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Versatile electronic behavior of the LixMn3−x−yFeyO4 spinels
Journal of Alloys and Compounds 577 (2013) 269–277
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Versatile electronic behavior of the LixMn3xyFeyO4 spinels D. Alonso-Domínguez a,b, I. Álvarez-Serrano a,⇑, M.L. López a, M.L. Veiga a, C. Pico a, F. Mompeán c, M. García-Hernández c, G.J. Cuello d a
Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain CEI Moncloa, UCM-UPM, Madrid, Spain c Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain d Institut Laue-Langevin, 6 rue Jules Horowitz, F-38042 Grenoble, France b
a r t i c l e
i n f o
Article history: Received 14 February 2013 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 25 May 2013 Keywords: Ferrites Neutron diffraction Magnetocaloric effect Dielectric behavior Li cathodes Spinel
a b s t r a c t The detailed structural and electronic characterization of microcrystalline powders of new spinels LixMn3xyFeyO4 (0.4 6 x 6 1.33; 0 6 y 6 1.30), obtained by the ‘‘liquid mix’’ method, is reported. Compositional characterization was carried out by means of thermogravimetric analysis, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy, and their structure was refined from neutron and X However, at temperray diffraction showing a cubic symmetry between 5 and 550 K, Space Group Fd3m. atures above ca. 950 K, a reversible transformation, probably implying the formation of an ordered vacant phase, has been detected. The magnetic behavior, analyzed from neutron diffraction data and magnetization measurements, is interpreted considering an ‘‘incomplete’’ ferrimagnetic response, due to magnetic frustration in the B sites. Potential applications have been evaluated from the magnetocaloric, electrochemical and dielectric behavior in selected compositional ranges. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ferrites constitute among the spinel type oxides a prominent family of materials which finds interesting applications in multiple hot topics of Materials Science. They show very attractive electrical, magnetic, electrochemical, etc. responses which make them potential candidates for high-density magnetic recording, magnetic fluids, ferrofluid technology, magnetocaloric refrigeration, magnetic resonance imaging enhancement and magnetically guided drug delivery [1–6]. In particular, lithium ferrites have received considerable attention due to their high Curie temperature, good thermal stability, high magnetization, low energy losses, etc. [7]. As soft magnetic materials, they show the basic magnetic characteristics such as high initial permeability and low coercivity with high saturation magnetization. The remarkable versatility, stability and low cost of these ferrites make them interesting in different prominent research fields. One of them is the optimization of the magnetocaloric response found in several lithium ferrites and their applications in magnetic refrigeration devices. Great effort has been focus in recent years on tuning parameters as composition and temperature of synthesis and sintering, in order to modify the lattice site occupation and therefore to achieve ⇑ Corresponding author. Tel.: +34 1 394 5237; fax: +34 1 394 4352. E-mail address: [email protected] (I. Álvarez-Serrano). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.043
an effective control on the electrical and magnetic properties [5,7–14]. Concerning the electrochemical properties of lithium ferrites, some interesting examples can be found in literature e.g., the LiMn1.99yLiyM0.01O4 (M = Al3+, Ni2+, Cr3+, Co3+) phases, which retain >90% of their capacity even at 5C rate; the LiMn1.5Ni0.5O4 and Lix(M0.1Mn0.9)2O4 (M = Ni, Ti) spinel, which shows good cycling and also high stability; and the Li(4x)/3Mn(52x)/3FexO4 system [11,15–17], reported by some of us, in which the dependence between Fe3+ content and crystallinity on the electrochemical response is studied. Also some relevant works dealing with manganese ferrites as anode materials have been reported [18]. Impedance measurements have been employed as a powerful tool in order to accurately characterize the electrical behavior of attractive manganese doped lithium ferrites as LiFe5xMnxO8 [19]. Other key features for many purposes are the stabilization of collinear arrangements avoiding frustrations effects and the introduction of different concentrations of paramagnetic cations [1,15]. Finally, the properties of these materials are markedly dependent on morphology and size of particles and therefore much attention has been paid to different synthesis and particularly to the preparation of micro and nanostructured materials [18,20]. In this scenario, the present work deals with the compositional and structural characterization and the above mentioned promi-
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nent electronic responses (magnetic, dielectric and electrochemical) of the LixMn3xyFeyO4 (0.4 6 x 6 1.33; 0 6 y 6 1.30) spinels, prepared by sol–gel techniques. Our aim was to analyze the influence of the most representative parameters and to establish the optimized specific compositional ranges for three main application fields: magnetocaloric refrigeration, cathodes for Li batteries and dielectrics.
2. Experimental Polycrystalline powders of LixMn3xyFeyO4 spinels were prepared by the ‘‘liquid-mix’’ (Pechini) method using stoichiometric amounts of LiNO3, Mn(NO3)24H2O and Fe(NO3)29H2O. These were dissolved in a 20% v/v HNO3 solution and after citric acid (up to 1 M) and ethyleneglycol (5% v/v) were added. The resulting solutions were heated under constant stirring until the decomposition of organic matter was completed. Then, final thermal treatments in the range 773–1023 K (for several days) were applied, with intermediate grindings, depending on the y value (higher temperatures are needed as iron content is increased), obtaining pure phases, black powders, in all cases. The required conditions for the different phases were: 773 K/1.5 days for the samples with 0 6 y 6 0.5; 873 K/ 2 days for spinels with 0.5 < y 6 0.1; 923 K/1 day for y = 1.20 and 1.25; and 1023 K/1 day for y = 1.30. X-ray powder diffraction (XRD) patterns were obtained at room temperature with a Siemens D-5000 diffractometer using X-ray powder diffraction experiments at high temperatures, up to 1123 K, were carried out in a PANalytical X’Pert MPD diffractometer. In all cases a Cu Ka radiation with k = 1.5418 Å was used and the measurements were performed in air and in dynamic vacuum. Neutron diffraction (ND) data were recorded at the D2B ILL Grenoble in the D2B (k = 1.5945 Å) and D20 (k = 1.8700 Å) diffractometers. The collected data were analyzed by the Rietveld [21] profile method using the Fullprof program [22]. Scanning electron microscope images were obtained with a JEOL JSM 6335F microscope, with an OXFORD INCA energy dispersive spectroscopy (EDS) analyzer. Semiquantitative chemical analyses were made from energy dispersive X-ray spectroscopy (EDS). Electron energy loss spectroscopy (EELS) experiments were performed using a JEOL JEM 3000F microscope operating at 300 kV, with an ENFINA spectrometer with an energy resolution of 1.3 eV. EELS spectra were acquired for the L2,3 edge of the transition metals. Magnetic experiments were carried out in a Quantum Design MPMS (Quantum Design, MPMS-XL model). The d.c. magnetization data were obtained in the temperature range 4.2–400 K in applied field of 500 Oe and isothermal magnetization curves were obtained with magnetic fields up to 6T. Powdered samples of LixMn3xyFeyO4 were pressed into pellets (at 6 kbar), which typical diameter of 5 mm and thickness of 1–2 mm, and then sintered at T = 1223 K. For a.c. electrical conductivity measurements, blocking electrodes were deposited on both sides of the as-pelletized samples by platinum paint (previously dried at 1073 K) for measurements in the 300–700 K temperature range, and silver paint for measurements below room temperature. The a.c. conductivity was obtained, in the temperature range 250–700 K, using a frequency analyzer (Solartron 1260) over a frequency range of 102 to 107 Hz. Resistance and permittivity values were derived from the complex impedance diagrams. Thermogravimetric analysis (TG) was carried out in a Cahn D200 balance at several heating rates under H2/He (2:3) and oxygen atmospheres.
