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Structural and morphological characterization of FeCo2O4 and CoFe2O4 spinels prepared by a coprecipitation method
Solid State Sciences 5 (2003) 383–392 www.elsevier.com/locate/ssscie
Structural and morphological characterization of FeCo2 O4 and CoFe2 O4 spinels prepared by a coprecipitation method T.A.S. Ferreira a,b , J.C. Waerenborgh a,c , M.H.R.M. Mendonça a , M.R. Nunes a , F.M. Costa a,∗ a CCMM, Depto Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Ed. C8, Piso 5, 1749-016 Lisboa, Portugal b Depto Química, Universidade de Évora, Colégio Luís Verney, Rua Romão Ramalho, 59, 7000 Évora, Portugal c Instituto Tecnológico e Nuclear, Química, Estrada Nacional 10, 2686-953 Sacavém, Portugal
Received 4 September 2002; accepted 21 October 2002
Abstract Due to the importance of cation distribution in the physical properties of spinels, a structural and morphological study of the FeCo2 O4 and CoFe2 O4 spinels prepared by a low-temperature coprecipitation method has been undertaken. Only spinel phases are observed after annealing at 570 K. However, up to 770 K the sample with global composition CoFe2 O4 was chemically heterogeneous. At 1170 K, a single homogeneous CoFe2 O4 spinel is obtained (average grain size ≈ 1 µm, inversion degree λ ≈ 0.76 ± 0.02). FeCo2 O4 could only be prepared by heating at ≈ 1170 K (average grain size ≈ 0.5 µm and λ approximately equal or slightly larger than 0.50, depending on heating time). At lower temperatures Co- and Fe-rich spinel phases are segregated. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Spinel; Co–Fe–O system; Coprecipitation method; Rietveld refinement; Mössbauer spectroscopy
1. Introduction Oxides with the spinel structure are some of the most studied compounds in solid state sciences due to their wide range of applications as magnetic materials, semiconductors, catalysts and pigments, among others. Oxide spinels may be described by the general formula AB2 O4 , where A and B stand for tetrahedral and octahedral cation sites in a cubic close packing of oxygens. The unit cell contains 8 unit formulas and the symmetry is cubic, ¯ space-group Fd3m. The anionic array is described by the monovariant equivalent position 32eu , point symmetry 3m; the actual values of the free parameter u (commonly known as the oxygen positional parameter) show a slight deviation from 1/4, the ideal value for cubic-closest packing if the ¯ unit cell origin is taken at a centre of symmetry 3m, equiposition 16c; this deviation usually increases the volume ratio between the occupied A and B sites, respectively, 8a ¯ ¯ (1/8, 1/8, 1/8), 43m, and 16d (1/2, 1/2, 1/2), 3m. The structure of spinel oxides is responsible for the variety of interesting physical and chemical properties exhibited * Corresponding author.
E-mail address: [email protected] (F.M. Costa).
by these compounds, since it can accommodate a very large number of different cations, some in more than one oxidation state, distributed in different ways among the A and B sites. Two extreme distributions may be defined: the normal and the inverse. For spinels where only divalent and trivalent cations are present, the inversion degree, λ, is defined as the fraction of A sites occupied by trivalent cations. Accordingly, for a normal spinel λ = 0 and for a completely inverted spinel λ = 1. Determination of the cation distribution is of considerable relevance because the theoretical interpretation of the chemical and physical properties of these compounds depends on this distribution. The cation octahedral and tetrahedral sites preferences as well as the temperature of equilibrium have a strong influence on the cation arrangement. Different thermodynamic models have been proposed to treat quantitatively this arrangement [1–3]. De Guire et al. [4] have emphasized the effect of the kinetics of cooling. Unless the spinels are characterized at high temperature, the cation arrangement measured at room temperature depends on the cooling rate, as the kinetics of cation diffusion is fast enough to allow some degree of ordering during quenching, leading to quenchedin distributions that differ from the equilibrium one at the annealing temperature.
1293-2558/03/$ – see front matter 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00011-6
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The investigation of the Co1+x Fe2−x O4 (0 x 1) system has been undertaken for long by several authors. Lotgering [5] and Blasse [6] using magnetic properties reported a normal cation distribution for FeCo2 O4 , i.e., Co2+ [Fe3+ Co3+ ]O4 where cations in square brackets are located on B sites. Takahashi and Fine [7] by extrapolating to 0 K the measured saturation magnetization of a number of cobalt–iron spinels, confirmed that Co3+ , in the lowspin state, replaces Fe3+ in the B site as it is introduced in Co1+x Fe2−x O4 . In contrast to Lotgering and Blasse and based on Mössbauer data, Smith et al. [8] have proposed that 45–50% of Fe3+ is located in the A sites in FeCo2 O4 annealed at 1170 K and cooled down to room temperature in 100 s. On the other hand, Murray and Linnett [9,10] obtained no more than 30% for Fe3+ in A sites for samples heated at 1193 K for three days and quenched in water, while Uzunova et al. [11], more recently, suggested a fraction of Fe3+ in A sites of 41% for samples prepared under mild conditions and submitted to a final temperature of 570 K. As far as CoFe2 O4 is concerned, Mössbauer spectroscopy measurements have suggested that samples annealed at 1520 K have λ ≈ 0.76 if they are water quenched or 0.93 if slowly cooled [12,13]; λ ≈ 0.62 was reported for the samples annealed at 1320 K and quenched in water [9,10] while λ ≈ 0.80 was suggested for CoFe2 O4 prepared at 870 K [11]. A synchrotron X-ray diffraction study, taking advantage of anomalous X-ray scattering, of a sample annealed at 1073 K suggested λ ≈ 0.78. No details were, however, given about the cooling conditions [14]. The study of the transport properties of Co containing spinels as versatile materials for electrocatalysis has been undertaken [15–17]. In order to avoid the drawbacks from high-temperature solid-state reactions, these oxides have been synthesized by wet-chemical methods. As referred above, cation distribution and, consequently, the physical properties of Co1+x Fe2−x O4 are strongly dependent on the heating treatment. Considering the scarce results published on Co1+x Fe2−x O4 prepared by these methods and particularly the report on the preparation of a FeCo2 O4 single phase at 570 K [11], while phase diagrams indicate a two-phase equilibrium at this temperature [7,10], a detailed investigation of the evolution with annealing temperature of morphological and structural features of the spinel phases FeCo2 O4 and CoFe2 O4 synthesized by a low-temperature coprecipitation method and followed by annealing at different temperatures, is reported in this work.
