12792
J. Phys. Chem. C 2010, 114, 12792–12799
Characterization of The Lithium-Manganese Ferrite LiFeMnO4 Prepared by Two Different Methods M. Gracia,*,† J. F. Marco,† J. R. Gancedo,† J. Ortiz,‡ R. Pastene,‡ and J. L. Gautier‡ Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, c/Serrano 119, 28006 Madrid, Spain, and LEQFS Departamento de Quı´mica de los Materiales, Facultad de Quı´mica y Biologı´a, UniVersidad de Santiago de Chile, AV. L.B. O’Higgins 3363, Santiago, Chile ReceiVed: April 19, 2010; ReVised Manuscript ReceiVed: June 17, 2010
Lithium-manganese ferrite LiFeMnO4 was prepared using two different methods: a conventional ceramic high-temperature solid-state reaction technique (Cer) and thermal decomposition of metal nitrate salts (NTD). The characterization of the compounds was carried out by SEM/EDX, XRD, XPS, Fe K- and Mn K-edge XANES, and Mo¨ssbauer spectroscopy. Both Cer and NTD LiFeMnO4 samples have the nominal expected Fe/Mn atomic ratio and show a homogeneous morphology, but they exhibit different particle sizes. Fe K-edge XANES and Mo¨ssbauer spectroscopy results show that the oxidation state of Fe ions is +3 in both samples, whereas the Mn K-edge XANES data indicate that the bulk average Mn oxidation state is +3 for the NTD sample and close to +4 for the Cer one. Thus, to maintain the charge neutrality, the NTD sample has to be nonstoichiometric in oxygen with a composition close to LiFeMnO3.5. The Mn 3s XPS data indicate that the average surface oxidation state of Mn is also lower in the NTD sample. The results suggest the occurrence of small Fe clusters inside the spinel-related structure in both samples. The Fe cluster occurrence can be due to the presence of diamagnetic Li+ ions, which share both tetrahedral and octahedral sites with the Fe3+ ions. This can result in different configurations around the Fe-occupied sites such that some of the Fe3+ ions located at the octahedral sites remain in a paramagnetic state at 298 K and, therefore, are responsible for the doublet observed in the corresponding Mo¨ssbauer spectrum. The neighboring Mn-rich regions containing Mn3+ or Mn4+ ions in octahedral positions also modify the existing magnetic interactions in such a way that they become more complex in the NTD sample than in the Cer one, which is reflected in a more marked superparamagnetic-like behavior. This suggests the existence in the NTD sample of a larger amount of Fe clusters, which can be explained by the different oxidation states of Mn, a larger number of oxygen vacancies, and a higher number of Fe sites with reduced coordination in this material. Finally, the differences found in the chemical and structural properties of Cer and NTD samples can be due not only to variations in thermal treatments used in each synthesis procedure but also to the different nature of the starting products in both methods. Introduction Spinel-type manganese-lithium ferrites are of interest in the design of positive electrodes for lithium-ion rechargeable batteries because the replacement of manganese ions (3d3/3d4) by lithium or transition-metal ions at the 16d octahedral sites of the spinel-related structure can be straightforwardly carried out via several trouble-free ways, giving rise to stable matrices with a variety of satisfactory properties for the lithium-ion insertion/extraction.1 Among transition metals, iron is a good candidate for such a replacement due to its abundance and the favorable electrochemical properties of the resulting frameworks.1-6 The electrochemical and magnetic properties of each single phase belonging to the Li-Fe-Mn-O system are connected not only with the range of its Li-Mn-Fe composition but also with both the cationic distribution in the spinel lattice and the Mn and Fe oxidation states. The oxidation states of the ions, their distribution over the tetrahedral (A) and * To whom correspondence should be addressed. E-mail: rocgracia@ iqfr.csic.es. † Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC. ‡ Universidad de Santiago de Chile.