3. Results and discussion 3.1. Compositional characterization The notation and composition of the studied samples, LixMn3(0.4 6 x 6 1.33; 0 6 y 6 1.30), together with other significant data, are included in Table 1. Hereinafter the samples are denoted by their ‘‘y’’ value, e.g. y = 0.1 stands for Li1.3Mn1.6Fe0.1O4. These samples were compositionally characterized from EDS, EELS, ND and TG experiments, the last ones made both under reductive H2/He (300 mbar/200 mbar) and oxidant (O2) atmospheres. The average cationic composition (Mn/Fe) for each sample was firstly established by EDS and was coincident with the expected one, within the experimental error (±1%). As an example, Fig. 1 gathers a micrograph and the correspondent EDS curve for the y = 1.20 sample. Mean values of Mn/Fe percentages measured in several crystals gave an atomic ratio equal to the expected one, i.e. 1.00(2). Besides, the stoichiometry of the samples was fully evaluated from further ND data and TG analysis. EELS experiments were carried out in order to obtain complementary information about the oxidation states of Mn and Fe. As a representative example, Fig. 2 shows the EELS spectra in both the L2,3 Mn and Fe edges (1a and 1b), for two samples, y = 0.40 and 0.5, nominally corresponding to a Mn oxidation state of +3.5 and +4, respectively. According to Ref. [23], an estimation based on the ratio of the peaks intensities is adequate to estimate the Mn oxidation state, but the chemical shift method is more accurate for analyzing the oxidation state of iron. Having in mind that typical values of the Fe L2,3 edge signals have been reported to be ca. 708/722 eV for Fe2+ and 711/724 eV for Fe3+ [23,24], the presence of Fe3+ in the title samples can be assumed. On the other hand, the L2,3 ratios for the Mn window, roughly estimated from intensity increments, together with the observed shift, are consistent with only Mn4+ for y = 0.5 (Dy – Dy0 = 2.0) and mixed valence Mn4+/Mn3+ for y = 0.4 (Dy – Dy0 = 2.8). In this sense, EELS experiments permitted both to ascribe a trivalent oxidation state for Fe cations and a mixed valence for Mn as expected. Fig. 3 shows the TG curves corresponding to reduction and oxidation processes in the 300–1000 K range for y = 0.5 and 0.2 samples, respectively. The analysis of the mass losses found in the TG experiments under reductive atmosphere and, complementarily, the examination of the resultant diffractograms, allow us to propose the decomposition sequence included in Fig. 2a, which is consistent with the expected stoichiometry. On the other hand, the TG experiments carried out under O2 atmosphere showed, as a general xyFeyO4
Table 1 Composition and notation of the LixMn3xyFeyO4 studied samples. Sample
y
a (Å)
x(O)a
Cation distribution (M)T[M]oO4
RB
TC (K)
Mean Mnn+
Li1.33Mn1.67O4 Li1.3Mn1.6Fe0.1O4 Li1.3Mn1.5Fe0.2O4 Li1.24Mn1.46Fe0.3O4 Li1.18Mn1.47Fe0.35O4 Li0.9Mn1.7Fe0.4O4 Li1.2Mn1.3Fe0.5O4 Li0.9Mn1.5Fe0.6O4 Li0.85Mn1.5Fe0.65O4 Li0.8Mn1.5Fe0.7O4 Li0.4Mn1.6Fe1O4 Li0.6Mn1.2Fe1.2O4 Li0.5Mn1.25Fe1.25O4 Li0.75Mn0.95Fe1.3O4
0 0.10 0.20 0.30 0.35 0.40 0.50 0.60 0.65 0.70 1.00 1.20 1.25 1.30
8.147(1) 8.158(7) 8.172(5) 8.187(2) 8.291(1) 8.383(2) 8.257(8) 8.414(1) 8.3760(2) 8.3045(4) 8.3846(3) 8.4062 8.3577 8.356(1)
0.261(1) 0.261(2) 0.261(5) 0.261(2) 0.262(1) 0.264(2) 0.258(6) 0.261 0.2622 0.2639 0.2621 0.2589 0.2630 0.257
(Li1)[Li0.33Mn1.67] (Li1)[Li0.3Mn1.6Fe0.1] (Li1)[Li0.26Mn1.54Fe0.2] (Li1)[Li0.24Mn1.46Fe0.3] (Li0.85 Fe0.15)[Li0.33Mn1.47Fe0.2] (Li0.25 Fe0.12 Mn0.63)[Li0.65Mn1.07Fe0.28] (Li1)[Li0.17Mn1.3Fe0.5] (Li0.5 Fe0.5)[Li0.4Mn1.5Fe0.1] (Li0.65 Mn0.10Fe0.25)[Li0.25Mn1.40Fe0.35] (Li0.75Mn0.19Fe0.06)[Li0.05Mn1.31Fe0.64] (Li0.47Mn0.28Fe0.25)[Li0.53Mn1.02Fe0.44] (Li0.56Fe0.44)[Mn1.2Fe0.76] (Li0.48Fe0.52)[Mn1.25Fe0.73] (Li0.28Fe0.72)[ Li0.46Mn0.94Fe0.60]
1.61 1.93 1.54 1.72 1.51 3.40 2.76 2.67 3.04/6.78b 5.58 8.40/7.20b ND3.17 2.33 6.40 2.82
<17 zone PM ok Griff Griffi Griff 210 245 Griff 350 275 340 390 475 530 625
+4 +4 +4 +4 +4 +3.47 +4 +3.53 +3.47 +3.4 +2.88 +3.17 +3 +3.52
S.G. Fd3m; Tetrahedral sites, hT at 8a(1/8,1/8,1/8); octahedral sites, hO at 16d (½,½,½). a Oxygen atoms at 32e(u,u,u). b Refinement from combined XRD and ND data.
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Fig. 1. Micrograph (left) and EDS spectrum (right) for y = 1.2.
Fig. 2. EELS spectra for y = 0.40 and 0.5; (a) Mn L2,3 and (b) Fe L2,3 edges.
feature, a great stability of the title ferrites under oxidative conditions and an oxygen stoichiometry nominally equal to that expected, within the experimental error. For example, in Fig. 3b it can be appreciated that, for the y = 0.2 sample, the maximum mass change percentage is ca. 0.15%, which represents a negligible variation in oxygen stoichiometry, d 0.01, leading to a composition nominally equal to that expected, i.e. Li1.3Mn1.5Fe0.2O3.99(1). Particular details observed in some curves (mass gaining and losses which alternate before stabilization) will be discussed below.
Fig. 3. Thermogravimetric profiles (a) under H2:He atmosphere for y = 0.5 and (b) under O2 atmosphere for y = 0.2.
3.2. Structural characterization The structural characterization has been carried out from XRD and ND data, by applying the Rietveld method with the Fullprof program. All the title samples present XRD diffraction patterns characteristic of a cubic symmetry, which maintains the essential features of the parent structure, x = 0 and y = 3 one, i.e. Fe3O4, without showing extra peaks characteristic of structural distortion. In
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Fig. 4. XRD profiles for LixMn3xyFeyO4, for 0 6 y 6 1.30.