2. Experimental FeCo2 O4 and CoFe2 O4 spinels were synthesized according to a previously described method [18]. Iron chloride, FeCl3 , and cobalt chloride, CoCl2 ·6H2 O were weighted in the adequate molar ratios Co/Fe, dissolved in distilled water and poured into a boiling hydroxide solution at 350– 370 K, keeping a constant vigorous stirring. The solution pH was confirmed to be higher than 12. Black precipitates were formed and heated at 350 K for one hour. After repeated filtering and washing with boiling distilled water, the precipitates were dried on a sand bath at 470 K. All the resulting materials were annealed in air for 1 h 30 at 570 K and 770 K, and for 6 h at 1170 K. In the case of FeCo2 O4 an annealing of 6 h at 1070 K was also performed. After annealing, the samples were quenched in air down to room temperature (RT). All the samples were checked by powder X-ray Diffraction (XRD). In order to examine the effect of the heating time and cooling rate on the cation distribution and phase stability of FeCo2 O4 , two additional heat treatments were performed on these samples. One was annealed at 1170 K for 17 h and quenched, FeCo2 O4 (17, q), and the other was annealed at 1170 K and slowly cooled, at a rate of 3 ◦ C/min, FeCo2 O4 (s.c.). The FeCo2 O4 sample annealed for 6 h and quenched will hereafter be referred to as FeCo2 O4 (6, q). The total Co and Fe contents of four selected samples (Table 1) were estimated by Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma Emission Spectroscopy (ICP), respectively. Scanning electron microscopy (SEM) and X-ray microanalysis, energy dispersive spectroscopy (EDS) were performed in a JEOL (JSM-35C) scanning electron microscope working at 22 keV, with an energy dispersive X-ray spectrometer Noram (Voyager). Powder XRD was carried out on a Philips X-ray diffractometer (PW 1700). Cu Kα radiation was used for the routine sample characterization. The diffractograms analysed by the Rietveld method were obtained using Co Kα radiation (λ1 = 1.78896 Å and λ2 = 1.79285 Å). Co was used instead of Cu radiation in the hope to improve the accuracy of the method. Fe and Co have similar atomic numbers, however, as the wavelength of Co Kα radiation is close to the Fe absorption K edge, the difference between the scattering factors of Co and Fe becomes larger for this radiation due to the anomalous scattering effect (cf. tables of dispersion corrections in Refs. [19,20]). The diffraction patterns were col-
Table 1 Chemical analysis for the FeCo2 O4 and CoFe2 O4 samples annealed at 570 K and 1170 K Sample FeCo2 O4 FeCo2 O4 CoFe2 O4 CoFe2 O4
Heating temperature (K)
% Fe (ICP)
% Co (AAS)
Actual composition (± 0.01 in Fe and Co contents)
570 1170 570 1170
22.2 23.4 43.0 44.7
46.6 49.4 23.4 23.9
Fe0.97 Co2.03 O4 Fe0.96 Co2.04 O4 Co1.06 Fe1.94 O4 Co1.05 Fe1.95 O4
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lected from 17 to 145◦ 2θ with a step of 0.02◦ 2θ . All peak positions were used in the determination of precise lattice parameters. Structural refinements were carried out using the Rietveld program FULLPROF [21]. Peak profiles were fitted with a pseudo-Voigt function and an asymmetry parameter was considered for peaks below 70◦ 2θ . The background was refined with a polynomial function. Scattering factors for Co2+ , Co3+ and Fe3+ cations and O2− anion were obtained from [22] and the anomalous dispersion corrections from Cromer et al. [19,20]. Mössbauer spectra were acquired using a spectrometer combined with a 512 multichannel analyser, with the absorber at RT and at 78 K. A 57 Co(Rh) source (Ritverc GmbH and V.G. Khlopin Radium Institut) with activity of about 50 mCi was used. The spectrometer was calibrated with a thin absorber of elemental iron. The isomer shifts are referred to iron metal. The absorbers were prepared in the form of a uniform layer of the crystalline powder samples dispersed in an appropriate varnish (G.E. Varnish), without iron. During data collection, temperature was controlled within ±1◦ C. The Mössbauer spectra were computer-processed and decomposed to single Lorentzians by the least-square method used by the Normos fitting program [23]. During refinement, for each sextet, the widths of the 6 peaks were kept equal while the ratio of the relative areas I1,6 /I2,5 /I3,4 were kept equal to 3:2:1. Further constraints were used as explained below.
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3. Results and discussion 3.1. Chemical analysis and morphological characterization Duplicate analyses of different fractions of the same sample were found to agree within 0.01 cations per formula-unit confirming the reproducibility of the AAS and ICP methods. The results obtained for the four selected samples (Table 1) are in good agreement with the expected stoichiometry. SEM micrographs show that the CoFe2 O4 grains have an average size of 2–3 µm and that there is no significant variation in grain size with heating temperature (Fig. 1a and b). On the other hand, for FeCo2 O4 samples there is a slight increase in the average grain size from ≈ 0.3 µm at 570 K up to ≈ 0.5 µm at 1170 K (Fig. 1c and d). 3.2. Phase analysis and structural characterization The powder XRD data of the CoFe2 O4 (Fig. 2) and all the FeCo2 O4 samples annealed at 1170 K (Fig. 3), either quenched or slowly cooled, reveal a well-crystallized cubic spinel-type phase. No impurity phases were detected. In contrast, in the powder X-ray diffractograms of FeCo2 O4 samples annealed between 570 K and 1070 K, although all the diffraction peaks are located at the positions expected for a spinel phase, they are very broad at low-θ angles and splitted for 2θ 50◦ (Fig. 3). These features indicate that in the temperature range 570–1070 K, two spinel
Fig. 1. SEM micrographs of samples annealed at different temperatures: CoFe2 O4 samples: (a) 570 K; (b) 1170 K; FeCo2 O4 samples: (c) 570 K; (d) 1170 K.
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Fig. 2. X-ray diffraction pattern (Co Kα) for CoFe2 O4 sample heated at 570 K (a) and heated at 1170 K (b) and quenched. The Miller indices refer to the spinel phase. The curves over the experimental points in (b) are the patterns calculated by the Rietveld analysis. The difference between the calculated and observed pattern is represented below.
phases are present, in agreement with the phase diagram reported in Murray and Linnet [10] and with the fact that a single spinel phase is observed by Takahashi [7] for FeCo2 O4 only above 1120 K. The unit-cell parameters estimated for these spinels from the 570 K data are a = 8.269 ± 0.001 Å and a = 8.1075 ± 0.0009 Å. Considering the Fe2+ , Co2+ , Fe3+ and Co3+ cation radii [1] the spinel phase with lower a should be richer in Co. The broadening of the diffraction peaks occurs fundamentally due to the existence of microdomains with slightly distinct compositions. This phase segregation probably takes place during the coprecipitation of hydroxides at 353 K and seems to remain up to 1170 K where a single-phase spinel is formed. As referred above, no broadening of diffraction peaks are observed in the XRD pattern of the FeCo2 O4 sample slowly cooled from 1170 K to RT. No impurity phases are observed, either. This suggests that once a homogeneous FeCo2 O4 phase is formed at 1170 K, it may withstand relative long exposures to temperatures where it is either thermodynamically unstable or where its formation as a single phase is kinetically very slow.
Fig. 3. X-ray diffraction pattern (Co Kα) of the FeCo2 O4 sample heated at 570 K (a), at 1170 K and quenched (b), at 1170 K and slowly cooled (c). The Miller indices in (a) refer to the spinel phase richer in iron; the symbol (F) refers to the peaks with the same Miller indices of the spinel phase richer in cobalt. The curves over the experimental points in (b) and (c) are the patterns calculated by the Rietveld analysis. The differences between the calculated and observed patterns are represented below the corresponding diffractograms.
Diffraction peaks of CoFe2 O4 annealed at 570 K (Fig. 2) are not splitted but are broadened. This broadening may not be attributed to small grain size effects since at 1170 K
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diffraction peaks are narrow although SEM analyses reveal no increase in grain size (Fig. 1a and b). Line broadening seems therefore to be related to crystal defects or chemical heterogeneity of the sample. Differences in composition are however not large enough to be detected by the present SEM/EDS results. 3.3. Structure refinement The crystal data and site occupation factors estimated from the Rietveld analysis for the single-phase samples are summarized in Table 2. The crystal refinements were performed assuming space ¯ group Fd 3m. In the final refinements 23 parameters were fitted to the 6400 data points: 1 scale factor, 6 background parameters, zeropoint for 2θ , 5 profile function parameters (2 for the Lorentzian/Gaussian mixing parameter and 3 for the width), 4 asymmetry correction parameters, the cell parameter a, the oxygen positional parameter u, 3 isotropic thermal parameters (B), one for each equiposition 8a, 16d and 32e, and 1 cation disorder parameter which allowed the occupation of the 8a and 16d sites by Fe3+ and Co2+ , respectively, to vary within the constraints of full site occupancy. In the case of FeCo2 O4 and based on the strong octahedral site preference of low-spin Co3+ [7,8,24], it was assumed that all the Co3+ is present on the B site. The estimated site occupation factors correspond to the cation distributions summarized in Table 2. In order to check if these cation distributions are consistent with the estimated a and u values, interatomic distances between O2− and the cations on the A sites and on the B sites, dt and do , respectively, are derived and compared with those deduced
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from the effective cation radii of O’Neill and Navrotsky [1]. These radii have been optimized for oxide spinels and have been found to be in better agreement with experimental data for these materials [25] than those of Shannon [26]. The interatomic distances between a cation and the nearest anion are given by [27] √ dt = a 3(u − 0.125) for the 8a site, (1) do = a(3u2 − 2u + 0.375)1/2
for the 16d site.
(2)
From these values cation radii (hereafter referred to as structural radii as in [25]) may be estimated by taking rt = dt − rO2−
and ro = do − rO2−
(3)
and assuming that the radius of tetrahedrally coordinated O2− is rO2− = 1.38 Å [26]. The average cation radii, rt and ro for the 8a and the 16d sites, respectively, were calculated according to [1,25]
rt = (1 − λ) rCo2+ ,t + λ rFe3+ ,t . (4)
ro =
(rCo3+ ,o ) + λ(rCo2+ ,o ) + (1 − λ)(rFe3+ ,o )
2 for the FeCo2 O4 samples or
ro =
λ(rCo2+ ,o ) + (2 − λ)(rFe3+ ,o )
(5) 2 for the CoFe2 O4 sample, where λ, the inversion degree, is the fraction of A sites occupied by Fe3+ according to the estimated site occupation factors (Table 2) and (rXn+ ,s ) are the effective cation radii [1]. The results are presented in Table 2.