octahedral (B) sublattices, and the electrochemical behavior of the host thermodynamically stable spinels have been extensively studied.1-17 It is known that, in the Mn-Li ferrite spinel system, Mn3+ and Mn4+ ions can substitute the available Fe3+ ions located at the B sites and also that, when manganese is introduced progressively in the lithium ferrite Li0.5Fe2.5O4 (having the cationic distribution (Fe3+)[Li+0.5Fe3+1.5]), Li+ ions are displaced from B to A sites to stay exclusively on tetrahedral sites in the last member of the series: the cubic spinel LiMn2O4,2,7 which exhibits the ionic distribution (Li+)[Mn3+Mn4+]O2-4. Conversely, when Fe enters LiMn2O4, once the iron content exceeds 50%, the lithium ions tend to occupy octahedral positions.7 Mn3+ and, especially, Mn4+ ions have preference for these positions, whereas Mn2+ can go to A sites.8,13,18-20 For a certain critical value of the Li and Mn content, Mn3+ ions located at B sites can move to A sites.18,19 Regarding Fe ion occupancy, it is known that, in the spinel structure of the lithium ferrite Li0.5Fe2.5O4, the Fe3+ ions are distributed over the B and A sites in a ratio of Fe3+(B)/Fe3+(A) ) 1.5.7 The fraction of Fe3+ occupying A sites in the compounds Li0.5FeMn1.5O4 and LiFe0.6Mn1.4O4 is approximately 40% and 9%, respectively,7,8 whereas in LiFe0.5Mn1.5O4, only Li+ ions occupy the A sites.4,5
10.1021/jp103507k 2010 American Chemical Society Published on Web 07/06/2010
LiFeMnO4 Prepared by Two Different Methods X-ray powder diffraction (XRD),1,2,4-17,20-24 57Fe-Mo¨ssbauer spectroscopy,1,4,5,8,10-12,14,15,25 X-ray photoelectron spectroscopy (XPS),10,12,21,26-30 and Mn K-edge and Fe K-edge XANES/ EXAFS,4,12,14 among others, appear to be adequate techniques for determining structural, magnetic, and oxidation state changes in this type of compound and have been largely used. In previous studies, we have carried out the bulk and surface characterization of the series Li1-0.5xFe1.5x+1Mn1-xO4 (0.2 e x e 1) by XRD, Mo¨ssbauer spectroscopy, XPS, and Mn K- and Fe K-edge XANES/EXAFS.10,12 We also studied the electrocatalytic activity of the series for oxygen evolution.10 The first compound of the series, LiFeMnO4 (x ) 0), which was not included in our previous studies, appears interesting because the three metals are simultaneously present with identical stoichiometry. From the literature data it can deduced1,7,14-19,23 that, in the spinel-related structure of LiFeMnO4, the Li+ and Fe3+ ions share the tetrahedral A sites, whereas the 16d octahedral B sites are occupied by Li+, Fe3+, and Mn4+ ions. The atomic metal occupancy reported for this oxide is (Li0.6Fe0.4)[Li0.4Fe0.6Mn]O4.10,14,18,19,23 However, data obtained from distinct synthetic samples showed discrepancies in the cation distribution over the spinel sublattices, the oxidation state of Mn, and the stoichiometry of the oxide. Quenched samples usually show both a larger lattice parameter and different electrochemical properties.15 Also, as it is usual in the preparation of the Li-Fe-Mn-O series, the synthesis conditions are not well-optimized;15 it is necessary to try different strategies of preparation because all structural parameters are highly dependent on the reaction conditions and the various processing steps.2,3,16,17,20 In general, synthesis procedures are based on a solid-solid state reaction induced by thermal treatments upon solid starting materials, such as mixtures of carbonates, oxalates, oxides, or hydroxides, in the adequate metal molar ratio for the desired oxide composition. Sol-gel, hydrothermal processes, ball-milling, or thermal decomposition of the proper metal nitrates are other common synthetic routes.31 The autoignition method of metal nitrates has been also tested.16,17 In this work, we have prepared the lithium-manganese ferrite LiFeMnO4 by two different methods and have carried out the characterization of the resulting samples by XRD, SEM/EDX, XPS, Mo¨ssbauer spectroscopy, and Mn K- and Fe K-edge XANES measurements. Experimental Methods As we have indicated above, the synthesis conditions of Li-Fe-Mn oxides are not well-optimized and it is not unusual to have difficulties in obtaining pure specimens of the spinel oxide LiFeMnO4. Thus, we carried out a first synthesis procedure of this compound by a conventional ceramic procedure10,12 consisting of a solid-state reaction involving Li2CO3, Fe2O3, and MnO2 powders (Fluka, refs 63470, 44956, and 63553, respectively) accurately weighted in the appropriate stoichiommetric ratio. The powders were rigorously mixed and ground in an agate mortar, sintered for 24 h in an oxygen gas flow at 800 °C, and then cooled to RT at a rate of 10 °C min-1. The resulting sample was named Cer. For the second oxide preparation, we used the well-known procedure based on thermal decomposition of metal nitrates (NTD). Starting materials were pure LiNO3, Fe(NO3)3 · 9H2O, and Mn(NO3)2 · 4H2O powdered salts (Merck, refs 1.12230.0250, 3883, and 1.05940.0500, respectively) in a stoichiommetric ratio. The nitrate salts were intimately mixed and heated at 90 °C to dissolve them into their own crystallization water. The heating was maintained until a dry solid residue was obtained. Further, this residue was ground