Fig. 4 the XRD profiles at room temperature (RT) for all the samples are shown. Relevant changes in intensity specially concerning the (1 1 1), (2 2 0) and (3 1 1) reflections can be appreciated, showing the differences taking place in cationic occupation of both sublattices when varying x and y in the title system [25,26]. As an example, Fig. 5 shows the Rietveld refinement profiles at 300 K from (a) combined ND and XRD data for y = 0.65, and (b) space group. Tafrom XRD data for y = 1.20, considering the Fd3m ble 1 gathers the most relevant structural parameters and Rietveld factors for all the samples. The agreement between the observed data and the proposed model let us assume the accuracy of this one in all cases; therefore we can conclude that XRD and ND data evidence the stabilization of pure phases with spinel-type structure and cubic symmetry at room temperature for every y value essayed. The cell parameter, a, evolves coherently with the mean size of cations located in both sublattices, especially concerning the mean oxidation state of Mn, as can be appreciated in Fig. 6. Finally, the occupation of site-type is graphically shown in Fig. 7 with clarifying purpose. It is also interesting to pay attention to the structural evolution with temperature. A reversible process of decomposition has been detected for y = 0.4 by means of XRD data obtained from the sample heated in an in situ experiment. In Fig. 8 the XRD profiles at temperatures between 25 and 850 °C are shown. Both a change in intensity of (1 1 1), (2 2 0), (3 1 1) and (4 0 0) diffraction peaks and a splitting of (2 2 0) and (4 0 0) reflections are clearly observed and these facts can be interpreted considering two possibilities, schematized by the following decomposition sequences:
Fig. 5. Rietveld profiles at 300 K for (a) y = 0.65, from XRD and ND combined data and (b) y = 1.20 profiles, from XRD data.
TP750 C
Li0:9 Mn1:7 Fe0:4 O4 ! Li0:8 Mn1:5 Fe0:4 O4d þ 0:05 Mn3 O4 O2
þ 0:05 Li2 MnO3
ð1Þ
TP750 C
Li0:9 Mn1:7 Fe0:4 O4 ! zLi2 MnO3 þ ð1 zÞLið0:92zÞ=ð1zÞ Mnð1:7zÞ=ð1zÞ Fe0:4=ð1zÞ O4 þ z=2O2
ð2Þ
whereas (Eq. (1)) leads to the formation of a vacant spinel without any mass loss, (Eq. (2)) implies the removal of a certain amount of oxygen and the stabilization of another spinel-type ferrite with different composition. The main facts observed are the following: the appearance of a subtle Bragg maximum attributable to Mn3O4, the very low value of mass increment obtained in the TG experiment under oxygen atmosphere (less than 1.2%, see Fig. 9) and the splitting of (2 2 0) and (4 0 0) maxima, which takes place at high temperatures (see
Fig. 6. Evolution of cell parameter (right) and mean oxidation state (left) with y.
Fig. 8). All of them suggest that the actual process suffered by the sample probably implies a complex sequence of different reactions or the coexistence of diverse and reversible decomposition paths. The splitting of the mentioned maxima could be effectively related to the stabilization of a vacant phase with decreasing symmetry, considering any S.G. of the previously ascribed ones to vacant spinels, as the tetragonal I41/amd, F41/ddm or the
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Fig. 7. Cationic distribution in octahedral and tetrahedral sites as a function of y.
Fig. 9. Thermogravimetric profiles for y = 0.4 under O2 atmosphere.
Fig. 8. Evolution of diffraction profiles with temperature for y = 0.4. (upper panel). Selected zones showing the splitting of (220) and (400) Bragg peaks (bottom panels).
orthorhombic Fddd [27,28]. Therefore, TG experiments carried out under reductive and O2 atmosphere and XRD data for intermediate phases allow establishing the decomposition sequence and the high stability of the title ferrites under oxidative conditions. However, our available data are not accurate enough to permit us discerning the actual decomposition sequence. Further experiments will be carried out in order to get deeper insight about this question, but it is clear the interest of such reversible transformations concerning oxygen insertion and removal processes.
Fig. 10. Variation of magnetization vs. temperature for LixMn3xyFeyO4.
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nearest neighbor (JAB) and next nearest neighbor (JAA and JBB) interactions, being the first ones the greatest [29]. Thus, all the bonds are not simultaneously satisfied and the system remains frustrated. Moreover, in the title system, in addition to this ‘‘intrinsic’’ frustration, both the magnetic dilution due to the presence of Li and the cationic disorder give rise to a complex magnetic behavior connected to different predominant situations: Yafet-Kittel canted state, localized canted state, spin glass, cluster spin glass, etc. [30,31] The observed general trend in the magnetic response, as a function of y, can be summarized as follows:
Fig. 11. Variation of inverse susceptibility vs. temperature for LixMn3xyFeyO4. Inset shows a zoomed view for y = 0.1 and 0.2.
3.3. Electronic behavior Potential applications in diverse materials research fields have been evaluated: magnetocaloric response, electrochemical and dielectric responses have been analized for selected samples in order to establish the optimized specific compositional ranges for each one. 3.3.1. Magnetic behavior Fig. 10 shows the variation of magnetization with temperature, for an applied magnetic field of 500 Oe, for the LixMn3xyFeyO4 ferrites. As expected, a strong enhancement in magnetization is obtained below a certain temperature, TC, and this temperature varies over a wide range, between ca. 250 and 650 K, for different y values. In spinel-type compounds, the cations occupying the tetrahedral (A) and the octahedral (B) sublattices experience competing
For small y values, a Griffits phase-cluster-type magnetic response is obtained. In Fig. 11 this can be appreciated from the form adopted by the 1/v (inverse susceptibility) vs. T curves near TC. ‘‘Partial’’ ferrimagnetic behavior is detected as a function of composition, above a certain y value. Fig. 12 includes the M vs. H cycles, for representative examples of samples displaying this behavior, showing well defined narrow cycles below the corresponding Curie temperatures. In order to analyze the magnetic ordering in both sublattices and its temperature dependence, some neutron diffraction experiments in a wide range of temperatures were carried out for selected samples. In spinel-type systems the nuclear and magnetic symmetries are the same and, then, the Bragg contributions of both appear at the same angular positions. The evolution of the neutron profiles with temperature and the identification of paramagnetic and ferromagnetic regimes can be followed considering the variation in intensity of (1 1 1) reflection, around 4.8 Å, as shown in Fig. 13 for two selected compositions, y = 1.20 and 1.25. The neutron diffraction data have been refined at low temperatures (below the Curie temperature in each case) considering the As an example, Fig. 14 typical ferrimagnetic arrangement F1. shows the Rietveld neutron diffraction profiles for y = 1. In Table 2 the obtained magnetic moments and R-factors are gathered. The experimental saturation magnetization values, obtained from M vs. H graphs, are also included with comparative aim.
Fig. 12. Magnetization vs. magnetic field curves for LixMn3xyFeyO4.
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a b
y
(A)[B]
lnet (BM)b
RB
lsat (BM)a
0.65 1.00 1.20 1.25
(Li0.65 Mn0.10Fe0.25)[Li0.25Mn1.40Fe0.35] (Li0.47Mn0.28Fe0.25)[Li0.53Mn1.02Fe0.44] (Li0.56Fe0.44)[Mn1.2Fe0.76] (Li0.48Fe0.52)[Mn1.25Fe0.73]
1.40 1.90 2.92 2.88
8.6 6.4 5.4 9.5
1.32 1.43 2.53 2.81
From magnetization data. From ND data at T = 4 K.
Fig. 13. Thermal evolution of ND profiles for (a) y = 1.20 and (b) y = 1.25.