Table 2 Crystallographic data, including lattice parameters, site occupancy and equivalent isotropic thermal factors (Beq ), estimated from the Rietveld analysis of the powder XRD data of FeCo2 O4 (17, q) and (s.c.) and CoFe2 O4 samples annealed at 1170 K. Structural radii (rt ; ro ) and effective cation radii ( rt ; ro ) [1], for tetrahedral and octahedral sites, calculated from a and u and from the cation site occupancy, respectively Sample
FeCo2 O4 (17,q)
FeCo2 O4 (s.c.)
CoFe2 O4
a (Å) Cell volume (Å3 ) u (Å) Site occupancy Co(II) 8a Fe(III) 8a Co(II) 16d Fe(III) 16d Co(III) 16d Btet (Å2 ) Boct (Å2 ) Boxy (Å2 ) RBragg (%); RF (%) Inversion degree obtained from site occupancy Formula obtained from site occupancy (cations in square brackets are located on B sites) rt ; ro (Å)
rt NN ; ro NN (Å)
8.2420(1) 559.88(1) 0.25918(6)
8.2436(1) 560.22(1) 0.25839(6)
8.3806(1) 588.60(1) 0.25664(8)
3.72(2) 4.28(2) 4.28(2) 3.72(2) 8.00 0.11(2) 0.62(1) 0.76(2) 2.17; 2.13
3.66(8) 4.34(8) 4.34(8) 3.66(8) 8.00 0.33(2) 1.08(1) 1.97(3) 3.58; 4.12
2.04(3) 5.96(3) 5.96(3) 10.04(3) – 0.59(2) 0.71(2) 1.33(3) 3.24; 3.59
λ = 0.54 ± 0.01 3+ 2+ 3+ 3+ (Co2+ 0.46 Fe0.54 )[Co0.54 Fe0.46 Co1.00 ]O4
λ = 0.54 ± 0.01 3+ 2+ 3+ 3+ (Co2+ 0.46 Fe0.54 )[Co0.54 Fe0.46 Co1.00 ]O4
λ = 0.75 ± 0.01 3+ 2+ 3+ (Co2+ 0.25 Fe0.75 )[Co0.75 Fe1.25 ]O4
0.535; 0.608 0.53; 0.61
0.525; 0.614 0.53; 0.61
0.531; 0.661 0.51; 0.67
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Considering that the standard deviations of the effective cation radii of O’Neill and Navrotsky [1] are ±0.01 Å, the values calculated for the structural radii and the corresponding average cation radii are the same within experimental error. Those deduced for the A site cations in CoFe2 O4 are the only pair that differ by more than 0.01 Å. Taking into account the propagation of errors associated with the calculation of the average cation radii, the difference between these rt and rt is still lower than twice the derived standard deviation (0.014 Å). The estimated values for the a and u parameters may therefore be considered consistent with the cation distribution deduced from the site occupation factors. 3.4. Mössbauer data of the CoFe2 O4 samples The RT Mössbauer spectra of the CoFe2 O4 samples annealed at 1170 K and 570 K are shown in Fig. 4. The spectrum of the sample annealed at 1170 K (Fig. 4a) is similar to that reported by Sawatzky et al. [12,13] and was fitted by the model proposed by these authors, according to which the supertransferred magnetic hyperfine fields of Fe3+ on the B sites increase with the number of Fe3+ next-nearest (NN) neighbours located on A sites. Each B site has 6 A sites as NN neighbours. Assuming a statistical occupation of the A sites by Fe3+ and Co2+ the probability of finding m Fe atoms in a shell of 6 NN neighbour A sites is given by the binomial distribution function 6! λm (1 − λ)6−m , P (m) = m!(6 − m)! where, as referred above, λ is the fraction of the A sites occupied by Fe3+ . According to XRD data in CoFe2 O4 annealed at 1170 K, λ ≈ 0.75 ± 0.01 (Table 2). The calculation of P (m) for λ ≈ 0.75 shows that: P (6) = 0.208;
P (5) = 0.374;
P (4) = 0.279;
P (3) = 0.111;
P (2) = 0.027;
P (1) = 0.001.
Considering that 1.25/2.00 ≈ 0.625 of the Fe3+ cations are on the B site, the relative areas of the sextets corresponding
to the above NN neighbour configurations are, respectively: I (6) = 12.8%;
I (5) = 23.0%;
I (4) = 17.2%;
I (3) + I (2) + I (1) = 9.5%.
Due to the strong overlapping of the sextets, the contributions of subspectra with I < 5%, i.e., those with two or less Fe3+ NN neighbours, were not refined as independent sextets. Their I were summed to that of the magnetic splitting corresponding to the Fe3+ cations on the B sites with 3 NN Fe3+ on the A sites. Therefore, only 4 configurations, hereafter referred to as B1, B2, B3 and B4, were considered in the fitting of the Mössbauer spectra. Only one magnetic splitting was attributed to Fe3+ on A sites, because Fe3+ on these sites is much less sensitive to the replacement of Fe3+ by Co2+ on the NN sites than Fe3+ on B the sites. While Fe3+ on the B site has 6 tetrahedral NN neighbours, Fe3+ on A sites has 12. As explained by Sawatzky et al. [12,13], the replacement of one Fe3+ by Co2+ among the 12 NN cations does not produce a large enough change in the total superexchange interaction to cause a significant difference in the ionic moments and consequently in the magnetic hyperfine fields of Fe3+ on A sites. The Mössbauer spectrum of CoFe2 O4 annealed at 1170 K was therefore fitted by 5 magnetic splittings corresponding, one to Fe3+ on A sites and four to four different NN neighbours configurations B1, B2, B3 and B4, of Fe3+ on B sites. In addition to the constraints described in the experimental section, the relative areas of the Fe3+ on the A sites and on the four B sites with different NN neighbour configurations were kept equal, during the refinement, to the values consistent with λ and the calculated P (m). Furthermore and since the number of overlapping sextets is large, the isomer shifts of Fe3+ on B sites were assumed to be equal for all the configurations. All the other parameters were adjusted freely. In order to estimate the value of λ that leads to the best fit of the spectrum, several refinements with ratios of the relative areas of the four B configurations corresponding to λ values varying by 0.01 from λ = 0.73 up to λ = 0.81 were performed. The minimum value of the final χ 2 was obtained
Fig. 4. Mössbauer spectra taken at RT of the CoFe2 O4 samples annealed at 1170 K (a) and 570 K (b).
