Long- and Short-Range Structure of Ferrimagnetic IronâChromium

Article pubs.acs.org/IC

Long- and Short-Range Structure of Ferrimagnetic Iron−Chromium Maghemites Marco García-Guaderrama,†,# María E. Montero-Cabrera,*,‡ Emilio Morán,† Miguel A. Alario-Franco,† Luis E. Fuentes-Cobas,‡ Edgar Macías-Ríos,‡ Hilda E. Esparza-Ponce,‡ and María E. Fuentes-Montero§ †

Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain (EU) Centro de Investigación en Materiales DIP-CUCEI, Universidad de Guadalajara, Av. Revolución 1500, Col. Olímpica, Guadalajara, México ‡ Centro de Investigación en Materiales Avanzados (CIMAV), Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua 31136, Chih, México § Facultad de Ciencias Químicas, Universidad Autónoma de Chihuahua, Av. Escorza 900, Zona Centro, Chihuahua 31000, México #

ABSTRACT: Maghemite-like materials containing Fe3+ and Cr3+ 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 (FeCrO3). 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-Fe2−xCrxO3 are worth being investigated, especially to get further insight into Fe3+ (high spin) and Cr3+ magnetic interactions. The motivation for these studies comes from the predictions made by Baettig and Spaldin on the properties of Bi2FeCrO6―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 multiferroic materials.18 X-ray absorption fine structure (XAFS) techniques allow understanding the local order and electronic structure of the nearest neighbors of an atom19 at distances normally from 0.5

1. INTRODUCTION Iron sesquioxide Fe2O3 presents two well-known, structurally different polymorphs, with completely different magnetic properties: γ-Fe2O3 (maghemite) is a ferrimagnetic cation deficient spinel, while α-Fe2O3 (hematite) shows the corundum structure and is antiferromagnetic. Chromium sesquioxide Cr2O3 is isostructural to hematite and is an antiferromagnet too. Consequently, the solid solution α-Fe2−xCrxO3, can be obtained in the whole compositional range.1 These materials have been extensively studied, aiming for catalytic applications.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 spintronics8 and magnetoelectricity.9 For both Fe2O3 polymorphs, crystal and magnetic structures have been determined;10−13 moreover, first-principles calculations have also been performed.14 In contrast to this, studies on the maghemite-like solid solution γ-Fe2−xCrxO3 are scarce, with © XXXX American Chemical Society

Received: July 19, 2015

A

DOI: 10.1021/acs.inorgchem.5b01624 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry to 0.8 nm. X-ray absorption near edge structure (XANES) offers information on the oxidation state of the absorbing element.20,21 Extended X-ray absorption fine structure (EXAFS) allows the determination of distances, coordination numbers, and species of the first neighbors of the absorbing atom.22 XAFS experiments require X-rays sources of high intensity and tunable energies, as is the case of synchrotron radiation (SR).19 XAFS studies display some advantages, as the measurement of the X-ray absorption coefficient energy dependence allows perfectly distinguishing neighboring elements in the Periodic Table. The solution-combustion method allows obtaining metastable phases as substituted maghemites.23 The objective of the present article is to show the synthesis, magnetic properties, and long- and short-range order structure of the maghemite-like γ-Fe2−xCrxO3 (x = 0.75, 1, and 1.25) dual oxide system. The tetra- and octahedral sites cation occupancy and the ordering trend of structural vacancies have been verified.

Figure 1. Spinel structure of γ-Fe2−xCrxO3 (x = 0.75, 1, and 1.25). Oxygen atoms, not represented, are located on the octahedral and tetrahedral vertices; selected coordination tetrahedra are shown in red, while octahedra around cations are respectively magenta and gray, representing different Wyckoff sites in the considered spinel model (see text below).