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12793 and slowly heated to 350 °C and kept at this temperature for nitrate decomposition until the emission of gaseous products stopped completely. The resulting product was ground again and calcined in a tubular furnace under air. The calcination treatment was performed by raising slowly the temperature to 800 °C, keeping a constant temperature of 800 °C for 20 h, and cooling to room temperature (RT) at a rate of 4 °C min-1. The end product (labeled NTD) was also ground and stored in a desiccator. The synthesis conditions used in both cases were those that led to the best results as far as the purity of the phases is concerned. SEM examination and EDX microanalysis were performed with a ZEISS DSM-960 microscope equipped with an EDX unit (ISIS-LINK, Oxford Inst.). Samples were prepared for this study by dispersing the powdered specimens on a conductive carbon tape. XRD data were recorded with a Siemens D5000 diffractometer using Ni-filtered Cu KR radiation (λ ) 0.154056) and a 99.99% pure Si powder as an internal standard. The diffractometer, equipped with a graphite monochromator and a stepped detector (2θ increment ) 0.05°), was operated at 40 kV and 20 mA, over a scanning range of 5 e 2θ e 80°. The cubic spinel lattice parameter, a, was determined using the Lattice program. XPS data were recorded with a triple channeltron CLAM-2 analyzer using Al KR radiation, a constant analyzer transmission energy of 20 eV, and an operating vacuum below 1 × 10-8 Torr. Spectra were recorded from powdered samples mounted on carbon double-sided adhesive tape. Binding energy values were charge-corrected by setting the adventitious C 1s peak at 284.6 eV. Relative atomic concentrations were calculated from spectral areas after Shirley background subtraction and atomic sensitivity factors correction.32,33 XANES measurements were performed at station BM25 (Fe K- and Mn K-edges) at the European Synchrotron Radiation Facility, ESRF, Grenoble, France. Data from the prepared oxides and from the reference compounds R-Fe2O3, MnO, and Mn2O3 were collected at room temperature in the fluorescence mode. The edge profiles were separated from the EXAFS data and, after subtraction of the linear pre-edge background, normalized to the edge step. The edge position was defined as the energy at which the first maximum of the first derivative of the absorption appears. 57 Fe Mo¨ssbauer measurements were carried out at various temperatures between RT and 17 K with a conventional spectrometer equipped with a 57Co (Rh) source and a closedcycle He cryogenerator (Air Products Inc.). The spectra were computer fitted to a sum of Lorentzian lines by applying the constraints of equal line width and area for the two peaks of doublets and equal line width and areas in the 3:2:1:1:2:3 ratio for the six peaks of sextets. The addition of a hyperfine magnetic field distribution was necessary in the majority of the fits, to account for the magnetic broad components. Isomer shifts were referred to the centroid of the spectrum of R-Fe at RT. Relative concentrations of the different iron species at any given temperature were calculated from their corresponding spectral areas, assuming equal recoil-free fractions for all of the iron species. Results SEM/EDX. SEM micrographs reveal that the particles of both samples have a similar cubic morphology but different sizes (Figure 1). Sizes of Cer particles are in the range of 1-3 µm, whereas those of the NTD sample are larger (5-10 µm). In general, a soft synthesis route using lower temperatures favors
12794
J. Phys. Chem. C, Vol. 114, No. 29, 2010
Gracia et al.
Figure 1. SEM images of the LiFeMnO4 samples.