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Table 3 Estimated parameters from the best fit of the RT Mössbauer spectrum of CoFe2 O4 heated at 1170 K Site
δ (mm s−1 )
ε (mm s−1 )
Bhf (T)
Γ (mm s−1 )
I
λ
A B1 B2 B3 B4
0.223(3) 0.406(3) 0.406(3) 0.406(3) 0.406(3)
0.00(1) 0.01(1) 0.01(1) −0.09(1) 0.02(4)
48.52(1) 51.08(4) 48.57(3) 45.87(7) 42.02(18)
0.32(1) 0.35(1) 0.36(2) 0.51(2) 0.71(5)
38.5 12.8 23.0 17.2 8.5
0.77
δ isomer shift relative to metallic Fe at room temperature; ε = (e2 VZZ Q/4)(3 cos2 θ − 1) quadrupole shift calculated from (φ1 + φ6 - φ2 - φ5 )/2, where φn is the shift of the nth line of the sextet due to quadrupole coupling; Bhf magnetic hyperfine field; Γ full width at half maximum; I relative areas calculated on the basis of the inversion degree λ (see text). Estimated standard deviations are < 0.2 T for Bhf and < 0.02 mm/s for the other parameters. A is relative to Fe3+ on A sites; B1, B2, B3 and B4 to Fe3+ ions on B sites with 6, 5, 4 and 3 Fe NN neighbours on A sites, respectively. Table 4 Estimated parameters from the best fit for the 78 K Mössbauer spectra of FeCo2 O4 (17, q), (6, q) and (s.c.). Parameters and standard deviations as defined in Table 3 ε (mm s−1 )
Bhf (T)
Γ (mm s−1 )
I
λ
0.29(1) 0.35(1) 0.35(1) 0.35(1) 0.35(1)
−0.04(2) 0.009 0.006 0.033 0.033
48.8(1) 52.6(2) 51.2(3) 50.4(4) 46.0(3)
0.47(2) 0.43(6) 0.34(8) 0.39(14) 0.30(7)
50.0 17.2 15.6 11.7 5.5
0.50
A B1 B2 B3 B4
0.269(8) 0.365(7) 0.365(7) 0.365(7) 0.365(7)
0.002 0.009 0.006 0.033 0.033
48.9(1) 52.2(1) 50.6 49.2 46.5(1)
0.616(3) 0.411(2) 0.354(3) 0.383(8) 0.155(3)
56.0 20.4 13.2 7.7 2.7
0.56
A B1 B2 B3 B4
0.30(1) 0.34(1) 0.34(1) 0.34(1) 0.34(1)
−0.02(2) 0.009 0.006 0.033 0.033
48.6 52.2 50.6 49.2 46.2
0.65(4) 0.41(4) 0.29(3) 0.30(6) 0.27(11)
55.0 19.9 13.6 8.4 3.1
0.55
Sample
Site
FeCo2 O4 (17, q)
A B1 B2 B3 B4
FeCo2 O4 (6, q)
FeCo2 O4 (s.c)
δ (mm s−1 )
A is relative to Fe3+ on A sites; B1, B2, B3 and B4 to Fe3+ ions on B sites with 4, 3, 2 and 1 Fe NN neighbours on A sites, respectively.
for λ = 0.77. Refinements for λ equal to 0.86 and 0.70 were also carried out in order to check if local minima of χ 2 could be found for λ values far from the 0.73–0.81 range. The χ 2 for λ = 0.77 was the lowest. The estimated parameters for this refinement (Table 3) are in good agreement with those reported by Sawatzky et al. [12,13]. Considering the small differences in χ 2 corresponding to the fittings with 0.76 λ 0.78 and the standard deviations of the estimated site occupation factors (Table 2), λ = 0.77 ± 0.01 which better describes the Mössbauer data is in very good agreement with the inversion degree estimated from powder XRD data (Table 2). The peaks in the spectrum of CoFe2 O4 annealed at 570 K are significantly broader (Fig. 4b). If domains with slightly different concentrations of Fe and Co are present, the hyperfine parameters of Fe3+ on the same crystallographic sites may present a distribution of slightly different values corresponding to each of the NN neighbours arrangements. A number of magnetic splittings larger than that analysed for the homogeneous CoFe2 O4 annealed at 1170 K and quenched is therefore expected and the Mössbauer spectrum in Fig. 4b agrees with the chemical heterogeneity of CoFe2 O4 annealed at 570 K suggested by
powder XRD data and the similar average grain sizes in the temperature range 570–1170 K observed by SEM. 3.5. Mössbauer data of the FeCo2 O4 samples The Mössbauer spectra of the FeCo2 O4 samples annealed at 1170 K are very similar. Typical spectra are shown in Fig. 5. The RT spectra show broad peaks (Fig. 5a). The XRD data suggests that these samples are single homogeneous phases. Furthermore, Mössbauer peaks are much better defined and narrow at 78 K (Fig. 5b). As the magnetic ordering temperature, Tord , of FeCo2 O4 is around 320 K [5,8], peak broadening in the RT Mössbauer spectra should be attributed to magnetic relaxation due to the proximity of Tord , rather than to chemical inhomogeneities. Since magnetic relaxation does not allow an accurate analysis of the relative fractions of Fe3+ in A and B sites from RT spectra, only the 78 K Mössbauer spectra were analysed. The refinements were performed considering the same model used for the CoFe2 O4 samples, described above. In this case the first fitting was performed assuming λ = 0.50 in agreement with the site occupation factors estimated from powder XRD data (Table 2). The P (m) corresponding to this
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Fig. 5. Mössbauer spectra taken at 298 K and 78 K, of the FeCo2 O4 samples annealed at 1170 K for 6 h and quenched ((a) and (b) taken at 298 K and 78 K, respectively) and annealed at 570 K ((c) and (d) taken at 298 K and 78 K, respectively).
inversion degree are: P (6) = 0.016;
P (5) = 0.094;
P (4) = 0.234;
P (3) = 0.312;
P (2) = 0.234;
P (1) = 0.094;
P (0) = 0.016. If 50% of the total Fe are on B sites the relative areas of the sextets are: I (6 + 5 + 4) = 17.2%; I (2) = 11.7%;
I (3) = 15.6%;
I (1 + 0) = 5.5%.
Again, contributions with relative areas lower than 5% were considered indistinguishable from those, with higher probability, differing by 1 or 2 Fe3+ NN neighbours. The Mössbauer spectra were therefore fitted with 5 magnetic splittings, A for the Fe3+ in A sites and B1, B2, B3 and B4. It should be noticed that due to the different stoichiometry and λ, the sextets referred to B1, B2, B3 and B4 in the case of FeCo2 O4 do not correspond to the same NN neighbour configurations as in the case of CoFe2 O4 . Refinements corresponding to λ values varying by 0.02 from λ = 0.57 down to λ = 0.37 were performed and, near the values corresponding to the lowest χ 2 , the refinements were performed for λ values differing by only 0.01. The minimum value of the final χ 2 was obtained for λ = 0.56 in the case of FeCo2 O4 annealed for 6 h and quenched, λ = 0.50 in the case of FeCo2 O4 annealed for 17 h and λ = 0.55 for FeCo2 O4 annealed for 6 h and slowly cooled.
The estimated parameters corresponding to these fittings are summarized in Table 4. In the case of the FeCo2 O4 samples, differences in the final χ 2 are less dependent on the λ value considered for the fitting of the spectra than in the case of CoFe2 O4 . λ values estimated from the FeCo2 O4 Mössbauer data by the above procedure are therefore expected to be less accurate than those obtained for CoFe2 O4 and it would not be surprising that agreement with the λ values deduced from the Rietveld refinement might be worse for the FeCo2 O4 samples. In fact, for FeCo2 O4 (17, q) the difference between the λ values deduced by both techniques is 0.04 (Tables 2 and 4). It should be noticed however that, although the site occupation factors are the same, within experimental error, for the (17, q) and the (s.c.) samples, the most probable values estimated for these parameters, as well as those calculated for rt and ro (Table 2), indicate a larger occupation of the A sites by Co2+ and of B sites by Fe3+ in the (17, q) sample in agreement with the lower λ deduced from Mössbauer data. In the RT Mössbauer spectrum for the FeCo2 O4 sample annealed at 570 K (Fig. 5c), a central quadrupole doublet overlaps a broad distribution of magnetic sextets. This result is consistent with the presence of two spinel phases as observed in the powder X-ray diffractogram. The spinel with higher Fe3+ concentration should order magnetically above 320 K, Tord of FeCo2 O4 . As Fe3+ is replaced by Co3+ , Tord is expected to decrease down to ≈ 30–40 K, temperature range where Co3 O4 orders magnetically [5,8]. It is there-
T.A.S. Ferreira et al. / Solid State Sciences 5 (2003) 383–392
fore not surprising that Tord of the phase with higher Co concentration, in the FeCo2 O4 sample annealed at 570 K, is lower than RT. In the 78 K spectrum (Fig. 5d), a better defined magnetically ordered component is observed but a low-intensity paramagnetic contribution is still detected, certainly corresponding to the domains where the Co concentration is the highest. 4. Conclusion The cation distribution in CoFe2 O4 and FeCo2 O4 spinels was determined from powder XRD and Mössbauer data (Tables 2, 3 and 4). Together with SEM morphological analysis, these data have also allowed a better characterization of the chemical homogeneity of the prepared samples. For both FeCo2 O4 and CoFe2 O4 compositions, only phases with the spinel structure are observed immediately after the coprecipitation of the hydroxides. The XRD patterns of samples annealed at temperatures as low as 570 K are already well defined although diffraction peaks are broad and, in the case of FeCo2 O4 , splitted. Mössbauer and SEM data suggest that the broadening of the diffraction peaks of the CoFe2 O4 samples annealed at 570 K is due to chemical heterogeneities rather than small grain size. Microdomains slightly richer in Fe and others in Co seem to be formed as the precipitation of the hydroxides occurs in the boiling hydroxide solution of the metal salts used as starting materials. Annealing at temperatures as high as 770 K for 6 h is not enough to homogenize the sample. A single homogeneous phase may however be obtained by annealing for the same time at 1170 K. In CoFe2 O4 a random cation distribution corresponds to λ = 0.67. Considering the analyses of Mössbauer (Table 3) and powder XRD data (Table 2), an inversion degree λ ≈ 0.76 ± 0.02 is deduced in the present study for CoFe2 O4 annealed at 1170 K and quenched. This value is similar to those reported for CoFe2 O4 quenched from 1520 K, λ ≈ 0.76 [12,13], and 1073 K, λ ≈ 0.78 [9,10]. All these experimental data agree with λ ≈ 0.8 predicted for 1473 K by the thermodynamic model described in [3] confirming the stronger preference of Co2+ for octahedral coordination as compared to Fe3+ . This preference is also consistent with the higher values of λ at lower equilibrium temperatures (λ ≈ 0.80 at 870 K [11] and 0.93 for slow cooled samples [12,13]), which clearly indicate that the inverse cation distribution is progressively favoured as the entropy term becomes less important with decreasing temperature [1,2,4]. λ should still decrease as the temperature increases above 1073 K. However, considering λ ≈ 0.78, 0.76 reported for 1070 K [9,10] and 1520 K [12,13], respectively, and λ ≈ 0.76 obtained in this study for 1170 K, it seems that in the range 1073–1520 K either the cooling rates are not fast enough to quench the cation distribution or the variation of λ with temperature is lower than the experimental uncertainties in the analysis of Mössbauer or XRD data.