Fig. 15. Graphs of permittivity (e0 ) and energy loss (d vs. frequency at room temperature for LixMn3xyFeyO4 (y = 0, 0.35, 0.40 and 0.60).
Fig. 14. Neutron diffraction profiles for y = 1 at 1.5 K. Observed pattern is denoted by dots, and the calculated and difference plots are shown in solid lines. Reflections positions for nuclear (upper row) and magnetic (lower row) phases are included.
The magnetic moments estimated from the assumption of totally collinear arrangement in each sublattice and the theoretical values of magnetic moments obtained from the cation occupation are clearly much higher than the obtained ones. The deviations observed between the experimental and the estimated net moments suggest the existence of non-collinear spin interactions. The complex magnetic structure as found in our system is a consequence of inhomogeneities arising from fluctuations of magnetic ion con-
centrations. Magnetic dilution in the two sublattices gives rise to frustrations in this perturbed magnetic ordering, in accordance with results previously obtained for similar compounds [32]. Unfortunately this fact constrains the potential high values of magnetic entropy and therefore a useful magnetocaloric response in this system, which remains restricted to certain y values. Nevertheless, the title spinels display a promising magnetocaloric behavior as we have previously reported on some members of the title solid solution [8].
3.3.2. Dielectric response Ferrites are, in general, insulating magnetic oxides. These materials are attractive for microwave device applications owing to their large magnetic permeability, high dielectric constant, low dielectric loss, etc. and chemical stability at relatively low frequencies. Therefore, it is interesting to explore the dielectric behavior of ferrites as a function of composition. For example, in Li-doped Mn/ Zn ferrites, saturation magnetization increases with Li substitution and enhances microwave properties because it plays an important
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Fig. 17. Trends in electronic behavior of LixMn3xyFeyO4.
(see Table 1), it seems that a great degree of polarization is related to a low concentration of Li cations in B sites and to the presence of both Fe and Mn cations in them. Also, it is interesting to note that this phase, y = 0.35, shows a ferromagnetic behavior with TC 210 K, suggesting that it could be a good candidate for magnetodielectric coupling; this property will be analyzed in future works.
Fig. 16. Charge/discharge (potential vs. specific capacity) curves at rate of C/10 between 2.5–3.5 V for y = 0.1, 0.2 and 0.3.
role in structural and magnetic properties. The sample with the highest Li content exhibits a great resistivity and minimal dielectric constant [33]. For selected compositions, the dielectric response was tested from impedance measurements. In Fig. 15 the variation of permittivity, e0 , vs. frequency at RT is shown for the 0 6 y 6 0.6 phases. The observed general trend consists in a relaxation-type plateau at low frequencies and a decreasing of e0 with increasing frequency. This fact can be related to a conducting process implying electron hopping Fe2+ M Fe3+ or Mnn+ M Mn(n+1)+ by electronic exchange between both transition metal cations. Among the samples investigated, it can be underlined the good dielectric response displayed by the y = 0.35 phase. Near RT a dielectric constant of about 5000 is obtained and it remains practically constant in the range 103 and 5 105 Hz, with the lowest dielectric losses registered, less than d = 1. Having in mind the cationic distribution in these phases
3.3.3. Electrochemical behavior In order to evaluate the possible applicability of the title ferrites as cathodes in Li-batteries, the electrochemical behavior of some members has also been studied. Taking into account the Li distribution among the A and B sublattices (Fig. 7) and the well-known mechanism of Li cations motion in the insertion–deinsertion processes [34], the electrochemical analysis was focussed on spinels with y < 1 and high values of x. In Fig. 16 the electrical potential vs. specific capacity graphs for y = 0.1, 0.2 and 0.3 are shown. A plateau around 2.7 V is visible, connected to the Mn4+/Mn3+ pair. The inserted Li amount increases as y does, i.e. Fe concentration increases, being of 0.79 for y = 0.3. Also the specific capacity and the cycling capability are improved as y increases up to the critical value of 0.3. Having in mind the great versatility of the electronic behavior displayed by the title series, different compositional ranges corresponding to the optimization of these materials for different application fields have been established. Fig. 17 summarizes the electronic behavior as a function of composition, taking the Curie temperature as a guiding parameter. Three color regions are highlighted:
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(a) Region e0 : corresponding to compositions in the range 0.35 < y < 0.70, which show the optimal behavior in Dielectrics. The obtained response can be related to the dilution of Mn and Fe in both A and B sublattices, which gives rise to an enhancement of polarization phenomena. (b) Region MCE: covering the compositional range 0.3 < y < 1.2, which gathers the best magnetocaloric responses, in which the Curie temperature can be tuned by changing the Fe content. (c) Region Batteries: containing samples with y < 0.5, which display good electrochemical behavior, linked to a high Li content, i.e. x > 1.2, and an exclusive and high oxidation state for manganese cations (+4), with potential interest in the Li batteries cathodes research field.
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good candidate for presenting eventual magnetoelectric coupling and this point will actually be analyzed in future works. In conclusion, the title system displays an electronic behavior which can find an attractive and wide variety of potential applications in fields of Materials Science related to magnetocaloric, dielectric and electrochemical properties. Acknowledgements Authors are grateful to the CAI centers of UCM (XRD and electron microscopy) and financial support from Spanish Ministerio de Ciencia e Innovación MAT2010-20117, MAT2011-27470-C0202 and CSD2009-00013. D. Alonso-Domínguez thanks the CEI Moncloa for financial facilities.
The intermediate areas enclose samples that potentially could exhibit coupled magneto-electrical properties and, thus, they point to future promising studies. Finally, it is interesting to note that the phase y = 0.35 presents a ferromagnetic behavior with TC 210 K together with relatively high values of permittivity at RT. Therefore it is a good candidate for eventual magnetoelectric coupling and this behavior will be undertaken in due course.