the formation of smaller particles. In both samples, the maximum temperature of the calcination treatment was 800 °C, but the permanence at this temperature was 4 h shorter in NTD than in Cer. However, the NTD sample was slowly cooled from 800 °C to RT, whereas a more rapid cooling rate was used for Cer. Thus, the slower cooling rate used in the NTD synthesis could have given place to the formation of larger particles. The particles have very rounded edges and appear to be assembled in groups formed by a particle syntherization. EDX analyses taken from several zones of the samples (see the Supporting Information) gave Fe/Mn atomic ratios that agree well with the expected nominal value (1.0). EDX and SEM results prove that samples are chemically and morphologically homogeneous. XRD. The main lines in the X-ray powder diffraction patterns recorded from Cer and NTD samples (see the Supporting Information) include the hkl peaks 311, 111, 220, 440, and 511 that are characteristic of the spinel-related structure. Both patterns can be assigned to LiFeMnO4,1,23 but the diffractogram corresponding to the Cer sample showed additional peaks at 37.06° and 44.75° 2θ positions (d-spacing ) 24.26 and 20.25 nm, respectively). The presence of similar XRD peaks in some samples with compositions close to LiFeMnO4, prepared from different methods, had been also observed.1,15,16 These peaks were assigned to impurities,1,15,16 mainly constituted by a mixture of spinel phases (likely to be an iron-enriched phase) and to Li2MnO3.16 We think that these small extra peaks would be due to the presence of the lithium iron manganese oxide Li2Fe0.5Mn1.5O4 as a minor constituent.5 No additional peaks appear in the NTD diffractogram, which corresponds to a LiFeMnO4 single phase. While the a cell parameter of the Cer sample (0.8298 nm) is consistent with published data, that corresponding to the NTD one (0.8344 nm) is not.1,23 This discrepancy could be linked to differences in the Mn3+/Mn4+ ratio (see the XPS and Mn K-edge XANES results) and to variations in the concentration of oxygen vacancies.15 We will return to this result later. It is relevant to remark that the intensity of the 111 peak is directly related to the amount of lithium occupying the A sites; the 111 reflection does not appear in the diffractogram when Li+ ions reside exclusively into the B sites of the spinel lattice.8,13 Therefore, it seems from our XRD data that some Li+ ions have to be placed into the A sites. The difference in the relative intensity of the 111 peak between both diffractograms can be due to the presence of Li2Fe0.5Mn1.5O4 or Li2MnO3 impurities in the Cer sample.5,16 It is well known that the intensity of the 220 reflection, which is usually observed in the powder XRD pattern of spinels, depends upon the atomic scattering factors of the ions occupying tetrahedral 8a sites;13,22-24,34 thus, the 220 peak vanishes in the XRD
Figure 2. Fe 3s and Mn 3s XP spectra recorded from the LiFeMnO4 samples (note the different ∆E values of the Mn 3s doublet).
pattern of LiMn2O4 because, in this structure, the A positions are exclusively occupied by light lithium atoms.15,21,24 Consequently, the presence of the 220 peak in the diffractogram (see the Supporting Information) indicates that, in both samples, a certain amount of Fe3+ ions is located at the A sites with a nearly similar occupation (relative intensity of the 220 reflection is 23.6% and 25.8% for Cer and NTD, respectively). XPS. The Fe 2p spectra recorded from the two samples (not shown) are characterized by binding energies of the Fe 2p3/2 and Fe 2p1/2 core levels of 710.2 and 723.9 eV, respectively, which is indicative of the presence of Fe3+. Although the oxidation state of Mn can be, in principle, deduced from the binding energies of the corresponding Mn 2p core levels, it is much more reliable to deduce it from the inspection of the Mn 3s core levels. Because of the occurrence of multiplet splitting, the Mn 3s core level (which otherwise should be a singlet) splits into a doublet. The separation between the two peaks of such a doublet (∆E3s) is very sensitive to the oxidation state of Mn (o.s.Mn),27-30 which can be estimated from the equation30
O.S.Mn ) 9.67 - 1.27∆E3s
(1)
In the present case (Figure 2 and Table 1), this equation yields a value of 3.5 and 2.3 for the Cer and NTD samples, respectively. This result implies that numerous surface oxygen vacancies have to be present in both samples, especially in the NTD one. We would also mention that the surface Fe/Mn ratios calculated from the XPS data were lower than the nominal value
LiFeMnO4 Prepared by Two Different Methods
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12795
TABLE 1: XPS Study. Fe/Mn Ratio (at.) and Mn Oxidation State Calculated from 2p and 3s Peak Areas and from ∆E3s, Respectively Fe/Mn (at.) exptl, 2p
Fe/Mn (at.) exptl, 3s
Cer NTD
0.714 0.714
0.315 0.379
Cer NTD
BE Mn 3s(1)/eV 83.3 83.0
BE Mn 3s(2)/eV 88.2 88.8
a
Fe/Mn (at.) nominal 1 1
∆E3s/eV 4.9 5.8
Mn oxidation statea 3.45 2.30
Calculated from eq 1.30
Figure 3. Fe K-edge (top) and Mn K-edge (bottom) XANES spectra of the LiFeMnO4 samples and R-Fe2O3, MnO, and Mn2O3 reference spectra.