391
As far as the samples with global average composition 1Fe:2Co:4O are concerned, the coexistence of two spineltype phases with broad but resolved diffraction peaks in the temperature range 570–1070 K is clearly detected by powder XRD. The overall average Fe/Co ratios of the samples are still 1:2 as revealed by ICP and AAS analyses (Table 1). Both the unit cell parameters estimated from XRD data as well as Tord deduced from Mössbauer data suggest that the two spinel phases formed correspond to phases with distinct concentrations of Fe and Co, a phase richer in Fe with higher a and Tord , and a phase with higher Co concentration with lower a and Tord . Chemical heterogeneities within each of these two main phases may also be inferred from the large widths of the diffraction peaks and from the large range of Tord detected by the Mössbauer effect. The relative areas of the magnetically ordered subspectra increase from RT down to 78 K but, at this temperature, a small fraction of the domains with higher Co concentration is still paramagnetic. With the synthetic procedures used in this study, it was not possible to prepare single phase FeCo2 O4 at temperatures, equal or lower than 1070 K, as reported by Uzunova et al. [11]. Only the samples annealed at 1170 K were obtained as single homogeneous FeCo2 O4 phases. Our data are, therefore, in agreement with the results of Kawano et al. [24] and the Co–Fe–O phase diagram reported by Murray et al. [10] which shows that FeCo2 O4 is only stable in a narrow temperature range around 1170 K. Furthermore, the kinetics of decomposition of this phase seems to be slow since the single phase FeCo2 O4 formed at 1170 K does not decompose when it is cooled down slowly (at 3 ◦ C/min) or even, as reported by Kawano et al. [24], if it is kept at 610 K long enough to obtain a powder neutron diffraction pattern. The powder XRD (Table 2) and Mössbauer data (Table 4) of the samples annealed for 6 h indicate that the fraction of Fe3+ in A sites is ≈ 55 ± 1%, whether the sample is slowly cooled or quenched. Although no significant differences are detected between samples annealed for 6 and for 17 hours, the Mössbauer analysis as well as the interatomic distances deduced from XRD data suggest a lower λ value for the sample (17, q), closer to λ = 0.50 reported by Smith et al. [8] for FeCo2 O4 annealed at the same temperature. Low-spin Co3+ has a very strong crystal-field stabilization energy in octahedral coordination. All the Co3+ is therefore expected to be on B sites even at temperatures higher than those at which our samples were subjected. Random cation distribution may only be attained between Fe3+ and Co2+ and would therefore correspond to λ ≈ 0.5, i.e., 50% of the A sites occupied by Fe3+ . The results obtained point out that at 1170 K the distribution of Fe3+ and Co2+ is close to random. A slightly higher λ for the sample annealed for 6 h may be explained by the fact that 6 h is not long enough for FeCo2 O4 to reach equilibrium. λ at equilibrium should still be higher than 0.50, if the stronger B site preference of Co2+ relative to Fe3+ observed in CoFe2 O4 is also detected in FeCo2 O4 .
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Acknowledgements This work was supported by Ph.D. grant PRAXIS XXI/ BD/11541/97 and Project No. 435—PRAXIS/PCEX/QUI/ 83/96 from Fundação para a Ciência e Tecnologia.
References [1] H. O’Neill, A. Navrotsky, Amer. Mineral. 68 (1983) 181. [2] H. O’Neill, A. Navrotsky, Amer. Mineral. 69 (1984) 703. [3] R. Subramanian, R. Dieckmann, G. Eriksson, A. Pelton, J. Phys. Chem. Solids 55 (1994) 391. [4] M.R. De Guire, R.C. O’Handley, G. Kalonji, J. Appl. Phys. 65 (1989) 3167. [5] F.K. Lotgering, Phil. Res. Rep. 11 (1956) 337. [6] G. Blasse, Phil. Res. Rep. 18 (1963) 383. [7] M. Takahashi, M. Fine, J. Appl. Phys. 43 (1972) 4205. [8] P.A. Smith, C.D. Spencer, R.P. Stillwell, J. Phys. Chem. Solids 39 (1978) 107. [9] P.J. Murray, J.W. Linnett, J. Phys. Chem. Solids 37 (1976) 1041. [10] P.J. Murray, J.W. Linnett, J. Phys. Chem. Solids 37 (1976) 619. [11] E.L. Uzunova, I.G. Mitov, D.G. Klissurski, Bull. Chem. Soc. Jpn. 70 (1997) 1985. [12] G.A. Sawatzky, F. van der Woude, A.H. Morrish, Phys. Rev. 187 (1969) 747.
[13] G.A. Sawatzky, F. van der Woude, A.H. Morrish, J. Appl. Phys. 39 (1968) 1204. [14] Y. Waseda, K. Shinoda, K. Sugiyama, Z. Naturforschung 50a (1995) 1199. [15] A.C. Tavares, M.I. Silva Pereira, M.H. Mendonça, M.R. Nunes, F.M.A. Costa, C.M. Sá, J. Electroanal. Chem. 449 (1998) 91. [16] A.C. Tavares, M.A.M. Cartaxo, M.I. Silva Pereira, F.M.A. Costa, J. Electroanal. Chem. 464 (1999) 187. [17] A.C. Tavares, M.A.M. Cartaxo, M.I. Silva Pereira, F.M.A. Costa, J. Solid State Electrochem. 5 (2001) 57. [18] A. Malats, I. Riera, G. Pourroy, P. Poix, J. Solid State Chem. 101 (1992) 195. [19] D.T. Cromer, Acta Crystallogr. A 32 (1976) 339. [20] D.T. Cromer, D.A. Liberman, Acta Crystallogr. A 37 (1981) 267. [21] J.R. Carvajal, FULLPROF—Rietveld, Profile Matching & Integrated Intensity, version: 0.2, LLB–IRC, 1998. [22] J.A. Ibers, W.C. Hamilton (Eds.), Revised and Supplementary Tables, International Tables for X-ray Crystallography, Vol. 4, Kynoch Press, Birmingham, England, 1974. [23] R.A. Brand, NORMOS Mössbauer Fitting Program, version 13.04, WissEl GmbH, 1996. [24] S. Kawano, N. Yamamoto, N. Achiwa, Japan J. Appl. Phys. 13 (1974) 1891. [25] J.C. Waerenborgh, M.O. Figueiredo, J.M.P. Cabral, L.C.J. Pereira, J. Solid State Chem. 111 (1994) 300. [26] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [27] R.J. Hill, J.R. Craig, G.V. Gibbs, Phys. Chem. Min. 4 (1979) 317.