References
4. Summary and conclusions
[10]
Microcrystalline powders of the new ferrites LixMn3xyFeyO4 (0.4 6 x 6 1.33; 0 6 y 6 1.30) have been prepared by the sol–gel method and their compositions have been obtained from TGA, EDS and EELS techniques. EELS experiments indicated a trivalent oxidation state for Fe cations and a mixed valence for Mn. ND and XRD data showed all the samples to present cubic symmetry between 5 and 550 K, S.G. Fd3m. At higher temperatures, above ca. 950 K, a reversible decomposition implying the formation of an ordered vacant phase has been detected. The magnetic behavior is compatible with a frustrated ferromagnetic response due to the atomic disorder in the B sites. The best dielectric response has been measured for the y = 0.35 phase. At temperatures near the ambient, a dielectric constant of about 5000 is obtained and it remains practically constant between 103 and 5 105 Hz, with the lowest dielectric losses registered, less than d = 1. In this sense, low concentration of Li cations in B sites and the presence of both Fe and Mn transitions metals in it seem to be connected to a great degree of polarization in the materials. The samples with y < 1 display a moderate good electrochemical behavior, linked to the Mn4+/Mn3+ pair and the lithium concentration, as expected. A maximum value of 0.79 for the amount of inserted Li is obtained for y = 0.3. Both specific capacity and cycling capability are improved as y increases being the critical value y = 0.3. Having in mind the great versatility of the electronic behavior displayed by the LixMn3xyFeyO4 the optimization of these materials for different application fields seem to reveal different compositional ranges in each case: (a) Dielectrics: 0.35 < y < 0.70, (b) magnetocaloric materials: 0.3 < y < 1.2, (c) Li batteries cathodes: y < 0.5. Finally, it is interesting to note that the phase with y = 0.35 presents a ferromagnetic behavior with TC 210 K, and therefore is a
[11]
[1] [2] [3] [4] [5] [6] [7] [8] [9]
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Versatile electronic behavior of the LixMn3xyFeyO4 spinels D. Alonso-Domínguez a,b, I. Álvarez-Serrano a,⇑, M.L. López a, M.L. Veiga a, C. Pico a, F. Mompeán c, M. García-Hernández c, G.J. Cuello d a
Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain CEI Moncloa, UCM-UPM, Madrid, Spain c Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain d Institut Laue-Langevin, 6 rue Jules Horowitz, F-38042 Grenoble, France b
a r t i c l e
i n f o
Article history: Received 14 February 2013 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 25 May 2013 Keywords: Ferrites Neutron diffraction Magnetocaloric effect Dielectric behavior Li cathodes Spinel
a b s t r a c t The detailed structural and electronic characterization of microcrystalline powders of new spinels LixMn3xyFeyO4 (0.4 6 x 6 1.33; 0 6 y 6 1.30), obtained by the ‘‘liquid mix’’ method, is reported. Compositional characterization was carried out by means of thermogravimetric analysis, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy, and their structure was refined from neutron and X However, at temperray diffraction showing a cubic symmetry between 5 and 550 K, Space Group Fd3m. atures above ca. 950 K, a reversible transformation, probably implying the formation of an ordered vacant phase, has been detected. The magnetic behavior, analyzed from neutron diffraction data and magnetization measurements, is interpreted considering an ‘‘incomplete’’ ferrimagnetic response, due to magnetic frustration in the B sites. Potential applications have been evaluated from the magnetocaloric, electrochemical and dielectric behavior in selected compositional ranges. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ferrites constitute among the spinel type oxides a prominent family of materials which finds interesting applications in multiple hot topics of Materials Science. They show very attractive electrical, magnetic, electrochemical, etc. responses which make them potential candidates for high-density magnetic recording, magnetic fluids, ferrofluid technology, magnetocaloric refrigeration, magnetic resonance imaging enhancement and magnetically guided drug delivery [1–6]. In particular, lithium ferrites have received considerable attention due to their high Curie temperature, good thermal stability, high magnetization, low energy losses, etc. [7]. As soft magnetic materials, they show the basic magnetic characteristics such as high initial permeability and low coercivity with high saturation magnetization. The remarkable versatility, stability and low cost of these ferrites make them interesting in different prominent research fields. One of them is the optimization of the magnetocaloric response found in several lithium ferrites and their applications in magnetic refrigeration devices. Great effort has been focus in recent years on tuning parameters as composition and temperature of synthesis and sintering, in order to modify the lattice site occupation and therefore to achieve ⇑ Corresponding author. Tel.: +34 1 394 5237; fax: +34 1 394 4352. E-mail address: [email protected] (I. Álvarez-Serrano). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.043
an effective control on the electrical and magnetic properties [5,7–14]. Concerning the electrochemical properties of lithium ferrites, some interesting examples can be found in literature e.g., the LiMn1.99yLiyM0.01O4 (M = Al3+, Ni2+, Cr3+, Co3+) phases, which retain >90% of their capacity even at 5C rate; the LiMn1.5Ni0.5O4 and Lix(M0.1Mn0.9)2O4 (M = Ni, Ti) spinel, which shows good cycling and also high stability; and the Li(4x)/3Mn(52x)/3FexO4 system [11,15–17], reported by some of us, in which the dependence between Fe3+ content and crystallinity on the electrochemical response is studied. Also some relevant works dealing with manganese ferrites as anode materials have been reported [18]. Impedance measurements have been employed as a powerful tool in order to accurately characterize the electrical behavior of attractive manganese doped lithium ferrites as LiFe5xMnxO8 [19]. Other key features for many purposes are the stabilization of collinear arrangements avoiding frustrations effects and the introduction of different concentrations of paramagnetic cations [1,15]. Finally, the properties of these materials are markedly dependent on morphology and size of particles and therefore much attention has been paid to different synthesis and particularly to the preparation of micro and nanostructured materials [18,20]. In this scenario, the present work deals with the compositional and structural characterization and the above mentioned promi-
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nent electronic responses (magnetic, dielectric and electrochemical) of the LixMn3xyFeyO4 (0.4 6 x 6 1.33; 0 6 y 6 1.30) spinels, prepared by sol–gel techniques. Our aim was to analyze the influence of the most representative parameters and to establish the optimized specific compositional ranges for three main application fields: magnetocaloric refrigeration, cathodes for Li batteries and dielectrics.
2. Experimental Polycrystalline powders of LixMn3xyFeyO4 spinels were prepared by the ‘‘liquid-mix’’ (Pechini) method using stoichiometric amounts of LiNO3, Mn(NO3)24H2O and Fe(NO3)29H2O. These were dissolved in a 20% v/v HNO3 solution and after citric acid (up to 1 M) and ethyleneglycol (5% v/v) were added. The resulting solutions were heated under constant stirring until the decomposition of organic matter was completed. Then, final thermal treatments in the range 773–1023 K (for several days) were applied, with intermediate grindings, depending on the y value (higher temperatures are needed as iron content is increased), obtaining pure phases, black powders, in all cases. The required conditions for the different phases were: 773 K/1.5 days for the samples with 0 6 y 6 0.5; 873 K/ 2 days for spinels with 0.5 < y 6 0.1; 923 K/1 day for y = 1.20 and 1.25; and 1023 K/1 day for y = 1.30. X-ray powder diffraction (XRD) patterns were obtained at room temperature with a Siemens D-5000 diffractometer using X-ray powder diffraction experiments at high temperatures, up to 1123 K, were carried out in a PANalytical X’Pert MPD diffractometer. In all cases a Cu Ka radiation with k = 1.5418 Å was used and the measurements were performed in air and in dynamic vacuum. Neutron diffraction (ND) data were recorded at the D2B ILL Grenoble in the D2B (k = 1.5945 Å) and D20 (k = 1.8700 Å) diffractometers. The collected data were analyzed by the Rietveld [21] profile method using the Fullprof program [22]. Scanning electron microscope images were obtained with a JEOL JSM 6335F microscope, with an OXFORD INCA energy dispersive spectroscopy (EDS) analyzer. Semiquantitative chemical analyses were made from energy dispersive X-ray spectroscopy (EDS). Electron energy loss spectroscopy (EELS) experiments were performed using a JEOL JEM 3000F microscope operating at 300 kV, with an ENFINA spectrometer with an energy resolution of 1.3 eV. EELS spectra were acquired for the L2,3 edge of the transition metals. Magnetic experiments were carried out in a Quantum Design MPMS (Quantum Design, MPMS-XL model). The d.c. magnetization data were obtained in the temperature range 4.2–400 K in applied field of 500 Oe and isothermal magnetization curves were obtained with magnetic fields up to 6T. Powdered samples of LixMn3xyFeyO4 were pressed into pellets (at 6 kbar), which typical diameter of 5 mm and thickness of 1–2 mm, and then sintered at T = 1223 K. For a.c. electrical conductivity measurements, blocking electrodes were deposited on both sides of the as-pelletized samples by platinum paint (previously dried at 1073 K) for measurements in the 300–700 K temperature range, and silver paint for measurements below room temperature. The a.c. conductivity was obtained, in the temperature range 250–700 K, using a frequency analyzer (Solartron 1260) over a frequency range of 102 to 107 Hz. Resistance and permittivity values were derived from the complex impedance diagrams. Thermogravimetric analysis (TG) was carried out in a Cahn D200 balance at several heating rates under H2/He (2:3) and oxygen atmospheres.