(Table 1); that is, the sample surface appears to be enriched in Mn with respect to the bulk. XANES. The edge positions of the Fe K-edge XANES recorded from both the Cer and the NTD samples are very similar to that of the XANES recorded from R-Fe2O3 (Figure 3, top). This indicates that the Fe oxidation state in both materials is +3. The intensity of the pre-edge feature at 7113.9 eV, which is related to the coordination of the Fe absorbing atom (it increases if the symmetry of the absorbing site is lowered from octahedral to tetrahedral),12,14
is noticeably more intense in the spectrum of the NTD sample. This result suggests that this latter material could contain a higher proportion of Fe3+ ions in tetrahedral positions or more distorted Fe3+ octahedral sites. The edge positions of the Mn K-edge XANES recorded from both samples (Figure 3, bottom) appear at quite different energies. Whereas that of the NTD sample almost matches that of the edge recorded from Mn2O3 (indicating that the Mn average oxidation state of this sample is +3), the edge recorded from the Cer sample appears at a much higher energy, which suggests that the average oxidation state of Mn in this material is close to +4. This is consistent with the lattice parameters calculated from the XRD data. The lattice parameter calculated for the NTD sample is higher than that calculated for the Cer sample, which can be understood on the basis of the different ionic radii for Mn3+ (higher) and Mn4+ (lower). It is also interesting to note from the comparison of the XPS and XANES data that the Mn average oxidation state in the surface and the bulk of these materials appears to be different. Although the general trend shown by these two techniques is the same, a higher average Mn oxidation state in the Cer sample than in the NTD sample, the oxidation states found by XPS are lower than those found by XANES. The results suggest that the average Mn oxidation state at the surface is lower than that in the bulk, which is surprising, because the average oxidation state of iron is the same both in the surface and in the bulk. If we trust the values obtained from the XPS equation given above, this result can only be explained assuming a really large number of surface oxygen vacancies, such that the charge neutrality can be maintained. Mo¨ssbauer Study. Although the RT Mo¨ssbauer spectra recorded from both samples (Figure 4, left) consist of a Fe3+ doublet and a magnetic sextet component, they present distinct features: (i) a different doublet-to-sextet area ratio and (ii) much broader lines for the sextets of the NTD sample. The parameters obtained for the doublet (see the Supporting Information) correspond to high-spin Fe3+ in octahedral oxygen coordination. This doublet is identical to that found in the RT spectra of compounds of related compositions.4,10-12,15 The magnetic component of the spectra was best fitted considering two discrete sextets and a hyperfine magnetic field distribution (DH). The parameters corresponding to the two sextets (see the Supporting Information) confirm that Fe3+ occupies both the A and the B sites of the spinel-related structure. It is difficult to assign unambiguously the hyperfine magnetic distribution to Fe3+ in either A or B sites on the basis of the isomer shift (δ) values obtained from the fitting. In both cases, they are low to be considered purely octahedral, and most probably, the distribution contains both octahedral and tetrahedral contributions. We have found previously10,12 that, when Mn/Li is inserted in the lithium ferrite, the doublet/sextet ratio at RT increases with increasing Mn content.10,12 Comparison of Cer and NTD RT spectra with the RT spectrum of Li0.9Fe1.3Mn0.8O412 shows that the minor difference in Li/Fe and Mn/Fe ratios existing between the two compositions does not change the ratio between the paramagnetic and magnetic components in the NTD spectrum but produces a notable broadening of the magnetic component and a decrease in the area of the sextet assigned to Fe3+ ions at tetrahedral sites (from 39% in Li0.9Fe1.3Mn0.8O4 to 17% in LiFeMnO4 NTD). For the Cer sample, this decrease is small (to 29%); its magnetic component exhibits no significant broadening, and the proportion of doublet to magnetic component increases moderately. According to the XRD data, this increase could be due to the presence of a minor amount of Li2Fe0.5Mn1.5O4, given that the RT Mo¨ssbauer spectrum of this
12796
J. Phys. Chem. C, Vol. 114, No. 29, 2010
Gracia et al.
Figure 4. Mo¨ssbauer spectra recorded at RT (left) and 17 K (right) from the LiFeMnO4 samples. The subspectra obtained after the computer fit are shown as solid lines.
Figure 5. Mo¨ssbauer spectra recorded at low temperatures from the Cer sample.
oxide is a doublet.5 Matheyshina et al.15 have reported the RT spectrum corresponding to the LiFe0.9Mn1.1O4 oxide to be constituted by a doublet (47%) and a magnetic sextet (53%), assigning these components to Fe3+ in octahedral oxygen coordination (doublet) and to Fe3+ in tetrahedral sites (sextet).
All of these facts suggest that, as stated above, the spectral component DH in our LiFeMnO4 samples include contributions from both A and B sites. The spectra of both samples showed also a different evolution with decreasing temperature (Figures 5 and 6). In the case of
LiFeMnO4 Prepared by Two Different Methods
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12797
Figure 6. Mo¨ssbauer spectra recorded at low temperatures from the NTD sample.
