A Heterogeneous Distribution of the Oxidation States - ACS Publications

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Magnetic and Magnetocaloric Properties of SrFe0.5Co0.5O3‑y upon Oxidation and Reduction: A Heterogeneous Distribution of the Oxidation States O. Toulemonde,*,† J. Abel,† C. Yin,† A. Wattiaux,† and E. Gaudin† †

ICMCB, CNRS, Université de Bordeaux, 87 avenue du Dr. A. Schweitzer, Pessac, F-33608, France ABSTRACT: The electrochemical oxidation of SrFe0.5Co0.5O3‑y in alkaline solution was used to enhance its oxygen content. As a result of chemical titrations and Mössbauer spectroscopy studies, it is shown that the distribution of cobalt and iron oxidation states are heterogeneous, indicating that oxidation of cobalt cations requires higher oxidation potential than for the iron ones. Consequently, the random distribution of Co4+ and Fe4+ often considered is wrong. When oxygen content increases, a paramagnetic to ferromagnetic transition is evidenced at 305 K for SrFe4+0.50Co3+0.34Co4+0.16O2.83±0.02. The study of its magnetocaloric effect was performed, and special attention was paid to the air aging effect. Despite the small amount of oxygen release that slightly shifts toward a lower temperature of the ferromagnetic transition, the remarkable stability of its refrigerant capacity is highlighted. It suggests that a high content of Co4+ oxidation state is kept. KEYWORDS: Mössbauer spectroscopy, ferromagnetism, SrFe1‑xCoxO3, oxygen deficiency, electrochemical oxidation, aging effects, magnetocaloric effects

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INTRODUCTION Transition metal oxides ABO3 and A2BO4 with perovskite-type and K2NiF4 type structures are fundamentally and technologically very attractive because of their ability to stabilize a wide range of transition metal with various valence states. Consequently, they exhibit remarkable physical properties that can be tuned varying and/or mixing the chemical composition at the A and B crystallographic sites. Such modulation of the physical properties is caused by the presence of competing interactions involving the interplay between the lattice and the orbital and/or the charge and/or the spin degrees of freedom of the constituent transition metal ions. In this view another interesting approach is to play with the oxygen stoichiometry for a given metallic framework. This opens not only a way toward oxygen storage applications1 but also toward a huge change of the physical properties such as metal/insulator,2 semiconductor to superconductor,3 and antiferromagnetic to ferromagnetic transitions.4 Recently, order/order or disorder/order magnetic phase transitions were the subject of extensive studies because of the demand for environmentally aware science. In fact, research into materials showing large magnetocaloric effects close to room temperature is one of the areas currently being explored to develop new refrigeration systems.5 In the present work, we focused on the electronic and magnetic properties of SrFe0.5Co0.5O3‑y oxides which exhibit a ferromagnetic transition around room temperature. Almost fully stoichiometric SrFe0.5Co0.5O3 was originally synthesized either by high oxygen pressure by T. Takeda and H. Watanabe6 or by electrochemical oxidation by P. Bezdicka and co-workers7 and is known to display a high Curie temperature. When y values are lower than about (0.20), the ferromagnetic transition is kept and the Tc value decreases when y increases. They are categorized as © 2012 American Chemical Society

perovskites with unusual high transition metal oxidation states. Interestingly, the large oxygen nonstoichiometry domain that ranges from y = 0 to 0.5 gives rise to superstructure phases due to the oxygen vacancies ordering as recently proposed with y = 0.11.8 Here, a phase SrFe0.5Co0.5O3‑y with a y-value adjusted to get a Tc value close to room temperature was prepared. It will be pointed out the remarkable stability of the magnetocaloric effect (MCE) of SrFe0.5Co0.5O3‑y around room temperature over time despite the small amount of oxygen release due to an air aging process. Furthermore, as a result of chemical titrations and Mössbauer spectroscopy, we report, for the first time, that homogeneous oxidation (or reduction) of Fe and Co does not occur when the oxygen content increases (or decreases).