2. EXPERIMENTAL SECTION Preparation and Characteristics of γ-Fe2−xCrxO3. Chemical Synthesis. Ferrimagnetic maghemite-like materials such as γFe2−xCrxO3 (x = 0.75, 1, and 1.25) have been prepared by the solution-combustion synthesis.23,24 Stoichiometric amounts of Fe(NO3)3·9H2O (Aldrich, 98%) and Cr(NO3)3·9H2O (Sigma-Aldrich, 99%) were dissolved in 2-methoxyethanol (Sigma-Aldrich, 99.8%) in a ratio of 2 g of final product to 20 mL of solvent. As an additive, in order to promote the combustion and increase the oxidizing power, NH4NO3 (Sigma 99.5%) was also put into the solution, in a weight equaling that of the chromium and iron nitrates together. The whole was stirred and heated at 125 °C, until complete solution was achieved and then transferred to a wide and flat porcelain crucible (in order to get better exposition to the ambient oxygen). This was rapidly heated up to 250 °C in a heating plate placed inside an exhaust hood. Along with the evaporation of the solvent, the viscosity increases, and suddenly, a strong combustion process, with flames, takes place in a few seconds, almost explosively. The resulting product is a dark, spongeous solid. Half of the material obtained was directly characterized while the second part was further treated at 230 °C in flowing oxygen for 24 h, in order to ensure full oxidation. Both materials, fresh and O2-treated, were studied and compared. The oxidized materials are the subject of the present work. Morphologic, Thermal, and Magnetic Characterization. Sample morphologies and grain sizes were examined by means of scanning electron microscopy (SEM). It was performed on a JEOL 6400 microscope equipped with an EDAX Inc. energy-dispersive X-ray detector for microanalysis. A second EDS analysis was performed by JEOL 5800LV scanning electron microscopy coupled to EDX DXprime EDAX. Thermal analysis was executed in a PerkinElmer DTA 7 set up, in flowing N2, up to 1000 °C with a heating speed of 10 °C/min. During synthesis and thermal characterization, conventional X-ray diffraction data were collected on a Philips X’Pert PRO ALPHA1 of Panalytical B.V. instrument operating at 45 kV and 40 mA, fitted with a primary curved Ge111 monochromator in order to get CuKα1 radiation (λ = 1.5406 Å); a speed X’Celerator detector was used. Isothermal magnetization curves were measured with a Cryogenic Ltd. CFMS vibrating sample magnetometer delivering a magnetic field up to 12 T. Structural Characteristics. Figure 1 schematically shows the spinel structure of the studied maghemite-like materials, in the following called simply maghemites. An important open question is the distribution of Fe, Cr, and vacancies among the octahedral and tetrahedral spinel sites. Concerning vacancies, for the case of pure maghemite γ-Fe2O3, Shmakov et al.25 propose they are located at the 4b octahedral sites of the P4332 space group, while Jorgensen et al.26 suggest they are distributed among tetrahedral and octahedral sites of Fd−3m. Regarding Fe and Cr, it is reasonable to expect that Cr3+, as a t2g3 cation, with few exceptions,27−29 should go to octahedral positions,