the NTD sample, the doublet decreases in intensity steadily from RT and it is not longer present below 30 K. In the Cer sample, the doublet is also absent below 30 K, but it starts decreasing in intensity around 53 K. However, the broadening of the magnetic pattern continues even at temperatures around 17 K. This line broadening was always higher in the NTD spectra than in the Cer ones. We fitted the low-temperature spectra using a combination of a doublet, two sextets, and a hyperfine magnetic field distribution (DH). Figure 4, right panel, depicts the 17 K spectra with the different fitted subspectra (Mo¨ssbauer parameters at 17 K are reported in the Supporting Information), and Figures 5 and 6 show some of the spectra recorded at different temperatures (the corresponding spectral areas of the different species obtained from the fits are given as the Supporting Information). Because of the complexity of the spectra and the similarity of the parameters corresponding to the different sextets, the difficulty in assigning the remaining distribution component at 17 K to tetrahedral or octahedral sites still persists. In fact, from the evolution of the different spectral areas with decreasing temperature and mainly from that of the doublet, which has RT parameters clearly assignable to octahedral Fe3+, it seems that the magnetic distribution at 17 K should contain contributions from the two different types of sites, especially in the case of the NTD sample. Generally speaking, the behavior of the spectra with decreasing temperature (coexistence of a magnetic part and a para-
magnetic part over a broad range of temperatures, broadened spectral lines, and, for the distribution component, lower hyperfine magnetic field values at any given temperature than those expected to occur in ferrites) resembles that of superparamagnetic-like systems. In the present case, the superparamagnetism-like behavior would not be linked to the existence of small particles but to the existence of Fe clusters of different sizes within the spinel-related structure. The presence of diamagnetic Li ions in both A and B positions, magnetic Mn3+and Mn4+ ions in B positions, and Fe3+ in both A and B sites and, in the case of the NTD sample, of oxygen vacancies would bring about the existence of different configurations around the different Fe3+ A and B sites. The combination of all of these facts suggests a complex magnetic structure such that, for some configurations, depending on the number and type of neighboring ions, the magnetic interactions can collapse at RT (as is the case for some Fe3+ in octahedral positions) or reduce the hyperfine magnetic field (which occurs in a considerable fraction of the Fe3+ octahedral and tetrahedral sites). From the variation of the magnetic/doublet area ratio with temperature, the blocking temperature for the superparamagnetic-like clusters in the different samples can be estimated (Figure 7): 44 and 88 K for Cer and NTD samples, respectively. The shape of the curves given in Figure 7 also indicates that, in the NTD sample, there is a much larger distribution of cluster
12798
J. Phys. Chem. C, Vol. 114, No. 29, 2010
Figure 7. Variation with temperature of the magnetic ratio obtained from the Mo¨ssbauer spectra. The blocking temperature for the doublet can be estimated as 44 and 88 K for Cer and NTD samples, respectively.
sizes (a much broader range of blocking temperatures) than in the Cer sample. Discussion SEM/EDX results prove that the samples of the spinel oxide LiFeMnO4 obtained by both Cer and NTD procedures have the expected Fe/Mn ratio, being also chemically and morphologically homogeneous. The differences observed between particle sizes of both samples can be attributed to dissimilarities of the thermal cycles used in each synthesis. The distinct average oxidation state of Mn ions found for both samples (bulk ) +4 and +3; surface ) +3.5 and +2.3 for Cer and NTD, respectively) can explain the different values obtained for the a lattice parameter, which was higher for the NTD sample. This is so because Mn4+ is a smaller cation than Mn3+ (ionic radii of 78.5 and 67.0 pm for Mn3+ and Mn4+, respectively). It has been proved34 that, in this type of spinel structure, the a lattice parameter depends on the thermal treatments undergone by the sample because the thermal history (calcination or subsequent annealing temperatures and cooling rates) can change the oxidation states of the existing cations and their distribution within A or B sites of the spinel lattice, resulting even in an oxygen deficiency.15 In both synthesis procedures, we used identical maximum temperatures, and from the shorter length of the calcination and slower cooling rate used for the NTD sample,34 a lower a lattice parameter for NTD than for Cer would be expected. For these reasons, the higher Mn3+/Mn4+ ratio and the oxygen deficiency existing in the NTD sample might probably be connected to the oxidation of Mn2+ to Mn3+, which arises through a thermal decomposition of the reagent Mn(NO3)2 · 4H2O, that takes place earlier than the