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EXPERIMENTAL SECTION

Synthesis and Oxidation. The first step of the synthesis was done by the Pechini route.9 This soft chemical process is based on the formation of a polycationic resin. Polycrystalline ceramic with an oxygen deficiency y is obtained after heat treatment of this precursor resin at 1173 K during 40 h under air with intermediates grinding. To produce the so-called as-synthesized sample, the powder was pressed into pellets and sintered at 1323 K under air. Then, in a second step, oxygen is intercalated into the oxide networks at room temperature thanks to electrochemical oxidation in a galvanostatic mode with a constant current intensity of 100 μA. Details of the experiments have already been published.7,10 A part of the as-synthesized product was also postannealed at 873 K in flowing O2 gas for 20 h to obtain a reference with a controlled oxygen deficiency y = 0.25 determined by potassium bichromate titration as previously described in ref 11. Received: November 23, 2011 Revised: February 16, 2012 Published: February 21, 2012 1128

dx.doi.org/10.1021/cm2035079 | Chem. Mater. 2012, 24, 1128−1135

Chemistry of Materials

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Powder X-ray Diffraction. A Philips PW 1820 apparatus equipped with a monochromatized Cu Kα1 source was used to evaluate crystalline purity and to determine the cell parameters. Diffraction patterns were collected from 7° to 130° with a 2θ step of 0.015° with a counting time of 1300 s per step for advanced structural investigations. A full pattern matching was performed on the four diffractograms, using the JANA2000 software, to check the purity of the samples and determine the cell parameters. The background was estimated by a Legendre function, and the peak shapes were described by a pseudo-Voigt function and asymmetric parameters. Chemical Titration and Mö ssbauer Spectroscopy. The concentration of anionic vacancies (y) was deduced from the chemical determination of the mean oxidation state of iron and cobalt. The samples were dissolved in an acidic medium (HC1 3N) with an excess of Mohr salt (Fe2+). TM4+ concentration was deduced from the amount of remaining Fe2+ ions determined by back-titration with a N/ 1O K2Cr207 solution11 according to the following reactions and assuming a chemical formula Sr2+Fe3+0.5‑τ′Fe4+τ′Co3+0.5‑τ′′Co4+τ′′O3‑y

τ′Fe4 + + τ′Fe2 + → 2τ′Fe3 + τ′′Co4 + + 2τ′′Fe2 + → τ′′Co2 + + 2τ′′Fe3 + (0.5‐τ′′)Co3 + + (0.5‐τ′′)Fe2 + → (0.5‐τ′′)Co2 + + (0.5‐τ′′)Fe3 + and to relation due to the electroneutrality y = (1‐τ′‐τ′′)/2 Because the Mohr salt titration only gives the sum of the content of Fe4+ and Co4+ concentration, we then perform Mössbauer spectroscopy as a chemical selective probe to access the relative percentage of Fe3+ and Fe4+ and to deduce the relative percentage of Co3+ and Co4+. Mössbauer measurements were performed with a constant acceleration Halder type spectrometer using a room temperature 57Co source [Rh matrix] the transmission geometry. Isomer shift values refer to α Fe at 293 K. The spectra were analyzed using standard computer software. Physical Properties. Magnetization measurements were performed using a Superconducting Quantum Interference Device (SQUID, quantum design) under a dc field. For both temperature and field dependence studies, the samples were introduced at room temperature and heated up in zero-field at 5 K/min until 375 K was reached. Then they were cooled down with or without any applied field magnetic field at a rate of 5 K/min until the targeted temperature. The measurements were performed on heating up mode at fixed applied magnetic field. The magnetic loops at fixed temperature were alternatively measured increasing and decreasing the applied magnetic field and collected by step of 5 K or 2.5 K around TC.

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RESULTS AND DISCUSSION The oxygen stoichiometry of the oxides synthesized after annealing under air and after oxygen intercalation was determined by potassium bichromate titration. It is shown that the atomic percentage of the 3d transition metal cation with a positive formal charge of 4 as-called TM4+ species increases from 30% to 66% during the electrochemical oxidation process. It gives the following nominal stoichiometry SrFe0.5Co0.5O2.68±0.02 and SrFe0.5Co0.5O2.83±0.02 respectively. Figure 1 presents the X-ray powder diffraction patterns of the as-synthesized sample with the nominal stoichiometry SrFe0.5Co0.5O2.68±0.02 (a) of the sample with the nominal stoichiometry SrFe0.5Co0.5O2.83±0.02 immediately after the oxygen intercalation process (b) and of the same sample kept in air for one year with the nominal stoichiometry SrFe0.5Co0.5O2.83‑y (c). All the phases show similar diffraction patterns, but the position and in a smaller manner the intensity of some peaks are slightly different. Despite the very small peak due to the nonmagnetic secondary SrCO3 phase (main peak at 2θ = 25.25°), the X-ray diffraction pattern reveals that the oxides are a single cubic phase with a Pm3̅m space group as previously