3. Results and discussion 3.1. Compositional characterization The notation and composition of the studied samples, LixMn3(0.4 6 x 6 1.33; 0 6 y 6 1.30), together with other significant data, are included in Table 1. Hereinafter the samples are denoted by their ‘‘y’’ value, e.g. y = 0.1 stands for Li1.3Mn1.6Fe0.1O4. These samples were compositionally characterized from EDS, EELS, ND and TG experiments, the last ones made both under reductive H2/He (300 mbar/200 mbar) and oxidant (O2) atmospheres. The average cationic composition (Mn/Fe) for each sample was firstly established by EDS and was coincident with the expected one, within the experimental error (±1%). As an example, Fig. 1 gathers a micrograph and the correspondent EDS curve for the y = 1.20 sample. Mean values of Mn/Fe percentages measured in several crystals gave an atomic ratio equal to the expected one, i.e. 1.00(2). Besides, the stoichiometry of the samples was fully evaluated from further ND data and TG analysis. EELS experiments were carried out in order to obtain complementary information about the oxidation states of Mn and Fe. As a representative example, Fig. 2 shows the EELS spectra in both the L2,3 Mn and Fe edges (1a and 1b), for two samples, y = 0.40 and 0.5, nominally corresponding to a Mn oxidation state of +3.5 and +4, respectively. According to Ref. [23], an estimation based on the ratio of the peaks intensities is adequate to estimate the Mn oxidation state, but the chemical shift method is more accurate for analyzing the oxidation state of iron. Having in mind that typical values of the Fe L2,3 edge signals have been reported to be ca. 708/722 eV for Fe2+ and 711/724 eV for Fe3+ [23,24], the presence of Fe3+ in the title samples can be assumed. On the other hand, the L2,3 ratios for the Mn window, roughly estimated from intensity increments, together with the observed shift, are consistent with only Mn4+ for y = 0.5 (Dy – Dy0 = 2.0) and mixed valence Mn4+/Mn3+ for y = 0.4 (Dy – Dy0 = 2.8). In this sense, EELS experiments permitted both to ascribe a trivalent oxidation state for Fe cations and a mixed valence for Mn as expected. Fig. 3 shows the TG curves corresponding to reduction and oxidation processes in the 300–1000 K range for y = 0.5 and 0.2 samples, respectively. The analysis of the mass losses found in the TG experiments under reductive atmosphere and, complementarily, the examination of the resultant diffractograms, allow us to propose the decomposition sequence included in Fig. 2a, which is consistent with the expected stoichiometry. On the other hand, the TG experiments carried out under O2 atmosphere showed, as a general xyFeyO4
Table 1 Composition and notation of the LixMn3xyFeyO4 studied samples. Sample
y
a (Å)
x(O)a
Cation distribution (M)T[M]oO4
RB
TC (K)
Mean Mnn+
Li1.33Mn1.67O4 Li1.3Mn1.6Fe0.1O4 Li1.3Mn1.5Fe0.2O4 Li1.24Mn1.46Fe0.3O4 Li1.18Mn1.47Fe0.35O4 Li0.9Mn1.7Fe0.4O4 Li1.2Mn1.3Fe0.5O4 Li0.9Mn1.5Fe0.6O4 Li0.85Mn1.5Fe0.65O4 Li0.8Mn1.5Fe0.7O4 Li0.4Mn1.6Fe1O4 Li0.6Mn1.2Fe1.2O4 Li0.5Mn1.25Fe1.25O4 Li0.75Mn0.95Fe1.3O4
0 0.10 0.20 0.30 0.35 0.40 0.50 0.60 0.65 0.70 1.00 1.20 1.25 1.30
8.147(1) 8.158(7) 8.172(5) 8.187(2) 8.291(1) 8.383(2) 8.257(8) 8.414(1) 8.3760(2) 8.3045(4) 8.3846(3) 8.4062 8.3577 8.356(1)
0.261(1) 0.261(2) 0.261(5) 0.261(2) 0.262(1) 0.264(2) 0.258(6) 0.261 0.2622 0.2639 0.2621 0.2589 0.2630 0.257
(Li1)[Li0.33Mn1.67] (Li1)[Li0.3Mn1.6Fe0.1] (Li1)[Li0.26Mn1.54Fe0.2] (Li1)[Li0.24Mn1.46Fe0.3] (Li0.85 Fe0.15)[Li0.33Mn1.47Fe0.2] (Li0.25 Fe0.12 Mn0.63)[Li0.65Mn1.07Fe0.28] (Li1)[Li0.17Mn1.3Fe0.5] (Li0.5 Fe0.5)[Li0.4Mn1.5Fe0.1] (Li0.65 Mn0.10Fe0.25)[Li0.25Mn1.40Fe0.35] (Li0.75Mn0.19Fe0.06)[Li0.05Mn1.31Fe0.64] (Li0.47Mn0.28Fe0.25)[Li0.53Mn1.02Fe0.44] (Li0.56Fe0.44)[Mn1.2Fe0.76] (Li0.48Fe0.52)[Mn1.25Fe0.73] (Li0.28Fe0.72)[ Li0.46Mn0.94Fe0.60]
1.61 1.93 1.54 1.72 1.51 3.40 2.76 2.67 3.04/6.78b 5.58 8.40/7.20b ND3.17 2.33 6.40 2.82
<17 zone PM ok Griff Griffi Griff 210 245 Griff 350 275 340 390 475 530 625
+4 +4 +4 +4 +4 +3.47 +4 +3.53 +3.47 +3.4 +2.88 +3.17 +3 +3.52
S.G. Fd3m; Tetrahedral sites, hT at 8a(1/8,1/8,1/8); octahedral sites, hO at 16d (½,½,½). a Oxygen atoms at 32e(u,u,u). b Refinement from combined XRD and ND data.
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271
Fig. 1. Micrograph (left) and EDS spectrum (right) for y = 1.2.
Fig. 2. EELS spectra for y = 0.40 and 0.5; (a) Mn L2,3 and (b) Fe L2,3 edges.
feature, a great stability of the title ferrites under oxidative conditions and an oxygen stoichiometry nominally equal to that expected, within the experimental error. For example, in Fig. 3b it can be appreciated that, for the y = 0.2 sample, the maximum mass change percentage is ca. 0.15%, which represents a negligible variation in oxygen stoichiometry, d 0.01, leading to a composition nominally equal to that expected, i.e. Li1.3Mn1.5Fe0.2O3.99(1). Particular details observed in some curves (mass gaining and losses which alternate before stabilization) will be discussed below.
Fig. 3. Thermogravimetric profiles (a) under H2:He atmosphere for y = 0.5 and (b) under O2 atmosphere for y = 0.2.
3.2. Structural characterization The structural characterization has been carried out from XRD and ND data, by applying the Rietveld method with the Fullprof program. All the title samples present XRD diffraction patterns characteristic of a cubic symmetry, which maintains the essential features of the parent structure, x = 0 and y = 3 one, i.e. Fe3O4, without showing extra peaks characteristic of structural distortion. In
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Fig. 4. XRD profiles for LixMn3xyFeyO4, for 0 6 y 6 1.30.