Gracia et al. calcination process and to the length of the proper calcination treatment, which was 4 h shorter in the NTD than in the Cer. In both samples, but especially in the NTD, the Fe3+ ions placed in the B sites appear to be in a very inhomogeneous situation due to the existence of some high valence Mn-rich regions (Mn4+ or Mn3+ in 16d sites), occupancy by diamagnetic Li+ ions of both A and B sites, and/or an oxygen deficiency around Fe ions. Even more, because the average oxidation state of iron is the same, both in the surface and in the bulk, and the Mn oxidation state at the surface is lower than in the bulk, as it has been evidenced from XPS and XANES results, it is possible to assume the existence in both samples of a large number of surface oxygen vacancies. All of these facts can be the origin of the line broadening of the magnetic components and/or of the presence of the Fe3+ doublet observed in the RT Mo¨ssbauer spectra. The superparamagnetic-like behavior exhibited by both samples appears to be related to the existence of small Fe clusters inside the spinel-related structure caused by concurrent occupation of both A and B sites by nonmagnetic Li+ ions, as XRD data proved. The blocking of these possible small Fe clusters (those giving place to the doublet in the RT spectra) occurs in a narrow range of temperatures in the Cer sample and in a much broader temperature range in the NTD one. Thus, it is clear from these data that the different synthesis conditions used have a large influence on the cation distribution and on the existence of oxygen vacancies and, therefore, in the magnetic properties displayed by both materials. Comparison of both series of Mo¨ssbauer spectra points to an A occupation of Fe lower in the NTD than in the Cer. Differing from the Mo¨ssbauer data, Fe K-edge XANES results indicate that the fraction of Fe3+ in a coordination lower than octahedral is higher in the NTD sample than in the Cer one. Finally, XRD gives almost equal amounts of Fe occupying the A sites in both samples. An occupation of the A sites higher than 40% Fe is not expected for the composition range of this spinel oxide.1,7,12 Unfortunately, as stated above, our Mo¨ssbauer data do not allow discerning the relative distribution of Fe over A and B sites (note that the best resolved 17 K Mo¨ssbauer spectrum of the Cer sample would give a fraction of Fe on A sites of approximately 40%, which is close to the nominal value calculated for the LiMnFeO4 oxide,1,14,18,23 only if the nonnegligible (35%) magnetic component DH was assigned to B sites). It might be possible that the anionic nonstoichiometry (large number of oxygen vacancies) existing in the NTD sample can lead to a variety of distinct oxygen coordinations around the Fe ions located at B sites. In this case, the coordination of some of them becomes nearly tetrahedral4 or, in other words, the existence of some oxygen pentacoordinated Fe3+ atoms cannot be discarded. This could explain the observed discrepancy in the Fe-occupation results obtained from the various techniques as well as the low isomer shift value that characterizes the distribution component in the Mo¨ssbauer spectra. Conclusions Both LiFeMnO4 samples synthesized by the Cer and the NTD methods show the expected Fe/Mn atomic ratio. Both oxide preparations contain only Fe3+ ions, but whereas for the Cer sample, the bulk average Mn oxidation state is close to +4, as expected, it is +3 for the NTD sample. This implies that the NTD sample has to be nonstoichiometric in oxygen with a composition close to LiFeMnO3.5. Both samples, but specially the NTD, show a complex magnetic behavior linked to the inhomogeneous neighborhoods of the Fe3+ ions, which occupy both the A and the B sites and
LiFeMnO4 Prepared by Two Different Methods share them with Li+ ions. The Fe3+ ions are also surrounded by Mn-rich parts because the Mn4+ or Mn3+ ions are placed at the B sites. Dissimilarities found in the Mn oxidation states, the cation distribution, and the oxygen deficiency affect considerably the magnetic properties of the samples, which can be explained by the differences in thermal synthesis treatments and by the distinct type of starting products of both methods. We would finish commenting that it has been reported recently16,17 that materials of related composition prepared by different methods show very different electrochemical properties. Acknowledgment. This work was supported by the CSICUSACH cooperation program and Fondecyt (Chile) 1050175. We also acknowledge financial support from Dicyt-USACH. We thank Fernando Pinto (The Electronic Microscopy Service of the Centro de Ciencias Medioambientales, CSIC, Madrid) for the SEM/EDX measurements. We are grateful to the Spanish Ministry of Science and Innovation for providing financial support to carry out the XANES measurements at the SPLINE beamline of the ESRF. We thank Dr. Germa´n Castro and Dr. I. Da Silva for their assistance during the XANES measurements. Supporting Information Available: EDX analyses, X-ray powder diffraction patterns, Mo¨ssbauer parameters at RT and 17 K, and relative spectral areas of the distinct components obtained from the fits. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ohzuku, T.; Ariyoshi, K.; Takeda, S.; Sakai, Y. Electrochim. Acta 2001, 46, 2327–2336. (2) Grygar, T.; Bezdicˇeka, P.; Vorm, P.; Jordanova, N.; Krtil, P. J. Solid State Chem. 2001, 161, 152–160. (3) Grygar, T.; Bezdicˇka, P.; Piszora, P.; Wolska, E. J. Solid State Electrochem. 2001, 5, 487–494. (4) Shigemura, H.; Sakaebe, H.; Kageyama, H.; Kobayashi, H.; West, A. R.; Kanno, R.; Morimoto, S.; Nasu, S.; Tabuchi, M. J. Electrochem. Soc. 2001, 148, A730–A736. (5) Chen, C. J.; Greenblatt, M.; Waszczak, J. V. J. Solid State Chem. 1986, 64, 240–248. (6) Fergus, J. W. J. Power Sources 2010, 195, 939–954. (7) Wolska, E.; Stempin, K.; Krasnowska-Hobbs, O. Solid State Ionics 1997, 101-103, 527–531. (8) Bonsdorf, G.; Scha¨ffer, K.; Langbein, H. Eur. J. Solid State Inorg. Chem. 1997, 34, 1051–1062.