while Fe3+, a t2g3 eg2 cation in a high spin configuration, would occupy either octahedral or tetrahedral positions. Considered maghemites were studied by X-ray absorption fine structure (XAFS) of the K-absorption edge of Cr and Fe elements and by synchrotron radiation high-resolution X-ray diffraction (XRD). Double-element XAFS analysis was applied to clarify the local configuration of maghemite systems. XAFS spectra are the average signal over all absorbing Cr or Fe atoms in a homogeneous sample, and thus the spectra for each element reflect the average effects of the environment in which they appear. That means, for example, that the fine structure of the Fe K-edge reflects the mixed effects of the tetrahedral and octahedral environments of the Fe cations, and the trend (or not) of the vacancies to follow a random distribution or a short-range order. The interpretation of the spectra allows clarifying the details which reveal this mixture. XAFS Attributes for Transition Metals Study. XANES analysis provides the possible identification of particular photoelectron transitions. This is the case when analyzing the edge position, which is identified by the inflection point in the absorption edge function30,31 (which is useful when checking the oxidation state of a cation) and the so-called pre-edge features that appear at energies lower than the absorption edge. The pre-edge region of the spectra before the K-edge jump contains peaks, explained by electronic transitions to unoccupied bound states located below the vacuum level.30,32,33 A perceptible peak exists as a result of the photoelectron transitions 1s → 3d. These dipolar transitions in the transition metals are prohibited when Δl = 2: a condition that is reinforced when a metal cation occupies an octahedral site, a very symmetrical environment. Nevertheless, in the case of tetrahedral or distorted octahedral geometries, bonds of a 3d state mixed with the 2p ligand oxygen are formed and an intensified dipole transition is possible. Thus, detecting and analyzing the preedge features allows recognizing the transition metal cations oxidation state and/or occupancy of several particular sites in the local structure. In the present investigation of compounds containing Cr and Fe cations, the study of the mentioned attributes of the XANES spectra provides key information about oxidation states and local environment. For the maghemite system γ-Fe2−xCrxO3, X-ray diffraction reports the positions for a “Cr−Fe average atom” and the oxygen. EXAFS, for Cr or Fe as absorbents, provides Fe−O, Cr−O, Cr−Fe, Fe−Fe, and Cr−Cr distances. The analysis of the first inflection shifts at about the energy of the main jump, calculated as the difference between its position in the pure transition metal (zero oxidation) and B


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Inorganic Chemistry

Figure 2. (a) SEM micrograph of γ-FeCrO3. Note the foamy texture and the surface porosity. (b) The corresponding EDS microanalysis. the corresponding position of the studied substance, as well as the study of the above-mentioned pre-edge features, are considered for the subsequent EXAFS analysis. The pre-edge feature present at the Crand Fe K-edge will reflect if the Cr occupies or not a tetrahedral site, as well as the “mixed” occupation of both tetra- and octahedral sites by the Fe cation. Synchrotron Radiation Experiments. The global structures of the studied materials were investigated by high-resolution X-ray powder diffraction SXRD. At Stanford Synchrotron Radiation Lightsource (SSRL) beamline 2-1, the samples were mounted on a zero background holder and data were collected in reflection geometry at 12 keV (λ = 1.03265 Å) from 10° to 110° in 2θ. The instrumental resolution was calibrated with a standard LaB6 sample. The experimental data were processed by the Fullprof program, using the microstructural characterization routine to estimate the crystallite size and heterogeneous deformations of the lattice parameters. The scanning step was 0.01° in 2θ. Oxide samples were investigated by XAFS on the Fe and Cr Kedges. Measurements were performed at the SSRL at room temperature, at beamlines 2-3 and 4-3, in fluorescence mode, using Lytle + ion-chamber detectors and a Si(220), φ = 90° monochromator. The SPEAR-3 storage ring was operated at 3.0 GeV with a beam current of 100 mA. Absorption coefficient measurements were performed using a pure metal reference between I1 and I2 chambers, with a Mn filter in the Lytle detector for the case of the Fe K-edge. For the Cr K-edge, a V filter in the Lytle was employed. Data were recorded on the XANES region with energy intervals ΔE = 0.25 eV, and in the EXAFS zone, the energy intervals corresponded to Δk = 0.04 Å−1. Spectra were measured up to k = 13 Å−1. Cr(III)2O3 (corundum), Cr(IV)O2 (rutile), and SrCr(VI)O4 were used as model compounds for the Cr K-edge main edge energy position and pre-edge features comparison, when checking the oxidation state of study samples was necessary. ATHENA and ARTEMIS interfaces for IFEFFIT35 and FEFF8.436 codes were employed for XAFS spectra interpretation. Pre-edge decomposition using the ATHENA algorithm for modeling these transitions and theoretical modeling of X-ray absorption near edge structure (XANES) spectra by FEFF8.4 were performed. The normalized EXAFS spectra were converted from energy to k-space and weighted by k, k2, and k3. The three kn weightings were employed simultaneously for the spectra fitting. These data were then Fourier transformed to R-space. N, R, and σ2 structural parameters (Rj being the distance to neighboring atoms, Nj the coordination number of neighboring atoms, and σj the quadratic deviation average of the distance to the neighbors) were determined by fitting the EXAFS spectra with theoretical standards.37−39

similar results, although other common fuels, such as urea or saccharose, do not produce the desired product. The enormous quantities of evolved gases (N2, CO2, and H2O, according to the combustion literature40) produce a porous structure, as usually happens for combustion-made materials. The microanalysis results presented in Table 1 are in good agreement with the nominal atomic compositions. Table 1. Atomic Concentrations of Cr and Fe, Obtained by EDS Microanalysis for Studied Samples x