Structural and morphological characterization of FeCo2 O4 and CoFe2 O4 spinels prepared by a coprecipitation method T.A.S. Ferreira a,b , J.C. Waerenborgh a,c , M.H.R.M. Mendonça a , M.R. Nunes a , F.M. Costa a,∗ a CCMM, Depto Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Ed. C8, Piso 5, 1749-016 Lisboa, Portugal b Depto Química, Universidade de Évora, Colégio Luís Verney, Rua Romão Ramalho, 59, 7000 Évora, Portugal c Instituto Tecnológico e Nuclear, Química, Estrada Nacional 10, 2686-953 Sacavém, Portugal
Received 4 September 2002; accepted 21 October 2002
Abstract Due to the importance of cation distribution in the physical properties of spinels, a structural and morphological study of the FeCo2 O4 and CoFe2 O4 spinels prepared by a low-temperature coprecipitation method has been undertaken. Only spinel phases are observed after annealing at 570 K. However, up to 770 K the sample with global composition CoFe2 O4 was chemically heterogeneous. At 1170 K, a single homogeneous CoFe2 O4 spinel is obtained (average grain size ≈ 1 µm, inversion degree λ ≈ 0.76 ± 0.02). FeCo2 O4 could only be prepared by heating at ≈ 1170 K (average grain size ≈ 0.5 µm and λ approximately equal or slightly larger than 0.50, depending on heating time). At lower temperatures Co- and Fe-rich spinel phases are segregated. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Spinel; Co–Fe–O system; Coprecipitation method; Rietveld refinement; Mössbauer spectroscopy
1. Introduction Oxides with the spinel structure are some of the most studied compounds in solid state sciences due to their wide range of applications as magnetic materials, semiconductors, catalysts and pigments, among others. Oxide spinels may be described by the general formula AB2 O4 , where A and B stand for tetrahedral and octahedral cation sites in a cubic close packing of oxygens. The unit cell contains 8 unit formulas and the symmetry is cubic, ¯ space-group Fd3m. The anionic array is described by the monovariant equivalent position 32eu , point symmetry 3m; the actual values of the free parameter u (commonly known as the oxygen positional parameter) show a slight deviation from 1/4, the ideal value for cubic-closest packing if the ¯ unit cell origin is taken at a centre of symmetry 3m, equiposition 16c; this deviation usually increases the volume ratio between the occupied A and B sites, respectively, 8a ¯ ¯ (1/8, 1/8, 1/8), 43m, and 16d (1/2, 1/2, 1/2), 3m. The structure of spinel oxides is responsible for the variety of interesting physical and chemical properties exhibited * Corresponding author.
E-mail address: [email protected] (F.M. Costa).
by these compounds, since it can accommodate a very large number of different cations, some in more than one oxidation state, distributed in different ways among the A and B sites. Two extreme distributions may be defined: the normal and the inverse. For spinels where only divalent and trivalent cations are present, the inversion degree, λ, is defined as the fraction of A sites occupied by trivalent cations. Accordingly, for a normal spinel λ = 0 and for a completely inverted spinel λ = 1. Determination of the cation distribution is of considerable relevance because the theoretical interpretation of the chemical and physical properties of these compounds depends on this distribution. The cation octahedral and tetrahedral sites preferences as well as the temperature of equilibrium have a strong influence on the cation arrangement. Different thermodynamic models have been proposed to treat quantitatively this arrangement [1–3]. De Guire et al. [4] have emphasized the effect of the kinetics of cooling. Unless the spinels are characterized at high temperature, the cation arrangement measured at room temperature depends on the cooling rate, as the kinetics of cation diffusion is fast enough to allow some degree of ordering during quenching, leading to quenchedin distributions that differ from the equilibrium one at the annealing temperature.
1293-2558/03/$ – see front matter 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00011-6
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The investigation of the Co1+x Fe2−x O4 (0 x 1) system has been undertaken for long by several authors. Lotgering [5] and Blasse [6] using magnetic properties reported a normal cation distribution for FeCo2 O4 , i.e., Co2+ [Fe3+ Co3+ ]O4 where cations in square brackets are located on B sites. Takahashi and Fine [7] by extrapolating to 0 K the measured saturation magnetization of a number of cobalt–iron spinels, confirmed that Co3+ , in the lowspin state, replaces Fe3+ in the B site as it is introduced in Co1+x Fe2−x O4 . In contrast to Lotgering and Blasse and based on Mössbauer data, Smith et al. [8] have proposed that 45–50% of Fe3+ is located in the A sites in FeCo2 O4 annealed at 1170 K and cooled down to room temperature in 100 s. On the other hand, Murray and Linnett [9,10] obtained no more than 30% for Fe3+ in A sites for samples heated at 1193 K for three days and quenched in water, while Uzunova et al. [11], more recently, suggested a fraction of Fe3+ in A sites of 41% for samples prepared under mild conditions and submitted to a final temperature of 570 K. As far as CoFe2 O4 is concerned, Mössbauer spectroscopy measurements have suggested that samples annealed at 1520 K have λ ≈ 0.76 if they are water quenched or 0.93 if slowly cooled [12,13]; λ ≈ 0.62 was reported for the samples annealed at 1320 K and quenched in water [9,10] while λ ≈ 0.80 was suggested for CoFe2 O4 prepared at 870 K [11]. A synchrotron X-ray diffraction study, taking advantage of anomalous X-ray scattering, of a sample annealed at 1073 K suggested λ ≈ 0.78. No details were, however, given about the cooling conditions [14]. The study of the transport properties of Co containing spinels as versatile materials for electrocatalysis has been undertaken [15–17]. In order to avoid the drawbacks from high-temperature solid-state reactions, these oxides have been synthesized by wet-chemical methods. As referred above, cation distribution and, consequently, the physical properties of Co1+x Fe2−x O4 are strongly dependent on the heating treatment. Considering the scarce results published on Co1+x Fe2−x O4 prepared by these methods and particularly the report on the preparation of a FeCo2 O4 single phase at 570 K [11], while phase diagrams indicate a two-phase equilibrium at this temperature [7,10], a detailed investigation of the evolution with annealing temperature of morphological and structural features of the spinel phases FeCo2 O4 and CoFe2 O4 synthesized by a low-temperature coprecipitation method and followed by annealing at different temperatures, is reported in this work.
2. Experimental FeCo2 O4 and CoFe2 O4 spinels were synthesized according to a previously described method [18]. Iron chloride, FeCl3 , and cobalt chloride, CoCl2 ·6H2 O were weighted in the adequate molar ratios Co/Fe, dissolved in distilled water and poured into a boiling hydroxide solution at 350– 370 K, keeping a constant vigorous stirring. The solution pH was confirmed to be higher than 12. Black precipitates were formed and heated at 350 K for one hour. After repeated filtering and washing with boiling distilled water, the precipitates were dried on a sand bath at 470 K. All the resulting materials were annealed in air for 1 h 30 at 570 K and 770 K, and for 6 h at 1170 K. In the case of FeCo2 O4 an annealing of 6 h at 1070 K was also performed. After annealing, the samples were quenched in air down to room temperature (RT). All the samples were checked by powder X-ray Diffraction (XRD). In order to examine the effect of the heating time and cooling rate on the cation distribution and phase stability of FeCo2 O4 , two additional heat treatments were performed on these samples. One was annealed at 1170 K for 17 h and quenched, FeCo2 O4 (17, q), and the other was annealed at 1170 K and slowly cooled, at a rate of 3 ◦ C/min, FeCo2 O4 (s.c.). The FeCo2 O4 sample annealed for 6 h and quenched will hereafter be referred to as FeCo2 O4 (6, q). The total Co and Fe contents of four selected samples (Table 1) were estimated by Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma Emission Spectroscopy (ICP), respectively. Scanning electron microscopy (SEM) and X-ray microanalysis, energy dispersive spectroscopy (EDS) were performed in a JEOL (JSM-35C) scanning electron microscope working at 22 keV, with an energy dispersive X-ray spectrometer Noram (Voyager). Powder XRD was carried out on a Philips X-ray diffractometer (PW 1700). Cu Kα radiation was used for the routine sample characterization. The diffractograms analysed by the Rietveld method were obtained using Co Kα radiation (λ1 = 1.78896 Å and λ2 = 1.79285 Å). Co was used instead of Cu radiation in the hope to improve the accuracy of the method. Fe and Co have similar atomic numbers, however, as the wavelength of Co Kα radiation is close to the Fe absorption K edge, the difference between the scattering factors of Co and Fe becomes larger for this radiation due to the anomalous scattering effect (cf. tables of dispersion corrections in Refs. [19,20]). The diffraction patterns were col-
Table 1 Chemical analysis for the FeCo2 O4 and CoFe2 O4 samples annealed at 570 K and 1170 K Sample FeCo2 O4 FeCo2 O4 CoFe2 O4 CoFe2 O4
Heating temperature (K)
% Fe (ICP)
% Co (AAS)
Actual composition (± 0.01 in Fe and Co contents)
570 1170 570 1170
22.2 23.4 43.0 44.7
46.6 49.4 23.4 23.9
Fe0.97 Co2.03 O4 Fe0.96 Co2.04 O4 Co1.06 Fe1.94 O4 Co1.05 Fe1.95 O4
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lected from 17 to 145◦ 2θ with a step of 0.02◦ 2θ . All peak positions were used in the determination of precise lattice parameters. Structural refinements were carried out using the Rietveld program FULLPROF [21]. Peak profiles were fitted with a pseudo-Voigt function and an asymmetry parameter was considered for peaks below 70◦ 2θ . The background was refined with a polynomial function. Scattering factors for Co2+ , Co3+ and Fe3+ cations and O2− anion were obtained from [22] and the anomalous dispersion corrections from Cromer et al. [19,20]. Mössbauer spectra were acquired using a spectrometer combined with a 512 multichannel analyser, with the absorber at RT and at 78 K. A 57 Co(Rh) source (Ritverc GmbH and V.G. Khlopin Radium Institut) with activity of about 50 mCi was used. The spectrometer was calibrated with a thin absorber of elemental iron. The isomer shifts are referred to iron metal. The absorbers were prepared in the form of a uniform layer of the crystalline powder samples dispersed in an appropriate varnish (G.E. Varnish), without iron. During data collection, temperature was controlled within ±1◦ C. The Mössbauer spectra were computer-processed and decomposed to single Lorentzians by the least-square method used by the Normos fitting program [23]. During refinement, for each sextet, the widths of the 6 peaks were kept equal while the ratio of the relative areas I1,6 /I2,5 /I3,4 were kept equal to 3:2:1. Further constraints were used as explained below.