Figure 1. Observed (cross), calculated (solid line), and difference (bottom) X-ray diffraction patterns for (a) the “as-synthesized, (b) the post oxygen intercalated, and (c) the one year air aging ones. Inset: zoom of the (111) and (103) peaks with the observed, calculated, and difference pattern.

reported.12,13 Full pattern matching analysis allows the determination of the lattices parameters that go from a = 3.86833(4) Å for the as-synthesized to a = 3.83611(11) Å after the electrochemical oxidation process and to a = 3.8385(2) Å one year after. Note that the crystallographic phase of SrFe0.5Co0.5O2.75±0.02 used as a reference (y = 0.25) is also cubic with a cell parameter a = 3.85794(5) Å, slightly lower than the one refine for the “as-synthesized” phase. Such variation of the parameters was expected considering the 1129



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Figure 2. Mössbauer spectra collected at room temperature: (a) for the “as-synthesized, (b) the post oxygen intercalated, (c) after room temperature air aging, and (d) after annealing under O2 at 873 K. Dot and solid lines are the experimental and fitted data using the parameters of Table 1 respectively. 1130



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Table 1. Selected Mössbauer Parametersa studied samples as-synthesized under air annealed under O2 electrochemically oxidized after aging process

DIS

δ (mm·s−1)

Δ or ε (mm·s−1)

Γ (mm·s−1)

Hhf (T)

relative abundance (%)

A B A B A B A B C

0.11 0.15 0.09 0.11 0.03 0.03 0.04 0.39 0.05

0.30 0.75 0.72 0.31 0.29 −0.0660 1.27 1.07 +0.0001

0.25 0.25 0.20 0.20 0.30 0.35 0.25 0.25 0.25

/ / / / / 10.8 / / 0.7

44 56 33 67 40 60 39 15 46

δ, isomer shift; Δ, quadrupole splitting; or ε, quadrupole interaction parameter; Γ, Full Width Half Maximum of the Lorentzian; Hhf, hyperfine field, derived from the evaluation of the data of studied samples. a

Table 2. Proposed Oxidation State for Cobalt Cationa studied samples as-synthesized under air annealed under O2 electrochemically oxidized after the aging effect

(mm·s−1) 0.13 0.10 0.03 0.10

± ± ± ±

0.01 0.01 0.01 0.01

average oxidation state for iron 3.72 3.80 4.00 3.80

± ± ± ±

0.03 0.03 0.03 0.03

concentration on ionic vacancies

deduced oxidation state for cobalt

± ± ± ±

3.00 ± 0.05 3.20 ± 0.05 3.32 ± 0.05 unknown (see text)

0.32 0.25 0.17 0.25

0.02 0.02 0.02 0.05b

a

Deduced combining the average oxidation state for the iron cations given by the Mössbauer spectroscopy and the concentration on anionic vacancies given by the Mohr salt titration. bThe value here is an estimate as discussed on the text.