Cr at %

Fe at %

Cr/Fe

nominal Cr/Fe

0.75 1 1.25

38.5 (1.1) 52.1 (1.1) 58.1 (1.1)

61.5 (2.2) 47.9 (2.2) 41.9 (2.2)

0.63 (0.03) 1.09 (0.05) 1.39 (0.06)

0.6 1 1.67

Thermal Differential Analysis (TDA), shows an exothermic peak around 700−725 °C (figure is not showed, see ref 23) corresponding to the transformation of γ-FeCrO3 (maghemitelike) to the more stable α-FeCrO3 (hematite-like) phase. Cell parameters obtained by conventional XRD performed on the mentioned compounds correspond to the hematite-like compound (S.G. R3̅c) with a = 5.012 Å and c = 13.663 Å values between those of α-Fe2O3 (5.035 Å and 13.747 Å)41 and α-Cr2O3 (4.951 Å and 13.566 Å);42 this result is coherent with the respective ionic radii for the solid solution. It has been reported that maghemite transforms to hematite (corundum) at temperatures between 300 and 500 °C depending on the preparation method;43,44 therefore, it seems that chromium markedly stabilizes the spinel-type phase, a fact which could lead to potential applications. Synchrotron X-ray Diffraction. Figure 3 shows the XRD patterns from samples γ-Fe2−xCrxO3, with nominal compositions (x = 0.75, 1.00, 1.25). All XRD patterns are typical of cubic (S.G. Fd3m) γ-Fe2O3. The inset shows a zoom of the 440 peak in the three tested samples. The observed peaks’ broadening and asymmetry is representative of the collected spectra. The peaks of the x = 1 sample (γ-FeCrO3) are sharp and correspond to the minimum lattice parameter of this study. For both x < 1 and x > 1, the lattice parameter increases and peaks broaden. Forbidden (superlattice) reflections were not observed. The XRD peak broadening is primarily associated with an inhomogeneity in the dimensions of the unit cells produced by spatial variations in the amount and distribution of Cr3+ and Fe3+ cations and vacancies. The Rietveld refinement results for the x = 1 composition are shown in Table 2. The peak broadening analysis led to the following microstructural results:

3. RESULTS AND DISCUSSION SEM, EDS, and TDA Characterization. In Figure 2 the characteristic morphology of the powders is shown. By this synthesis method, dark brown, sponge-like samples were obtained. It is interesting to note that the addition of ammonium nitrate is indispensable for the combustion. Using glycine as the fuelinstead of methoxyethanolproduces C


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Inorganic Chemistry

Figure 3. XRD of γ-Fe2−xCrxO3 samples with (x = 0.75, 1.00, 1.25). Inset: Details of the reflection 440.

Table 2. Structure of γ-FeCrO3a O Fe Fe Cr a

X(σx)

Y(σy)

Z(σz)

B(σB)

Occ (σocc)

Mult

0.2573(3) 0.1250

0.2573(3) 0.1250

0.2573(3) 0.1250

0.65(9) 0.56(6)

32 8T

0.5000

0.5000

0.5000

0.44(5)

32 7.76(3) 2.90(3) 10.66667

16 O

Space group Fd−3m, lattice parameter a = 8.2881 (1) Å.

