Direct evidence of Fe/Fe charge order in the ferrimagnetic hematite- ilmenite Fe1.5Ti0.5O3-δ solid solution

A strong interplay between charge, lattice, orbital and spin degrees of freedom exists in transition-metal oxides yielding the emergence of a large spectrum of functionalities such as hightemperature superconductivity, metal/insulator transition, multiferroicity, thermoelectricity... More specifically, mixed valence states of 3d elements issue as a driving force in such interplay. In addition, valence state modulations can also originate from the oxygen content variation. Recently, new physical effects were revealed at the interfaces of complex transition-metal oxides demonstrating how a precise knowledge of the atomic and electronic structures at the atomic scale becomes a pivotal point to understand better the observed physical properties at the macroscopic scale (see e.g. [1] and [2]). Today modern technological breakthroughs in electron microscopy and spectroscopy pave the way toward (i) localizing atom positions at the sub-Angström resolution, (ii) obtaining 2D elemental maps and (iii) mapping the electronic structure at the atomic scale. For instance, studies of L2,3 transition metal fine structures by Electron Energy-Loss Spectroscopy (EELS) can now yield direct valence state quantification at such a sub-Angström resolution. Very recently, Tan et al. successfully mapped for the first time the Mn/Mn order in Mn3O4 at the atomic scale [3]. This proof of concept opens new perspectives to understand deeper the structure-properties relationship in complex transition-metal oxides. Here we study a solid solution based on titanohematite (Fe2−xTixO3), which presents interests in (i) geomagnetism as a responsible for the observed earth remanent magnetism based on the atomic scale spinodal decomposition inducing coherent interfaces between Fe2O3/FeTiO3 exsolution lamellae [4, 5] and (ii) as a potential spintronic material as predicted by theoretical ab initio spinresolved density of states calculations [6]. All these works have stimulated several studies on the microstructure of mineral rocks as well as the controlled elaboration of epitaxial thin films at the atomic scale [7]. However no evidence for cationic and charge orders at the atomic scale in these phases has been provided yet. Using a new-generation of aberration-corrected scanning transmission electron microscope, the NION UltraSTEM200, we imaged and mapped the atomic structure of Fe1.5Ti0.5O3 demonstrating a cationic order by means of the signature of a clear anti-correlation between pure Fe and mixed Fe/Ti plans over few unit cells (Fig. 1). More interestingly, relying on high-energy resolution EELS measurements on the Fe-L3 edge, we provide the direct evidence of a possible Fe/Fe charge ordering. This is exemplified in Figure 2 where atomically-resolved Fe, Ti, Fe and Fe maps are displayed. Linear combinations of Fe (siderite) – Fe (hematite) ELNES reference spectra were used to fit each experimental spectrum leading to the Fe and Fe reconstructed maps.