Fig. 4 the XRD profiles at room temperature (RT) for all the samples are shown. Relevant changes in intensity specially concerning the (1 1 1), (2 2 0) and (3 1 1) reflections can be appreciated, showing the differences taking place in cationic occupation of both sublattices when varying x and y in the title system [25,26]. As an example, Fig. 5 shows the Rietveld refinement profiles at 300 K from (a) combined ND and XRD data for y = 0.65, and (b) space group. Tafrom XRD data for y = 1.20, considering the Fd3m ble 1 gathers the most relevant structural parameters and Rietveld factors for all the samples. The agreement between the observed data and the proposed model let us assume the accuracy of this one in all cases; therefore we can conclude that XRD and ND data evidence the stabilization of pure phases with spinel-type structure and cubic symmetry at room temperature for every y value essayed. The cell parameter, a, evolves coherently with the mean size of cations located in both sublattices, especially concerning the mean oxidation state of Mn, as can be appreciated in Fig. 6. Finally, the occupation of site-type is graphically shown in Fig. 7 with clarifying purpose. It is also interesting to pay attention to the structural evolution with temperature. A reversible process of decomposition has been detected for y = 0.4 by means of XRD data obtained from the sample heated in an in situ experiment. In Fig. 8 the XRD profiles at temperatures between 25 and 850 °C are shown. Both a change in intensity of (1 1 1), (2 2 0), (3 1 1) and (4 0 0) diffraction peaks and a splitting of (2 2 0) and (4 0 0) reflections are clearly observed and these facts can be interpreted considering two possibilities, schematized by the following decomposition sequences:
Fig. 5. Rietveld profiles at 300 K for (a) y = 0.65, from XRD and ND combined data and (b) y = 1.20 profiles, from XRD data.
TP750 C
Li0:9 Mn1:7 Fe0:4 O4 ! Li0:8 Mn1:5 Fe0:4 O4d þ 0:05 Mn3 O4 O2
þ 0:05 Li2 MnO3
ð1Þ
TP750 C
Li0:9 Mn1:7 Fe0:4 O4 ! zLi2 MnO3 þ ð1 zÞLið0:92zÞ=ð1zÞ Mnð1:7zÞ=ð1zÞ Fe0:4=ð1zÞ O4 þ z=2O2
ð2Þ
whereas (Eq. (1)) leads to the formation of a vacant spinel without any mass loss, (Eq. (2)) implies the removal of a certain amount of oxygen and the stabilization of another spinel-type ferrite with different composition. The main facts observed are the following: the appearance of a subtle Bragg maximum attributable to Mn3O4, the very low value of mass increment obtained in the TG experiment under oxygen atmosphere (less than 1.2%, see Fig. 9) and the splitting of (2 2 0) and (4 0 0) maxima, which takes place at high temperatures (see
Fig. 6. Evolution of cell parameter (right) and mean oxidation state (left) with y.
Fig. 8). All of them suggest that the actual process suffered by the sample probably implies a complex sequence of different reactions or the coexistence of diverse and reversible decomposition paths. The splitting of the mentioned maxima could be effectively related to the stabilization of a vacant phase with decreasing symmetry, considering any S.G. of the previously ascribed ones to vacant spinels, as the tetragonal I41/amd, F41/ddm or the
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Fig. 7. Cationic distribution in octahedral and tetrahedral sites as a function of y.
Fig. 9. Thermogravimetric profiles for y = 0.4 under O2 atmosphere.
Fig. 8. Evolution of diffraction profiles with temperature for y = 0.4. (upper panel). Selected zones showing the splitting of (220) and (400) Bragg peaks (bottom panels).
orthorhombic Fddd [27,28]. Therefore, TG experiments carried out under reductive and O2 atmosphere and XRD data for intermediate phases allow establishing the decomposition sequence and the high stability of the title ferrites under oxidative conditions. However, our available data are not accurate enough to permit us discerning the actual decomposition sequence. Further experiments will be carried out in order to get deeper insight about this question, but it is clear the interest of such reversible transformations concerning oxygen insertion and removal processes.
Fig. 10. Variation of magnetization vs. temperature for LixMn3xyFeyO4.
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nearest neighbor (JAB) and next nearest neighbor (JAA and JBB) interactions, being the first ones the greatest [29]. Thus, all the bonds are not simultaneously satisfied and the system remains frustrated. Moreover, in the title system, in addition to this ‘‘intrinsic’’ frustration, both the magnetic dilution due to the presence of Li and the cationic disorder give rise to a complex magnetic behavior connected to different predominant situations: Yafet-Kittel canted state, localized canted state, spin glass, cluster spin glass, etc. [30,31] The observed general trend in the magnetic response, as a function of y, can be summarized as follows:
Fig. 11. Variation of inverse susceptibility vs. temperature for LixMn3xyFeyO4. Inset shows a zoomed view for y = 0.1 and 0.2.
3.3. Electronic behavior Potential applications in diverse materials research fields have been evaluated: magnetocaloric response, electrochemical and dielectric responses have been analized for selected samples in order to establish the optimized specific compositional ranges for each one. 3.3.1. Magnetic behavior Fig. 10 shows the variation of magnetization with temperature, for an applied magnetic field of 500 Oe, for the LixMn3xyFeyO4 ferrites. As expected, a strong enhancement in magnetization is obtained below a certain temperature, TC, and this temperature varies over a wide range, between ca. 250 and 650 K, for different y values. In spinel-type compounds, the cations occupying the tetrahedral (A) and the octahedral (B) sublattices experience competing
For small y values, a Griffits phase-cluster-type magnetic response is obtained. In Fig. 11 this can be appreciated from the form adopted by the 1/v (inverse susceptibility) vs. T curves near TC. ‘‘Partial’’ ferrimagnetic behavior is detected as a function of composition, above a certain y value. Fig. 12 includes the M vs. H cycles, for representative examples of samples displaying this behavior, showing well defined narrow cycles below the corresponding Curie temperatures. In order to analyze the magnetic ordering in both sublattices and its temperature dependence, some neutron diffraction experiments in a wide range of temperatures were carried out for selected samples. In spinel-type systems the nuclear and magnetic symmetries are the same and, then, the Bragg contributions of both appear at the same angular positions. The evolution of the neutron profiles with temperature and the identification of paramagnetic and ferromagnetic regimes can be followed considering the variation in intensity of (1 1 1) reflection, around 4.8 Å, as shown in Fig. 13 for two selected compositions, y = 1.20 and 1.25. The neutron diffraction data have been refined at low temperatures (below the Curie temperature in each case) considering the As an example, Fig. 14 typical ferrimagnetic arrangement F1. shows the Rietveld neutron diffraction profiles for y = 1. In Table 2 the obtained magnetic moments and R-factors are gathered. The experimental saturation magnetization values, obtained from M vs. H graphs, are also included with comparative aim.
Fig. 12. Magnetization vs. magnetic field curves for LixMn3xyFeyO4.
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a b
y
(A)[B]
lnet (BM)b
RB
lsat (BM)a
0.65 1.00 1.20 1.25
(Li0.65 Mn0.10Fe0.25)[Li0.25Mn1.40Fe0.35] (Li0.47Mn0.28Fe0.25)[Li0.53Mn1.02Fe0.44] (Li0.56Fe0.44)[Mn1.2Fe0.76] (Li0.48Fe0.52)[Mn1.25Fe0.73]
1.40 1.90 2.92 2.88
8.6 6.4 5.4 9.5
1.32 1.43 2.53 2.81
From magnetization data. From ND data at T = 4 K.
Fig. 13. Thermal evolution of ND profiles for (a) y = 1.20 and (b) y = 1.25.
Fig. 15. Graphs of permittivity (e0 ) and energy loss (d vs. frequency at room temperature for LixMn3xyFeyO4 (y = 0, 0.35, 0.40 and 0.60).
