationic ordering in the oxygen deficient FTO thin films, i.e. between rich Fe- and mixed Fe/Ti planes (see Fig. (b)). At higher energy resolution, this real-space technique enables us to map directly the Fe2+/Fe3+ valence states on each atomic columns by probing locally the evolution of the Fe-L2,3 fine structures (see Fig. (d) and (e)). Hence, we demonstrate a strong modulation of the Fe2+ valence state along the c axis (see Fig. (c)). Density functional theory calculations provide crucial information on the key role of oxygen vacancies in the observed charge distributions. Their presence at significant levels leads to the localization of extra electrons onto reduced Fe2+ sites, while Ti remains solely +4. The magnetic and transport properties of these films are reviewed in the light of the present results regarding their ferrimagnetic character correlated with the Fe2+ modulation and their semiconducting behavior interpreted by a variable-range hopping conduction regime via Fe2+ and Fe3+ centers.

These results issue from a collaboration between experimental physicists of the GEMaC, CEMES, and LPS labs with a theoretician team from the Tyndall National Institute of Cork University College. Specifically, the recent instrumental developments in spectro-microscopy techniques at the LPS laboratory yields for the first time the valence state mapping in a complex transition-metal oxide systems composed of several cations, each of which has different possible valence states. This opening new horizons for direct visualization of charge ordering mechanisms.

(a) Fe1.5Ti0.5O3 structural model system, (b) reconstructed elemental map combining the O-K, Ti-L2,3, and Fe-L2,3 edges (red, green, and blue), and (c) and the corresponding map of the Fe2+ spectral weight, (d) and (e) examples of Fe-L2,3 fine structures probed at different atomic sites.

Reference :

“Direct evidence of Fe2+/Fe3+ Charge Ordering in the Ferrimagnetic Hematite-Ilmenite Fe1.35Ti0.6503-d Thin Films” - L. Bocher, E. Popova, M. Nolan, A. Gloter, E. Chikoidze, K. March, B. Warot-Fonrose, B. Berini, O. Stéphan, N. Keller, and Y. Dumont.

Contact :

Laura Bocher (laura.bocher@u-psud.fr)