Average crystallite size: 758.4 (3) Å Average microstrain: 11.38 (5) %% (parts in ten thousands) Research on the distribution of Fe3+ cations between tetraand octahedral sites led to an interesting result given in Table 1: About 3% (=0.24/8) vacancies are in tetrahedral sites. This result is consistent with that reported by Jorgensen26 for pure iron maghemite and represents a subtle difference to the approximate model proposed by Grau-Crespo et al.45 Consideration of the x = 0.75 and 1.25 samples, taking into account the asymmetric peaks’ broadening represented by the inset of Figure 3, leads to the conclusion that these samples are formed by heterogeneous distributions of continuously varying microstructures, presumably associated with compositional inhomogeneity. This result prevents a reliable characterization of their ordering condition by diffraction. Magnetic Properties. Regarding the magnetic properties, the magnetization of these materials has been measured as a function of the applied field at temperatures 2, 100, and 300 K; in all cases the narrow hysteresis loops characteristic of soft ferrimagnetic materials were obtained. As a representative example, the curves obtained for the three composition materials at 2 K are shown in Figure 4: it can be observed that Ms, the saturation values for the magnetization, decrease and the hysteresis loops become wider with increasing Cr3+ contents. The experimental and the calculated values for magnetization are compared in Table 3 and are in quite good agreement, except for the x = 1.25 composition. This suggests that, in this particular sample, there is some disorder in the Fe

Figure 4. Magnetization versus field for the three different samples.

Table 3. Experimental and Calculated Magnetization of γFe2−xCrxO3 Samples Composition

μexp (B.M.)

μcalc (B.M.)

x = 0.75 x = 1.00 x = 1.25

1.27 0.63 0.26

1.33 0.67 0.00

and Cr sublattices. This statement is consistent with the conclusions of the Rietveld analysis. The theoretical values have been calculated using the simple collinear Néel model applied to the more likely distribution of Fe and Cr in the octahedral and tetrahedral sites of the cationic-deficient spinel-type structure: (Fe3+)t[□1/3Fe3+(5/3−4x/3)Cr3+4x/3]O4. According to D


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Inorganic Chemistry

Figure 5. (a) Comparison of XANES Cr K edge spectra of γ-FeCrO3 and model compounds. (b) Comparison of FEFF theoretical and experimental Cr K-edge XANES spectra of γ-FeCrO3 for x = 1.0. (c) Comparison of the FEFF theoretical average of the three modeled phases for octahedral and tetrahedral iron absorbing sites, and experimental γ-FeCrO3 Fe K-edge XANES spectra of γ-FeCrO3. When close observation of the experimental pre-edges features was carried out, the theoretical doublet at 7118−19 eV was also detected.

this model, the Cr3+ cation should go to octahedral positions (3 μB ↑), on the grounds of higher crystal field stabilization, while Fe3+, a cation in a high spin configuration, would be in either octahedral (5 μB ↑) or tetrahedral sites (5 μB ↓).46 X-ray Absorption Fine Structure. Figure 5 presents a selection of the results from the XAXES experiments and their corresponding interpretation. Both pre-edge analysis and theoretical modeling strongly support the XRD results presented above. Figure 5a shows the general view of obtained Cr K-edge XANES spectra and Cr model compounds. In measurements of the XANES spectra, performed in the same session, it was verified that the first inflections of the main edge in maghemite samples are at the energies 5998.2, 5999.4, and 5998.8 eV, for Cr contents of x = 0.75, 1.00 and 1.25, respectively; for Cr2O3 (corundum-III) and CrO2 (rutile-IV), the main edge positions are at 5999.4 and 6001.3 eV, respectively. As the measurements in the XANES region were perfomed with steps ΔE = 0.25 eV, the uncertainty of the main edge determination may be considered to be about 0.5 eV, and the energies 5998.2, 5999.4, and 5998.8 eV, as well as 5999.4, are essencially equal, corresponding to the oxidation state Cr3+. Also, the pre-edge peak in the maghemite spectra has a low intensity, below that of the CrO2 (rutile-IV) model compound. Thus, the Cr3+ oxidation state in the Oh environment at the γ-Fe2−xCrxO3 compounds18,47,48 is confirmed. In the theoretical modeling of Cr and Fe K-edge XANES spectra, computed by FEFF8.4,36 some simplifying considerations were adopted. In the model, vacancies are distributed so that a representative array of atom-vacancy distances is generated around the absorbing atoms. Octahedral positions were considered as occupied by Cr and vacancies. Tetrahedral sites were fully occupied by Fe cations. Vacancies were