reference. To better understand the observed changes, we considered a distribution of isomer shifts to fit the Mössbauer spectra as previously reported.15 Table 1 shows the selected Mössbauer parameters derived from the fits of the data at room temperature for the studied oxide samples. From these values, the Fe3+/Fe4+ ratio can be deduced for the as-synthesized oxide, and the following formula Sr2+Fe3+0.14Fe4+0.36Co3+0.50O2.68±0.02 is proposed in relation with the chemical titration (Table 2). For the calculation of the oxidation states of iron the average isomer shifts for Fe3+ and Fe4+ were considered equal to 0.39 and 0.03 mm·s−1 respectively given for the following equation x = −2.875 + 4.11 where is the average chemical shift estimated by a weighted sum of the two iron cation abundance. Contrary to previous assumptions (see for example the work performed by Swierczek et al.8), this result suggests a heterogeneous oxidation of the iron and cobalt cations where the iron is preferentially oxidized. This behavior is unambiguously confirmed for the reference sample annealed under oxygen and for the electrochemically oxidized one in light of their charge balance Sr2+Fe3+0.10Fe4+0.40Co3+0.40Co4+0.10O2.75±0.02 and Sr2+Fe4+0.50Co3+0.34Co4+0.16O2.83±0.02 (Table 1). These cationic distributions shed light that cobalt requires higher oxidation potential to fully adopt the tetravalent state and that the solid oxido-reduction reaction Co4+ + Fe3+ → Co3+ + Fe4+ likely occurs until all iron species are fully tetravalent. It provides new evidence for solid state oxido-reduction competition as recently observed on LaMn0.5Co0.5O316and La0.7Sr0.3Mn1‑xCuxO3 (0.0 ≤ x ≤ 0.30)17 compounds. Another point that can be relevant through these results is the evolution of the magnetic properties at room temperature. After the electrochemical oxidation, the phase exhibits two components according to the fitted data: (i) a broad paramagnetic single line with a relative abundance of 40% and (ii) an ordered quadruplet with a magnetic hyperfine field of 11.5T accounting for 60% (Figure 2b and table 1). Both show an isomer shift δ = 0.03 ± 0.01 mm/s characteristic of the presence of only Fe4+. After the aging process, contrary to the “fresh” oxidized, only a dissymmetric line appears at room

lowering (increase) of the average ionic radii upon oxidation (reduction). However, it is not possible to get a direct relation between the a cell-parameter and the y-value of the oxygen stoichiometry because of the inhomogeneous distribution between Fe4+ and Co4+ which is dependent on the synthesis routes that will be discussed later. If a cubic cell is observed for the three samples, it can be noticed that a broadening of some peaks is observed for both the “fresh” and the “old” oxidized samples with respect to the as-synthesized one. This is especially the case for the (111) and (002) peaks; a zoom of the first one is given in Figure 1 for each refinement. The broadening of these peaks does not concern the same crystallographic directions and cannot be attributed to an usual cell distortion. Indeed, with the tetragonal symmetry of Sr8Fe8O23 proposed by several authors8,14 there is no splitting of the (0 4 2) peak which corresponds to the (111) peak in the cubic symmetry, whereas, for instance, a splitting of the peak corresponding to the (103) reflection is observed. For this latter reflection, as shown in Figure 1, it is clear that for our samples such distortion is not observed. An explanation of this behavior could be given considering thermodynamic arguments. To reveal any oxygen vacancies ordering in the ́ Sr8Fe4Co4O23 phase, Swierczek and co-workers8 performed an oxygenation under 180−200 bar of oxygen pressure at 410− 500 °C. Within our work, the oxygen vacancies are likely disordered on the as-synthesized phase. Consequently, they cannot be filled in an ordered manner during the intercalation process at room temperature. Transmission Electron Microscopy studies would be helpful to check if diffuse scattering are observed and in which crystallographic directions. Therefore, our analysis demonstrates the relatively small impact of the oxygen release taking place at ambient temperature in air within one year on the X-ray powder diffraction pattern. As a local probe, the Mössbauer spectroscopy study was carried out on the different phases at room temperature to identify the local environment of the iron cations and to determine their average oxidation state (Figure 2). For the discussion, we also add the spectrum measured on the SrFe0.5Co0.5O2.75±0.02 phase used as 1131