385
3. Results and discussion 3.1. Chemical analysis and morphological characterization Duplicate analyses of different fractions of the same sample were found to agree within 0.01 cations per formula-unit confirming the reproducibility of the AAS and ICP methods. The results obtained for the four selected samples (Table 1) are in good agreement with the expected stoichiometry. SEM micrographs show that the CoFe2 O4 grains have an average size of 2–3 µm and that there is no significant variation in grain size with heating temperature (Fig. 1a and b). On the other hand, for FeCo2 O4 samples there is a slight increase in the average grain size from ≈ 0.3 µm at 570 K up to ≈ 0.5 µm at 1170 K (Fig. 1c and d). 3.2. Phase analysis and structural characterization The powder XRD data of the CoFe2 O4 (Fig. 2) and all the FeCo2 O4 samples annealed at 1170 K (Fig. 3), either quenched or slowly cooled, reveal a well-crystallized cubic spinel-type phase. No impurity phases were detected. In contrast, in the powder X-ray diffractograms of FeCo2 O4 samples annealed between 570 K and 1070 K, although all the diffraction peaks are located at the positions expected for a spinel phase, they are very broad at low-θ angles and splitted for 2θ 50◦ (Fig. 3). These features indicate that in the temperature range 570–1070 K, two spinel
Fig. 1. SEM micrographs of samples annealed at different temperatures: CoFe2 O4 samples: (a) 570 K; (b) 1170 K; FeCo2 O4 samples: (c) 570 K; (d) 1170 K.
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Fig. 2. X-ray diffraction pattern (Co Kα) for CoFe2 O4 sample heated at 570 K (a) and heated at 1170 K (b) and quenched. The Miller indices refer to the spinel phase. The curves over the experimental points in (b) are the patterns calculated by the Rietveld analysis. The difference between the calculated and observed pattern is represented below.
phases are present, in agreement with the phase diagram reported in Murray and Linnet [10] and with the fact that a single spinel phase is observed by Takahashi [7] for FeCo2 O4 only above 1120 K. The unit-cell parameters estimated for these spinels from the 570 K data are a = 8.269 ± 0.001 Å and a = 8.1075 ± 0.0009 Å. Considering the Fe2+ , Co2+ , Fe3+ and Co3+ cation radii [1] the spinel phase with lower a should be richer in Co. The broadening of the diffraction peaks occurs fundamentally due to the existence of microdomains with slightly distinct compositions. This phase segregation probably takes place during the coprecipitation of hydroxides at 353 K and seems to remain up to 1170 K where a single-phase spinel is formed. As referred above, no broadening of diffraction peaks are observed in the XRD pattern of the FeCo2 O4 sample slowly cooled from 1170 K to RT. No impurity phases are observed, either. This suggests that once a homogeneous FeCo2 O4 phase is formed at 1170 K, it may withstand relative long exposures to temperatures where it is either thermodynamically unstable or where its formation as a single phase is kinetically very slow.
Fig. 3. X-ray diffraction pattern (Co Kα) of the FeCo2 O4 sample heated at 570 K (a), at 1170 K and quenched (b), at 1170 K and slowly cooled (c). The Miller indices in (a) refer to the spinel phase richer in iron; the symbol (F) refers to the peaks with the same Miller indices of the spinel phase richer in cobalt. The curves over the experimental points in (b) and (c) are the patterns calculated by the Rietveld analysis. The differences between the calculated and observed patterns are represented below the corresponding diffractograms.
Diffraction peaks of CoFe2 O4 annealed at 570 K (Fig. 2) are not splitted but are broadened. This broadening may not be attributed to small grain size effects since at 1170 K
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diffraction peaks are narrow although SEM analyses reveal no increase in grain size (Fig. 1a and b). Line broadening seems therefore to be related to crystal defects or chemical heterogeneity of the sample. Differences in composition are however not large enough to be detected by the present SEM/EDS results. 3.3. Structure refinement The crystal data and site occupation factors estimated from the Rietveld analysis for the single-phase samples are summarized in Table 2. The crystal refinements were performed assuming space ¯ group Fd 3m. In the final refinements 23 parameters were fitted to the 6400 data points: 1 scale factor, 6 background parameters, zeropoint for 2θ , 5 profile function parameters (2 for the Lorentzian/Gaussian mixing parameter and 3 for the width), 4 asymmetry correction parameters, the cell parameter a, the oxygen positional parameter u, 3 isotropic thermal parameters (B), one for each equiposition 8a, 16d and 32e, and 1 cation disorder parameter which allowed the occupation of the 8a and 16d sites by Fe3+ and Co2+ , respectively, to vary within the constraints of full site occupancy. In the case of FeCo2 O4 and based on the strong octahedral site preference of low-spin Co3+ [7,8,24], it was assumed that all the Co3+ is present on the B site. The estimated site occupation factors correspond to the cation distributions summarized in Table 2. In order to check if these cation distributions are consistent with the estimated a and u values, interatomic distances between O2− and the cations on the A sites and on the B sites, dt and do , respectively, are derived and compared with those deduced
387
from the effective cation radii of O’Neill and Navrotsky [1]. These radii have been optimized for oxide spinels and have been found to be in better agreement with experimental data for these materials [25] than those of Shannon [26]. The interatomic distances between a cation and the nearest anion are given by [27] √ dt = a 3(u − 0.125) for the 8a site, (1) do = a(3u2 − 2u + 0.375)1/2
for the 16d site.
(2)
From these values cation radii (hereafter referred to as structural radii as in [25]) may be estimated by taking rt = dt − rO2−
and ro = do − rO2−
(3)
and assuming that the radius of tetrahedrally coordinated O2− is rO2− = 1.38 Å [26]. The average cation radii, rt and ro for the 8a and the 16d sites, respectively, were calculated according to [1,25]
rt = (1 − λ) rCo2+ ,t + λ rFe3+ ,t . (4)
ro =
(rCo3+ ,o ) + λ(rCo2+ ,o ) + (1 − λ)(rFe3+ ,o )
2 for the FeCo2 O4 samples or
ro =
λ(rCo2+ ,o ) + (2 − λ)(rFe3+ ,o )
(5) 2 for the CoFe2 O4 sample, where λ, the inversion degree, is the fraction of A sites occupied by Fe3+ according to the estimated site occupation factors (Table 2) and (rXn+ ,s ) are the effective cation radii [1]. The results are presented in Table 2.