described by null occupancies. In part b of Figure 5 are presented the theoretical XANES Cr K-edge spectrum of γFeCrO3, together with the experimental one. All the features of the XANES experimental spectra are reproduced by the model. Results corroborate the Cr3+ character of this ion in these samples, as well as their octahedral occupation. The study of Fe3+ in octahedral and tetrahedral sites using XANES spectra was performed also by both pre-edge decomposition49 and theoretical modeling via FEFF8.4.36 Experimental XANES spectra are very similar to those reported in XANES studies of γ-Fe 2 O 3 . 49−51 Pre-edge analysis information practically coincides with the results presented in the literature49 for natural maghemite. The experimental spectra for the three studied compositions, together with a theoretical spectrum, are presented in Figure 5c. The theoretical spectrum was obtained by weighting FEFF simulated spectra using the relative population of each modeled configuration. The pre-edge feature of the Fe K-edge for the γFe2−xCrxO3 compounds shows relatively wide shape and high intensity in the experimental spectra, corresponding to the doublet observed in the spectrum modeled by FEFF8.4. This fact confirms the “mixed” occupation of both tetra- and octahedral sites by the Fe cation. Taking into account some smoothing introduced by the experiment, it may be noticed that all features of experimental spectra have the corresponding signal in the FEFF8.4 theoretical spectrum. The results of XANES analysis about the occupancy of Fe3+ and Cr 3+ required fitting the EXAFS spectra with a configuration where the central absorbing atom of Fe is able to occupy both tetrahedral and octahedral positions, together with Cr absorbing atoms occupying only the octahedral sites. As XRD patterns do not show any superstructural peaks meaning that the long-range order in the iron−chromium E


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Inorganic Chemistry

Figure 6. Left: Succesive models for EXAFS interpretation and vacancies occupation. In the third model, S.G. P4332, the octahedral sites L1, L4, L7, and L10 at Wyckoff 4b sites, shown in blue, are always occupied by either Cr3+ or Fe3+. Right: (a) Octahedral and tetrahedral sites, with interatomic distances from the corresponding cations to first neighbor oxigen. (b) Interatomic distances from tetrahedral Fe3+ core to surrounding ions. (c) Interatomic distances from the octahedral 4b sites Fe3+ core to surrounding ions. (d) Interatomic distances from the octahedral Cr3+ core to surrounding ions.

Figure 7. Final fitting of Fe and Cr K-edges EXAFS Fourier transform (left column) and k3 weighted (right column) experimental spectra with the P4332 cubic structure theoretical standard, as described by Grau-Crespo et al.45

vacancies. Therefore, the correctness of the model of disordered or ordered vacancies in the octahedral sites as a trend in the local order of the maghemite γ-FeCrO3 was tested. N, R and σ2 were determined for x = 1 by the processing of average spectra in the EXAFS region in both Fe and Cr K-edges simultaneously. The EXAFS spectra were fitted with theoretical standards considering the facts that the central absorbing Fe

maghemite (γ-FeCrO3) is cubicthe examination of order at local scale is required. Grau-Crespo et al. in45 provided theoretical arguments about a favorable electrostatic contribution of Fe3+ cations in the pure iron maghemite, with an ordered supercell configuration based on the P4332 cubic structure, which exhibits the maximum possible homogeneity in the distribution of iron cations and F


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Inorganic Chemistry

Table 4. Comparison of XRD and EXAFS Fitting Results for Distances (given in Å) between Absorbing Cores and the Corresponding 1st or 2nd Neighbors in FeCrO3 Maghemitea EXAFS common fitting parameters S02

core

Fe K-edge first neighbors second neighbors

Cr K-edge first neighbors second neighbors

E

0.245 (0.105)b 0.273 (0.094)b 0.620 (0.041)b EXAFS - P4332 Ordered vacancies at sites 4b