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temperature as shown in Figure 2c. The Mössbauer parameters present then two significant changes with respect to the fresh one (Table 1). First, the nonmagnetic contribution (A site) is now split into the two sites A and B corresponding to the oxidation state Fe4+ (isomer shift δ = 0.04 ± 0.01 mm/s) and Fe3+ (δ = 0.39 ± 0.01 mm/s), respectively. The oxidation state of the iron goes from 100% of Fe 4+ just after the electrochemical process to 85% of Fe4+ when room temperature air aging process occurs. Second, the relative abundance of the Fe4+ magnetic contribution (site B immediately after the electrochemical oxidation and site C after the aging process) drops from 60 to 46% in parallel with the magnetic hyperfine field that drops from 10.8T to 0.7T. The second novel point that can consequently be underlined from our Mössbauer spectroscopy study is that Fe3+ cations do not participate in the magnetic ordering, only the Fe4+ ions are magnetically active. Therefore, the oxygen release under air condition impacts significantly the electronic distribution of the TM cation. Unfortunately, the amount of oxidized sample powder kept for one year is too small to process any chemical titration. The determination of its nominal stoichiometry was not doable, and only a range of it can be given considering the two extreme cases (i) in parallel to the reduction of the Fe4+ cation, all the Co4+ cation are also reduced in Co3+ for a given nominal stoichiometry Sr2+Fe3+0.10Fe4+0.40Co3+0.5O2.70 (ii) the Co4+/Co3+ ratio is kept constant and only the Fe4+ is reduced for a given nominal stoichiometry Sr2+Fe3+0.10Fe4+0.40Co3+0.34Co4+0.16O2.78 An average oxygen deficiency y = 0.25 can be roughly deduced from these two limit compositions and considering the value in Table 2. That is why the sample with the nominal composition Sr2+Fe3+0.10Fe4+0.40Co3+0.40Co4+0.10O2.75±0.02 was synthesized. Interestingly, the cubic lattice parameter a = 3.8385(2) Å of the aged phase is closer to the one obtained just after the electrochemical oxidation (a = 3.83611(11) Å for y = 0.17) than to the one used as reference (a = 3.85794(5) Å for y = 0.25). Furthermore, no magnetic contribution was observed o n t h e M ö s s b a u e r s p e c t r u m o f o u r r e f e r e n c e Sr2+Fe3+010Fe4+0.40Co3+0.40Co4+0.10O2.75±0.02 contrary to what is observed on the aged oxide. Consequently, the oxygen deficiency is likely smaller than 0.25 on the aged oxide. It would suggest that the iron cations are affected by the reduction process more than the cobalt cation. This assumption would lead to the following charge balance Sr2+Fe3+0.10Fe4+0.40Co3+0.34Co4+0.16O2.78. In-situ chemical selective probe as X-ray spectroscopy experiment are necessary to better understand this point. From a practical standpoint, magnetization temperature dependence for SrFe0.5Co0.5O3‑y phases was carried out to get a better indication of the impact of the oxygen release/intake on the order/disorder magnetic transition (Figure 3a,b). Both the amplitude and the order/disorder temperature phase transition are enhanced when oxygen is intaken. Turning first to the as-synthesized sample Sr2+Fe3+0.14Fe4+0.36Co3+0.50O2.68±0.02 (bottom Figure 3a), a peak is observed at Tg ∼ 80 K on the ZFC process, and it is followed by a plateau on the FC process. This strongly suggests the magnetic phase’s coexistence with ferromagnetic clusters in a weakly magnetic matrix or a spin glass phenomenon when a concentrated nanoparticles system is considered.18 The thermal hysteretic behavior associated with the first feature significant of

Figure 3. (top): Temperature-dependent magnetization for SrFe0.5Co0.5O3‑y (y = 0.32 at bottom and 0.25 at top) on both “zero field cooling” (ZFC) and “field cooling” (FC) process under 100 Oe. The vertical dot line is a guide for eyes underlining the lowering of Tg. (bottom): Temperature-dependent magnetization for SrFe0.5Co0.5O3‑y (y = 0.17 and after the aging process) on both “zero field cooling” (ZFC) process under 200 Oe. Left bottom inset: isothermal magnetization at 5 K for the electrochemically oxidized and after the aging process. Right top inset: derivative curve of the temperaturedependent magnetization for the electrochemically oxidized and after the aging process.

short-range ordering and defined by the minimum of dM/dT at TSRO ∼ 235 ± 5 K was expected in view of similar measurements carried out on the SrFeO3‑y system.19 But, whereas it is associated with a mixture of tetragonal Sr8Fe8O23 and orthorhombic Sr4Fe4O11 phases on the SrFeO3‑y system,15 our XRD analysis does not reveal any phase separation within the X-ray limit. Such magnetic behaviors are likely due to the large distribution of the local crystallographic environment (octahedral, square based pyramid, and tetrahedral site) at the perovskite B site associated with the heterogeneous distribution of cobalt and iron cations on these environments and with their own oxidation state distribution as previously seen by M ö s s b a u e r s p e c t r o s c o p y . I n t e r e s t i n g l y , f o r t h e Sr2+Fe3+0.10Fe4+0.40Co3+0.40Co4+0.10O2.75±0.02 phase (top Figure 3a), the flattening of the FC curve is slightly lost in parallel with the lowering of Tg and the increase of TSRO to ∼255 ± 5 K and the increasing of the magnetic amplitude. All these points indicate a lower intensity in cluster interactions benefit to longer ordering range with increasing oxygen content. From our coupled Mössbauer spectroscopy and chemical titration studies showing a continuous increase of the Co4+/Fe4+ ratio when oxygen is intaken (see the proposed nominal stoichiometry Sr 2+ Fe 3+ 0.14 Fe 4+ 0.36 Co 3+ 0.50 O 2.68±0.02 and 1132



