Insight from X‑ray Absorption Spectroscopy to Octahedral/Tetrahedral Site Distribution in Sm-Doped Iron Oxide Magnetic Nanoparticles

A set of Sm-doped iron oxide magnetic nanoparticles (Fe x O y NPs) of different sizes as an example of ferromagnetic NPs at room temperature was synthesized by microware-assisted methods. Powder X-ray diffraction and transmission electron microscopy showed Fe x O y NPs with an inverse spinel structure. Mössbauer and X-ray absorption spectroscopy (XAS) were used to study the local atomic and electronic structure of iron in the NPs. Linear combination of XAS spectra of reference iron oxides failed in reproducing the spectra of Fe x O y NPs [ Piquer, C. ; J. Phys. Chem. C 2014, 118, 1332−1346 ]. We attribute this fact to the nonstoichiometric distribution of tetrahedral, Td 3+, and octahedral, Oh 3+ and Oh 2+, sites in NPs compared to bulk references. This distribution was successfully reproduced by linear combination of theoretical XAS spectra obtained for clusters where iron was in Td 3+, Oh 3+, and Oh 2+ defined oxidation and symmetry states. This approach allowed us to obtain the quantitative speciation of the fraction of Td 3+, Oh 3+, and Oh 2+ sites as a function of the different size of the Sm-doped Fe x O y NPs based on the XAS data. In contrast to the standard XAS reference compounds analysis where experimental spectra fits are limited to constant Td 3+/Oh 3+ and Td 3+/Oh 3+/Oh 2+ ratios and to Mössbauer spectroscopy that requires liquid helium temperatures, the proposed method opens a new possibility to quantitatively estimate the amount of each independent inequivalent iron site in terms of symmetry and oxidation state at room temperature.