Fe_T Fe_O Cr_O

−7.035 (3.740)b −1.884 (2.316)b 2.380 (0.647)b Fd3̅m (Rietveld) a = 8.2881

core

site

r

Δr

Δr/r

r

Tetrahedral Octahedral Tetrahedral Tetrahedral Tetrahedral Tetrahedral Octahedral Octahedral Octahedral Tetrahedral Octahedral

O1 O6 CrOc1 FeOc1 O2 FeTe1 CrOc3 FeOc3 FeTe3 O3 O9

1.824c 1.927c 3.389 3.423 3.265c 3.578 3.044 2.882 3.473 4.324 4.501

-0.075c -0.086c −0.047 −0.013 -0.191c −0.010 0.114 −0.049 0.037 −0.123 −0.106

-0.039c -0.043c −0.014 −0.004 -0.055c −0.003 0.039 −0.017 0.011 −0.028 −0.023

1.8992 2.0133 3.4361 3.4361 3.4558 3.5889 2.9303 2.9303 3.4361 4.4472 4.6072

Octahedral Octahedral Octahedral Octahedral Octahedral Octahedral

O11 CrOc6 FeOc5 FeTe5 O12 O13

1.971c 2.963 2.943 3.443 3.581 3.577

-0.042c 0.032 0.013 0.007 0.097 −0.048

-0.021c 0.011 0.004 0.002 0.028 −0.013

2.0133 2.9303 2.9303 3.4361 3.4841 3.6251

a The information is shown for the Fe3+ absorbent cation in tetrahedral or octahedral positions and for the Cr3+ absorbent core. σ2 = 0.001 Å2 fixed in all dispersion paths. bUncertainties for S02 and E are shown in parentheses. cDistances given in bold and italic are those with relative uncertainties 22% or less and about 50%, respectively.

occupied by either Cr3+ or Fe3+, and the rest of 4b sites in the P4332 cubic structure contain the vacancies. In Figure 6 right, distances FeOc3 (which is equal to CrOc3) and CrOc6 (= FeOc5) are not shown, neither is presented the distance FeTe1 = 3.5889 Å; site O10 is not generated by the P4332 symmetry. When successive models were introduced, the R factor of the fitting was improved from 0.009 to 0.004, while other parameters of fitting were improved five times, with the final reduced χ2 = 498. In Table 3 we can observe that the cations’ distance to the nearest oxygen contracts. This contraction is 4% and 2% in iron and chromium, respectively, with confidence. The distance of Fe cations to their neighbors in the second sphere is contracted about 0.3 to 3%, as a trend. The exception is the distance to the nearest Cr, which moves away from 0.4 to 4%. As tendencies, chromium cation distances to the nearest cations expand, in 0.2% to tetrahedral Fe sites and in 0.4% when the neighbor is an octahedral Fe. The distance between chromium cations expands about 1.1% of the average Fe−Cr atom at the octahedral position. These results can be compared to the ionic radii, whether crystalline or effective, of oxygen, Cr3+, and Fe3+ in high-spin configuration, which are given by Shannon.52 According to this author, the sums of the radii of the cation with the corresponding first neighbor oxygen are equal to 2.015 and 2.045 Å for Cr3+ and Fe3+, respectively. Assuming these values, the relative contraction of the interatomic distance of Fe3+ draws attention. This model is, however, supersimplified, and is only pinpointing that this topic might be an interesting subject for quantum-mechanical investigations.