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Figure 4. Isothermal magnetic field dependence of the magnetization at various temperatures for SrFe0.5Co0.5O2.83 (a) and after room temperature air aging process (b) with their respective Arrott formalism plot on log/log scale (c) and (d). The temperature intervals are shorter around TC.

dT.21,22 Consequently, given that Fe3+ do not participate in the magnetic ordering as pointed out by the Mössbauer spectroscopy study, the ferromagnetic Fe4+-O(2p)-Co4+ interactions compete with Fe4+-O(2p)-Co3+ and Fe4+-O(2p)-Fe4+ antiferromagnetic super exchange interactions when the air aging effect occurs. Such ferromagnetic/antiferromagnetic transition has already been described when x increases for SrCo1‑xFexO3.23 Furthermore, the divergence of the magnetic properties between Sr2+Fe3+0.10Fe4+0.40Co3+0.40Co4+0.10O2.75±0.02 and the old oxidized phase further suggest that, as the oxygen intake act first on the iron cations during the oxidation process, the oxygen release would also essentially act on them to keep a significant number of Co4+ cations in support of the ferromagnetic Fe4+-O(2p)-Co4+ interactions accounting for the observed ferromagnetic properties. Magnetocaloric properties of SrFe0.50Co0.5O2.83 were determined for two reasons. First, it is expected to show a large MCE near Curie temperature since it shows a relatively abrupt jump at TC under low magnetic field. Second, a magnetization of 1.05 emu/gr at 273 K larger than most of the manganites on about the same experimental conditions (see the value 0.35 emu/gr for La0.7Sr0.3Mn0.98Ni0.02O324 and those in ref 25) reinforces that assumption. To explore the capability of the SrFe0.5Co0.5O2.83 phase as magnetocaloric material, isothermal magnetizations were measured with steps of 500 Oe in the range of 0−50 K Oe with an interval of 2.5 K around TC as shown in Figure 4. They exhibit an almost zero magnetic hysteresis (about 2 Oe), and the largest change in the magnetization occurs at relative low field. All these parameters are desirable if any refrigeration household application is wanted. Another important parameter is the nature of the

Sr2+Fe3+0.10Fe4+0.40Co3+0.40Co4+0.10O2.75±0.02), the volume fraction of ferromagnetic component increases with the appearance of ferromagnetic Fe4+-O(2p)-Co4+, Co4+-O(2p)-Co4+, and Co3+-O(2p)-Co4+ interactions. If one focuses on the electrochemical oxidized phases, then drastic changes are shown, and a paramagnetic to ferromagnetic phase transition is now pointed out as previously observed.12 The Curie temperature (TC) defined by the minimum of dM/ dT (right top inset of Figure 3b) and estimated to be 305 ± 5 K just after the oxygen intercalation is somewhat lower than the one expected for fully oxidized compounds,12,20 and this means that a controlled oxidation of the sample allowed us to adjust the TC around room temperature. It is estimated to 293 ± 5 K after the room temperature air aging process. The magnetic saturation (Msat) is also affected by the aging process since a loss of about 3.5% is displayed on the left bottom inset of Figure 3. The shift toward the lower temperature of the Curie temperature with room temperature air aging was expected in close relation with the Mössbauer spectroscopy studies showing a decrease of the magnetic contribution. The observed trend is believed to be attributed to an enhancement of the magnetic frustration arising from the coexistence of antiferromagnetic and ferromagnetic interactions. Indeed, ferromagnetic super exchange interactions are expected in between Fe4+-O(2p)Co4+, Co4+-O(2p)-Co4+, and Co3+-O(2p)-Co4+ within an ionic model as already discussed in the previous paragraph. However, Co4+-O(2p)-Co4+ and Co3+-O(2p)-Co4+ interactions are not believed to occur for such temperature around 300 K. Indeed, the ferromagnetic states for the fully oxidized samples SrCo4+O3 and La0.20Sr0.80Co3+0.20Co4+0.80O3 are both exhibited at only TC = 275 ± 5 K as estimated from the minimum of dM/ 1133