Fig. 14. Neutron diffraction profiles for y = 1 at 1.5 K. Observed pattern is denoted by dots, and the calculated and difference plots are shown in solid lines. Reflections positions for nuclear (upper row) and magnetic (lower row) phases are included.
The magnetic moments estimated from the assumption of totally collinear arrangement in each sublattice and the theoretical values of magnetic moments obtained from the cation occupation are clearly much higher than the obtained ones. The deviations observed between the experimental and the estimated net moments suggest the existence of non-collinear spin interactions. The complex magnetic structure as found in our system is a consequence of inhomogeneities arising from fluctuations of magnetic ion con-
centrations. Magnetic dilution in the two sublattices gives rise to frustrations in this perturbed magnetic ordering, in accordance with results previously obtained for similar compounds [32]. Unfortunately this fact constrains the potential high values of magnetic entropy and therefore a useful magnetocaloric response in this system, which remains restricted to certain y values. Nevertheless, the title spinels display a promising magnetocaloric behavior as we have previously reported on some members of the title solid solution [8].
3.3.2. Dielectric response Ferrites are, in general, insulating magnetic oxides. These materials are attractive for microwave device applications owing to their large magnetic permeability, high dielectric constant, low dielectric loss, etc. and chemical stability at relatively low frequencies. Therefore, it is interesting to explore the dielectric behavior of ferrites as a function of composition. For example, in Li-doped Mn/ Zn ferrites, saturation magnetization increases with Li substitution and enhances microwave properties because it plays an important
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Fig. 17. Trends in electronic behavior of LixMn3xyFeyO4.
(see Table 1), it seems that a great degree of polarization is related to a low concentration of Li cations in B sites and to the presence of both Fe and Mn cations in them. Also, it is interesting to note that this phase, y = 0.35, shows a ferromagnetic behavior with TC 210 K, suggesting that it could be a good candidate for magnetodielectric coupling; this property will be analyzed in future works.
Fig. 16. Charge/discharge (potential vs. specific capacity) curves at rate of C/10 between 2.5–3.5 V for y = 0.1, 0.2 and 0.3.
role in structural and magnetic properties. The sample with the highest Li content exhibits a great resistivity and minimal dielectric constant [33]. For selected compositions, the dielectric response was tested from impedance measurements. In Fig. 15 the variation of permittivity, e0 , vs. frequency at RT is shown for the 0 6 y 6 0.6 phases. The observed general trend consists in a relaxation-type plateau at low frequencies and a decreasing of e0 with increasing frequency. This fact can be related to a conducting process implying electron hopping Fe2+ M Fe3+ or Mnn+ M Mn(n+1)+ by electronic exchange between both transition metal cations. Among the samples investigated, it can be underlined the good dielectric response displayed by the y = 0.35 phase. Near RT a dielectric constant of about 5000 is obtained and it remains practically constant in the range 103 and 5 105 Hz, with the lowest dielectric losses registered, less than d = 1. Having in mind the cationic distribution in these phases
3.3.3. Electrochemical behavior In order to evaluate the possible applicability of the title ferrites as cathodes in Li-batteries, the electrochemical behavior of some members has also been studied. Taking into account the Li distribution among the A and B sublattices (Fig. 7) and the well-known mechanism of Li cations motion in the insertion–deinsertion processes [34], the electrochemical analysis was focussed on spinels with y < 1 and high values of x. In Fig. 16 the electrical potential vs. specific capacity graphs for y = 0.1, 0.2 and 0.3 are shown. A plateau around 2.7 V is visible, connected to the Mn4+/Mn3+ pair. The inserted Li amount increases as y does, i.e. Fe concentration increases, being of 0.79 for y = 0.3. Also the specific capacity and the cycling capability are improved as y increases up to the critical value of 0.3. Having in mind the great versatility of the electronic behavior displayed by the title series, different compositional ranges corresponding to the optimization of these materials for different application fields have been established. Fig. 17 summarizes the electronic behavior as a function of composition, taking the Curie temperature as a guiding parameter. Three color regions are highlighted:
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(a) Region e0 : corresponding to compositions in the range 0.35 < y < 0.70, which show the optimal behavior in Dielectrics. The obtained response can be related to the dilution of Mn and Fe in both A and B sublattices, which gives rise to an enhancement of polarization phenomena. (b) Region MCE: covering the compositional range 0.3 < y < 1.2, which gathers the best magnetocaloric responses, in which the Curie temperature can be tuned by changing the Fe content. (c) Region Batteries: containing samples with y < 0.5, which display good electrochemical behavior, linked to a high Li content, i.e. x > 1.2, and an exclusive and high oxidation state for manganese cations (+4), with potential interest in the Li batteries cathodes research field.
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good candidate for presenting eventual magnetoelectric coupling and this point will actually be analyzed in future works. In conclusion, the title system displays an electronic behavior which can find an attractive and wide variety of potential applications in fields of Materials Science related to magnetocaloric, dielectric and electrochemical properties. Acknowledgements Authors are grateful to the CAI centers of UCM (XRD and electron microscopy) and financial support from Spanish Ministerio de Ciencia e Innovación MAT2010-20117, MAT2011-27470-C0202 and CSD2009-00013. D. Alonso-Domínguez thanks the CEI Moncloa for financial facilities.
The intermediate areas enclose samples that potentially could exhibit coupled magneto-electrical properties and, thus, they point to future promising studies. Finally, it is interesting to note that the phase y = 0.35 presents a ferromagnetic behavior with TC 210 K together with relatively high values of permittivity at RT. Therefore it is a good candidate for eventual magnetoelectric coupling and this behavior will be undertaken in due course.
References
4. Summary and conclusions
[10]
Microcrystalline powders of the new ferrites LixMn3xyFeyO4 (0.4 6 x 6 1.33; 0 6 y 6 1.30) have been prepared by the sol–gel method and their compositions have been obtained from TGA, EDS and EELS techniques. EELS experiments indicated a trivalent oxidation state for Fe cations and a mixed valence for Mn. ND and XRD data showed all the samples to present cubic symmetry between 5 and 550 K, S.G. Fd3m. At higher temperatures, above ca. 950 K, a reversible decomposition implying the formation of an ordered vacant phase has been detected. The magnetic behavior is compatible with a frustrated ferromagnetic response due to the atomic disorder in the B sites. The best dielectric response has been measured for the y = 0.35 phase. At temperatures near the ambient, a dielectric constant of about 5000 is obtained and it remains practically constant between 103 and 5 105 Hz, with the lowest dielectric losses registered, less than d = 1. In this sense, low concentration of Li cations in B sites and the presence of both Fe and Mn transitions metals in it seem to be connected to a great degree of polarization in the materials. The samples with y < 1 display a moderate good electrochemical behavior, linked to the Mn4+/Mn3+ pair and the lithium concentration, as expected. A maximum value of 0.79 for the amount of inserted Li is obtained for y = 0.3. Both specific capacity and cycling capability are improved as y increases being the critical value y = 0.3. Having in mind the great versatility of the electronic behavior displayed by the LixMn3xyFeyO4 the optimization of these materials for different application fields seem to reveal different compositional ranges in each case: (a) Dielectrics: 0.35 < y < 0.70, (b) magnetocaloric materials: 0.3 < y < 1.2, (c) Li batteries cathodes: y < 0.5. Finally, it is interesting to note that the phase with y = 0.35 presents a ferromagnetic behavior with TC 210 K, and therefore is a
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