cation is able to occupy both types of sites, while Cr occupies only the octahedral ones. Each occupation weight was adjusted by the amplitude parameter S02. In the present EXAFS modeling of experimental Fe and Cr K-edges spectra, fitting of successive coordination distances up to 6.00 Å, coordination number of neighboring atoms and site occupancies-vacancies were strategically being constrained. On the left side of Figure 6 the successive steps of the EXAFS modeling are presented. The model constrains all vacancies in octahedral sites, giving 16.6% as open sites. The first model consisted of an average occupation of Fe3+-vacancy or Cr3+- vacancy at any octahedral site. The second model consisted of a “gray” Fe3+-vacancy or Cr3+- vacancy, occupying only the Wyckoff 4b sites of the F4132 symmetry. These sites are shown in Figure 6 as gray atoms. The third and final step considered that the cation vacancies tend to be located orderly within the structure of the iron−chromium maghemite (γFeCrO3) at Wyckoff 4b sites L2, L3, L5, L6, L8, L9, L11, and L12,45 and especially removed from the possible dispersion path. They are given in the Figure 6 as blue atoms. The images showing the experimental EXAFS spectra and theoretical standards are given in Figure 7. The last fitting results are presented in Table 4. The absorbing core-neighbor interatomic distances in the maghemite spinel are presented on the right side of Figure 6. Identification of distances corresponds to those presented in Table 4. Cr−Fe cations in octahedral sites are presented always in magenta, the Cr−Fe Wyckoff 4b octahedral sites are always in gray and Fe tetrahedral sites are represented in red. In the final model, the octahedral Wyckoff 4b sites L1, L4, L7 and L10 are always G


Article

Inorganic Chemistry

Nacional de Ciencia y Tecnologıá of Mexico, Project CONACYT CB-166366 are gratefully acknowledged by M. Garcı ́a-Guaderrama, and Projects CONACYT CB-46515 and CONACYT-CNPQ 174391 are gratefully acknowledged by M. E. Montero-Cabrera and L Fuentes Cobas. M. E. Montero Cabrera also wishes to thank Joshua Kas from the University of Washington and Rodrigo Domı ́nguez from CIMAV for their help with FEFF calculations.

The potential of Mossbauer spectroscopy (MS) for the study of magnetic materials based on iron is well-known. The application of the MS to the studied compounds could corroborate the oxidation state of iron in the samples and the occupation of tetrahedral and octahedral sites for Fe3+. From the study presented by da Costa et al.53 in pure and aluminum substituted maghemites, a complex Mossbauer spectrum can be expected: the occupation of both types of sites would produce MS spectra with several overlapped sextets. However, information on the short-range order of Cr, Fe, and vacancies is not expected, nor are details of the occupation of the tetrahedral sites by Fe or Cr.

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4. CONCLUSIONS By using solution-combustion synthesis, maghemite-like materials containing iron and chromium γ-Fe2−xCrxO3 (0.75 ≤ x ≤ 1.25) have been prepared. Their magnetic properties for x = 0.75 and x = 1.00 samples agree with the collinear Neel model for a ferrimagnetic material. These maghemites with Fe3+ and Cr3+ are more thermally stable than the chromiumfree ones. The study of chromium maghemites γ-Fe2−xCrxO3 in the vicinity of x = 1, using high resolution diffraction and XANES on a synchrotron light source, has allowed the characterization of the oxidation states and the crystal structure at local and global levels. On the whole, by XRD, it has been found that the symmetrical composition x = 1 produces the most uniform and unstrained samples. Chromium ions occupy only octahedral sites, while iron occupies octa- and tetrahedral positions. Vacancies are located mostly at the octahedral sites, with a small but detectable proportion of tetrahedral locations. For the case of the symmetrical composition of the Fe−Cr maghemites, both pre-edge analysis and theoretical modeling by the FEFF 8.4 code of XANES spectra for Fe- and Cr-K edges strongly support the XRD results. The results of fitting the experimental EXAFS spectra with theoretical standards obtained by the FEFF 8.4 code showed that the maghemite cation vacancies tend to follow a regular pattern within the structure, as in the model suggested by Grau-Crespo et al.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. The oxides’ synthesis was performed at Departamento de Quı ́mica Inorgánica I, Facultad de Ciencias Quı ́micas, Universidad Complutense de Madrid. E. Morán and M. A. Alario-Franco gratefully acknowledge both the “Comunidad de Madrid” (S2013MIT-2753 program) and the Spanish Ministerio de Economiá y Competitividad (Project MAT2013-44964-R) for financial support. Funds from Consejo H


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