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Figure 5a gives the thermal variation of the ΔSM curves with magnetic field change from 0T to various applied fields in the [1T−5T] range. Even if it is not shown, the linear dependence of ΔSM(H) as a function of H2/3 further emphasized the second-order character of the magnetic transition under the framework of the mean field theory.29 Indeed, the absolute values of ΔSM(H) are the same on both increasing or decreasing modes. The negative sign on the increasing mode means that applied magnetic field heats up the compound. The MCE at 305 K is not as large as the one shown by the manganites but still significant.30 The Relative Cooling Power (RCP)31 indicates how much heat can be transferred from the cold end to the hot end when a refrigerator describes a thermodynamic cycle. It is estimated from the product of ΔSM(T)max × δTfwhm where δTfwhm is the full width at halfmaximum of ΔS(T) and related to the refrigerant capacity in the appropriate temperature span that is given by the following T2 integration Q = ∫ T1 ΔSM(T,H)∂T. In a first approximation ΔSM(K,T) are fitted using a Gaussian function in order to calculate both RCP and Q. Such an approach was recently used to consider the effect of heterogeneous chemical distribution on the magnetocaloric effect in second-order magnetic phase transition.32,33 Here, the RCP of SrFe0.5Co0.5O2.83 varied with the field from 30 J/kg for ΔH = 1T to 179 J/kg for ΔH = 5T. The average RCP normalized to the applied magnetic field is then estimated at 33.5 J/kg·T which is on the same order of magnitude than most of the manganite showing a magnetic transition in the [305 K−310 K] temperature range but significantly smaller than the best candidates that couple a firstorder magnetic transition with a structural transition.34 However, to our knowledge, no aging effect has been probed yet on such compounds. Here, Figure 5a shows an average shift of −5.5 K in the position of the maximum field-induced magnetic entropy change when air aging effect occurs. This result was expected from the lowering of the Curie temperature. However, when air aging occurs, ΔSM(T)max is slightly lowered whereas δTfwhm is slightly increased likely due to the increase of the heterogeneous chemical distribution as supported by our Mössbauer spectroscopy study. Therefore, both the RCP and the refrigerant capacity are kept constant within the error bar range after room temperature air aging as highlighted in Figure 5b. Such very low drift of the refrigerant capacity emphasized the potentiality of the oxide materials and the interest of the aging effect studies. Finally, it should be noticed that no effort was made to optimize the density of the studied pellet. Since a high degree of crystallinity significantly enhanced the MCE,35 further studies are needed to achieve the maximum performance.

magnetic transition. Isothermal magnetizations measurements were then treated using the mean field theory Arrott formalism H/M versus M2 plots (Figure 4c).26 According to the Barnajee criterion,27 the observed positive gradient means that the magnetic transition is second-order which was indeed supported by the almost zero hysteresis. Since the line at 310 K passes through the origin, a Curie temperature of 310 K is estimated that agrees with the ones defined by the minimum of dM/dT. When an aging effect takes place, the almost zero magnetic hysteresis, the larger change at low magnetic field, and the second-order magnetic transition are kept (Figure 4 b,d). To quantitatively calculate the MCE, we use the Maxwell formalism. From the magnetization data seen in Figure 4, the isothermal field-induced magnetic entropy change from 0 to H around TC (Figure 5) is given by the expression ⎛ T − T2 ⎞ 1 ⎡ μ0H ⎟ = ΔSM ⎜ 1 M(T2 , μ0H )μ0∂H ⎝ 2 ⎠ T1 − T2 ⎢⎣ 0 μ0H ⎤ − M(T1, μ0H )μ0∂H ⎥ ⎦ 0

∫

∫

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CONCLUSION The Curie temperature of SrFe0.5Co0.5O3‑y can be tuned around room temperature by adjusting the oxygen content. Here, our work highlights that a heterogeneous distribution of the oxidation states upon oxidation and reduction takes place. We consequently propose the occurrence of the following solid oxido-reduction reaction Co4+ + Fe3+ → Co3+ + Fe4+ on the oxidation process until all the iron cations are fully oxidized. Even if drastic consequences on the magnetic properties are unambiguously reported, further studies are needed to definitively conclude on the charge balance for the reduction process. Finally, our study shed light that the oxido reduction reactions are greatly important for the understanding of perovskite related oxides.