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12799 (9) Woodley, S. M.; Catlow, C. R. A.; Piszora, P.; Stempin, K.; Wolska, E. J. Solid State Chem. 2000, 153, 310–316. (10) Rı´os, E.; Chen, Y.-Y.; Gracia, M.; Marco, J. F.; Gancedo, J. R.; Gautier, J. L. Electrochim. Acta 2001, 47, 559–566. (11) Tsuji, T.; Nagao, M.; Yamamura, Y.; Tai, N. T. Solid State Ionics 2002, 154-155, 381–386. (12) Gracia, M.; Marco, J. F.; Gancedo, J. R.; Gautier, J. L.; Rı´os, E. I.; Mene´ndez, N.; Tornero, J. J. Mater. Chem. 2003, 13, 844–851. (13) Wende, C.; Langbein, H. Cryst. Res. Technol. 2006, 41, 18–26. (14) Wende, C.; Olimov, Kh.; Modrow, H.; Wagner, F. E.; Langbein, H. Mater. Res. Bull. 2006, 41, 1530–1542. (15) Mateyshina, Y. G.; Uvarov, N. F.; Ulihin, A. S.; Pavlyukhin, Y. T. Solid State Ionics 2006, 177, 2769–2773. (16) Mateyshina, Y. G.; Lafont, U.; Uvarov, N. F.; Kelder, E. M. Solid State Ionics 2008, 179, 192–196. (17) Mateyshina, Y. G.; Lafont, U.; Uvarov, N. F.; Kelder, E. M. Russ. J. Electrochem. 2009, 45, 602–605. (18) Blasse, G. Philips Res. Rep., Suppl. 1964, 3, 1–139. (19) Blasse, G. Philips Res. Rep. 1965, 20, 528–555. (20) Piszora, P.; Nowicki, W.; Darul, J.; Wolska, E. Mater. Lett. 2004, 58, 1321–1326. (21) Wei, Y. J.; Yan, L. Y.; Wang, C. Z.; Xu, X. G.; Wu, F.; Chen, G. J. Phys. Chem. B 2004, 108, 18547–18551. (22) Robertson, A. D.; Tukamoto, H.; Irvine, J. T. S. J. Electrochem. Soc. 1999, 146, 3958–3962. (23) Liu, Y. F.; Feng, Q.; Ooi, K. J. Colloid Interface Sci. 1994, 163, 130–136. (24) Wickham, D. G.; Croft, W. J. J. Phys. Chem. Solids 1958, 7, 351– 360. (25) Sakai, Y.; Ariyoshi, K.; Ohzuku, T. Hyperfine Interact. 2002, 139/ 140, 67–76. (26) Gautier, J. L.; Rı´os, E.; Gracia, M.; Marco, J. F.; Gancedo, J. R. Thin Solid Films 1997, 311, 51–57. (27) Marco, J. F.; Gracia, M.; Gancedo, J. R.; Gautier, J. L.; Berry, F. J. Recent Research DeVelopments in Inorganic and Organometallic Chemistry; Research Signpost: Trivandrum, India, 2001; Vol. 1, pp 4560. (28) Kosova, N. V.; Asanov, I. P.; Devyatkina, E. T.; Avvakumov, E. G. J. Solid State Chem. 1999, 146, 184–188. (29) Galakhov, V. R.; Demeter, M.; Bartkowski, S.; Neumann, M.; Ovechkina, N. A.; Kurmaev, E. Z.; Lobachevskaya, N. I.; Mukovskii, Ya. M.; Mitchell, J.; Ederer, D. L. Phys. ReV. B 2002, 65, 113102. (30) Beyreuther, E.; Grafstro¨m, S.; Eng, L. M.; Thiele, Ch.; Do¨rr, K. Phys. ReV. B 2006, 73, 155425. (31) Taniguchi, I.; Bakenov, Zh. Powder Technol. 2005, 159, 55–62. (32) Sherwood, P. M. A. In Practical Surface Analysis: by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, U.K., 1990; Vol. 1, p 574. (33) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211. (34) Tarascon, J. M.; McKinnon, W. R.; Coowar, F.; Bowmer, T. N.; Amatucci, G.; Guyomard, D. J. Electrochem. Soc. 1994, 141, 1421–1431.
JP103507K
