Long- and Short-Range Structure of Ferrimagnetic Iron-Chromium Maghemites Marco García-Guaderrama, †,# María E. Montero-Cabrera,* ,‡ Emilio Mora ́ n, † Miguel A. Alario-Franco, † Luis E. Fuentes-Cobas, ‡ Edgar Macías-Ríos, ‡ Hilda E. Esparza-Ponce, ‡ and María E. Fuentes-Montero § † Departamento de Química Inorga ́ nica, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain (EU) # Centro de Investigació n en Materiales DIP-CUCEI, Universidad de Guadalajara, Av. Revolució n 1500, Col. Olímpica, Guadalajara, Me ́ xico ‡ Centro de Investigació n en Materiales Avanzados (CIMAV), Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua 31136, Chih, Mé xico § Facultad de Ciencias Químicas, Universidad Autó noma de Chihuahua, Av. Escorza 900, Zona Centro, Chihuahua 31000, Mé xico ABSTRACT: Maghemite-like materials containing Fe 3+ and Cr 3+ in comparable amounts have been prepared by solution-combustion synthesis. The conditions of synthesis and the magnetic properties are described. These materials are ferrimagnetic and are much more stable than pure iron maghemite since their maghemite-hematite transformation takes place at about ∼700 °C instead of ∼300 °C, as usually reported. These materials were studied by synchrotron radiation X-ray diffraction (XRD) and by X-ray absorption fine structure (XAFS) of the K-absorption edge of two elements. High-resolution XRD patterns were processed by means of the Rietveld method. Thus, maghemites were studied by XAFS in both Fe and Cr K-edges to clarify the short-range structure of the investigated systems. Pre-edge decomposition and theoretical modeling of X-ray absorption near edge structure transitions were performed. The extended X-ray absorption fine structure (EXAFS) spectra were fitted considering the facts that the central atom of Fe is able to occupy octahedral and tetrahedral sites, each with a weight adjustment, while Cr occupies only octahedral sites. Interatomic distances were determined for x = 1, by fitting simultaneously both Fe and Cr K-edges average EXAFS spectra. The results showed that the cation vacancies tend to follow a regular pattern within the structure of the iron-chromium maghemite (FeCrO 3 ). 1. INTRODUCTION Iron sesquioxide Fe 2 O 3 presents two well-known, structurally different polymorphs, with completely different magnetic properties: γ-Fe 2 O 3 (maghemite) is a ferrimagnetic cation deficient spinel, while α-Fe 2 O 3 (hematite) shows the corundum structure and is antiferromagnetic. Chromium sesquioxide Cr 2 O 3 is isostructural to hematite and is an antiferromagnet too. Consequently, the solid solution α-Fe 2-x Cr x O 3 , can be obtained in the whole compositional range. 1 These materials have been extensively studied, aiming for catalytic applica- tions. 2-5 One of the possible applications in the field of catalysis is the water-gas shift reaction. 6 Because of various properties, such as their high surface area and highly flexible surface oxygen species, chrome spinels also find application in catalysis for methane combustion. 7 Spinels exhibit properties that make them viable for other potential applications in spintronics 8 and magnetoelectricity. 9 For both Fe 2 O 3 poly- morphs, crystal and magnetic structures have been deter- mined; 10-13 moreover, first-principles calculations have also been performed. 14 In contrast to this, studies on the maghemite-like solid solution γ-Fe 2-x Cr x O 3 are scarce, with the compositional range explored being quite narrow (x ≤ 0.15). 15,16 To our knowledge, its structural and magnetic study has not yet been performed for x > 0.15. On the other hand, the magnetic properties of the solid solution γ-Fe 2-x Cr x O 3 are worth being investigated, especially to get further insight into Fe 3+ (high spin) and Cr 3+ magnetic interactions. The motivation for these studies comes from the predictions made by Baettig and Spaldin on the properties of Bi 2 FeCrO 6 ―a material where fascinating multiferroic or magnetoelectric properties should appear were those cations arranged in certain order. 17 Last but not least, it is important to note that materials similar to those described in this paper, that is, ferrimagnetic maghemites, frequently appear as tiny impurities when performing the synthesis of Fe-based multi- ferroic materials. 18 X-ray absorption fine structure (XAFS) techniques allow understanding the local order and electronic structure of the nearest neighbors of an atom 19 at distances normally from 0.5 Received: July 19, 2015 Published: November 17, 2015 Article pubs.acs.org/IC © 2015 American Chemical Society 11200 DOI: 10.1021/acs.inorgchem.5b01624 Inorg. Chem. 2015, 54, 11200-11208