Figure 5. top: Field-induced magnetic entropy change as function of the temperature for SrFe0.5Co0.5O2.83 (solid symbols and lines) and its change after room temperature air aging (open symbols and dashed lines). bottom: Linear dependence of the estimated refrigerant capacity versus magnetic field. The solid line is a guide to the eyes.

It is important to note that the as-calculated estimation of ΔSM(T) has been confirmed within the framework of the Landau theory minimizing the Gibbs free energy and giving the magnetic equation of state H/M = A+BM2 where A and B are temperature-dependent parameters.28 The parameters A and B were obtained fitting the Arrott plot seen in Figure 4. 1134



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Second, our magnetic property study display that only Fe4+ cations contribute to the parallel magnetic interaction. It turns out that the appreciable magnetocaloric effect observed around room temperature is kept after the room temperature air aging even if TC is slightly shifted toward lower temperature. It is because a significant number of Co4+ cations are kept to support ferromagnetic Fe4+-O(2p)-Co4+ interactions accounting for the observed ferromagnetic properties.

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(22) Pralong, V.; Caignaert, V.; Hebert, S.; Marinescu, C.; Raveau, B.; Maignan, A. Solid State Ionics 2006, 177, 815−820. (23) Okamoto, J.; Mamiya, K.; Fujimori, S.-I.; Okane, T.; Saitoh, Y.; Muramatsu, Y.; Yoshii, K.; Fujimori, A.; Tanaka, A.; Abbate, M.; Koide, T.; Ishiwata, S.; Kawasaki, S.; Takano, M. Phys. Rev. B 2005, 71, 104401. (24) Phan, M. H.; Tho, N. D.; Chau, N; Yu, S. C.; Kurisu, M. J. Appl. Phys. 2005, 97, 103901. (25) Phan, M. H.; Yu, S. C.; Hur, N. H. Appl. Phys. Lett. 2005, 86, 072504. (26) Arrott, A. S. J. Magn. Magn. Mater. 2010, 322, 1047 and references within. (27) Banerjee, S. K. Phys. Lett. 1964, 12, 16. See also, for example: Mira, J.; Rivas, J.; Rivadulla, F.; Vázquez-Vázquez, C.; López-Quintela, M. A. Phys. Rev. B 1999, 60, 2998. (28) Lévy, L. P. Magnetism and Superconductivity; Springer: Berlin, 2000. (29) Franco, V.; Blázquez, J. S.; Conde, A. Appl. Phys. Lett. 2006, 89, 222512. Oesterriecher, H.; Parker, F. T. J. Appl. Phys. 1984, 55, 4334. (30) Phan, M. H.; Yu, S. C. J. Magn. Magn. Mater. 2007, 308, 325. (31) Tishin, A. M.; Spichkin, Y. I. The Magnetocaloric Effect and its Applications; Institute of Physics Publishing: Bristol, 2003. (32) Amaral, J. S.; Tavares, P. B.; Reis, M. S.; Araújo, J. P.; Mendonça, T. M.; Amaral, V. S.; Vieira, J. M. J. Non-Cryst. Solids 2008, 354, 5301. (33) Toulemonde, O.; Roussel, P.; Isnard, O.; André, G.; Mentré, O. Chem. Mater. 2010, 22, 3807. (34) Trung, N. T.; Biharie, V.; Zhang, L.; Caron, L.; Buschow, K. H. J.; Brück, E. Appl. Phys. Lett. 2010, 96, 162507. (35) Venkatesh, R.; Pattabiraman, M.; Sethupathi, K.; Rangarajan, G.; Narayana Jammalamadaka, S. J. Appl. Phys. 2007, 101, 09C506.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the European project “SOPRANO” under Marie Curie actions (Grant No. PITNGA2008-214040), the French CNRS project “PR Réfrigération Magnétique”, and the Agence Nationale de la Recherche (ANR, project FUSTOM).

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A Heterogeneous Distribution of the Oxidation States - ACS Publications

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