Table 2 Crystallographic data, including lattice parameters, site occupancy and equivalent isotropic thermal factors (Beq ), estimated from the Rietveld analysis of the powder XRD data of FeCo2 O4 (17, q) and (s.c.) and CoFe2 O4 samples annealed at 1170 K. Structural radii (rt ; ro ) and effective cation radii ( rt ; ro ) [1], for tetrahedral and octahedral sites, calculated from a and u and from the cation site occupancy, respectively Sample
FeCo2 O4 (17,q)
FeCo2 O4 (s.c.)
CoFe2 O4
a (Å) Cell volume (Å3 ) u (Å) Site occupancy Co(II) 8a Fe(III) 8a Co(II) 16d Fe(III) 16d Co(III) 16d Btet (Å2 ) Boct (Å2 ) Boxy (Å2 ) RBragg (%); RF (%) Inversion degree obtained from site occupancy Formula obtained from site occupancy (cations in square brackets are located on B sites) rt ; ro (Å)
rt NN ; ro NN (Å)
8.2420(1) 559.88(1) 0.25918(6)
8.2436(1) 560.22(1) 0.25839(6)
8.3806(1) 588.60(1) 0.25664(8)
3.72(2) 4.28(2) 4.28(2) 3.72(2) 8.00 0.11(2) 0.62(1) 0.76(2) 2.17; 2.13
3.66(8) 4.34(8) 4.34(8) 3.66(8) 8.00 0.33(2) 1.08(1) 1.97(3) 3.58; 4.12
2.04(3) 5.96(3) 5.96(3) 10.04(3) – 0.59(2) 0.71(2) 1.33(3) 3.24; 3.59
λ = 0.54 ± 0.01 3+ 2+ 3+ 3+ (Co2+ 0.46 Fe0.54 )[Co0.54 Fe0.46 Co1.00 ]O4
λ = 0.54 ± 0.01 3+ 2+ 3+ 3+ (Co2+ 0.46 Fe0.54 )[Co0.54 Fe0.46 Co1.00 ]O4
λ = 0.75 ± 0.01 3+ 2+ 3+ (Co2+ 0.25 Fe0.75 )[Co0.75 Fe1.25 ]O4
0.535; 0.608 0.53; 0.61
0.525; 0.614 0.53; 0.61
0.531; 0.661 0.51; 0.67
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Considering that the standard deviations of the effective cation radii of O’Neill and Navrotsky [1] are ±0.01 Å, the values calculated for the structural radii and the corresponding average cation radii are the same within experimental error. Those deduced for the A site cations in CoFe2 O4 are the only pair that differ by more than 0.01 Å. Taking into account the propagation of errors associated with the calculation of the average cation radii, the difference between these rt and rt is still lower than twice the derived standard deviation (0.014 Å). The estimated values for the a and u parameters may therefore be considered consistent with the cation distribution deduced from the site occupation factors. 3.4. Mössbauer data of the CoFe2 O4 samples The RT Mössbauer spectra of the CoFe2 O4 samples annealed at 1170 K and 570 K are shown in Fig. 4. The spectrum of the sample annealed at 1170 K (Fig. 4a) is similar to that reported by Sawatzky et al. [12,13] and was fitted by the model proposed by these authors, according to which the supertransferred magnetic hyperfine fields of Fe3+ on the B sites increase with the number of Fe3+ next-nearest (NN) neighbours located on A sites. Each B site has 6 A sites as NN neighbours. Assuming a statistical occupation of the A sites by Fe3+ and Co2+ the probability of finding m Fe atoms in a shell of 6 NN neighbour A sites is given by the binomial distribution function 6! λm (1 − λ)6−m , P (m) = m!(6 − m)! where, as referred above, λ is the fraction of the A sites occupied by Fe3+ . According to XRD data in CoFe2 O4 annealed at 1170 K, λ ≈ 0.75 ± 0.01 (Table 2). The calculation of P (m) for λ ≈ 0.75 shows that: P (6) = 0.208;
P (5) = 0.374;
P (4) = 0.279;
P (3) = 0.111;
P (2) = 0.027;
P (1) = 0.001.
Considering that 1.25/2.00 ≈ 0.625 of the Fe3+ cations are on the B site, the relative areas of the sextets corresponding
to the above NN neighbour configurations are, respectively: I (6) = 12.8%;
I (5) = 23.0%;
I (4) = 17.2%;
I (3) + I (2) + I (1) = 9.5%.
Due to the strong overlapping of the sextets, the contributions of subspectra with I < 5%, i.e., those with two or less Fe3+ NN neighbours, were not refined as independent sextets. Their I were summed to that of the magnetic splitting corresponding to the Fe3+ cations on the B sites with 3 NN Fe3+ on the A sites. Therefore, only 4 configurations, hereafter referred to as B1, B2, B3 and B4, were considered in the fitting of the Mössbauer spectra. Only one magnetic splitting was attributed to Fe3+ on A sites, because Fe3+ on these sites is much less sensitive to the replacement of Fe3+ by Co2+ on the NN sites than Fe3+ on B the sites. While Fe3+ on the B site has 6 tetrahedral NN neighbours, Fe3+ on A sites has 12. As explained by Sawatzky et al. [12,13], the replacement of one Fe3+ by Co2+ among the 12 NN cations does not produce a large enough change in the total superexchange interaction to cause a significant difference in the ionic moments and consequently in the magnetic hyperfine fields of Fe3+ on A sites. The Mössbauer spectrum of CoFe2 O4 annealed at 1170 K was therefore fitted by 5 magnetic splittings corresponding, one to Fe3+ on A sites and four to four different NN neighbours configurations B1, B2, B3 and B4, of Fe3+ on B sites. In addition to the constraints described in the experimental section, the relative areas of the Fe3+ on the A sites and on the four B sites with different NN neighbour configurations were kept equal, during the refinement, to the values consistent with λ and the calculated P (m). Furthermore and since the number of overlapping sextets is large, the isomer shifts of Fe3+ on B sites were assumed to be equal for all the configurations. All the other parameters were adjusted freely. In order to estimate the value of λ that leads to the best fit of the spectrum, several refinements with ratios of the relative areas of the four B configurations corresponding to λ values varying by 0.01 from λ = 0.73 up to λ = 0.81 were performed. The minimum value of the final χ 2 was obtained
Fig. 4. Mössbauer spectra taken at RT of the CoFe2 O4 samples annealed at 1170 K (a) and 570 K (b).
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389
Table 3 Estimated parameters from the best fit of the RT Mössbauer spectrum of CoFe2 O4 heated at 1170 K Site
δ (mm s−1 )
ε (mm s−1 )
Bhf (T)
Γ (mm s−1 )
I
λ
A B1 B2 B3 B4
0.223(3) 0.406(3) 0.406(3) 0.406(3) 0.406(3)
0.00(1) 0.01(1) 0.01(1) −0.09(1) 0.02(4)
48.52(1) 51.08(4) 48.57(3) 45.87(7) 42.02(18)
0.32(1) 0.35(1) 0.36(2) 0.51(2) 0.71(5)
38.5 12.8 23.0 17.2 8.5
0.77
δ isomer shift relative to metallic Fe at room temperature; ε = (e2 VZZ Q/4)(3 cos2 θ − 1) quadrupole shift calculated from (φ1 + φ6 - φ2 - φ5 )/2, where φn is the shift of the nth line of the sextet due to quadrupole coupling; Bhf magnetic hyperfine field; Γ full width at half maximum; I relative areas calculated on the basis of the inversion degree λ (see text). Estimated standard deviations are < 0.2 T for Bhf and < 0.02 mm/s for the other parameters. A is relative to Fe3+ on A sites; B1, B2, B3 and B4 to Fe3+ ions on B sites with 6, 5, 4 and 3 Fe NN neighbours on A sites, respectively. Table 4 Estimated parameters from the best fit for the 78 K Mössbauer spectra of FeCo2 O4 (17, q), (6, q) and (s.c.). Parameters and standard deviations as defined in Table 3 ε (mm s−1 )
Bhf (T)
Γ (mm s−1 )
I
λ
0.29(1) 0.35(1) 0.35(1) 0.35(1) 0.35(1)
−0.04(2) 0.009 0.006 0.033 0.033
48.8(1) 52.6(2) 51.2(3) 50.4(4) 46.0(3)
0.47(2) 0.43(6) 0.34(8) 0.39(14) 0.30(7)
50.0 17.2 15.6 11.7 5.5
0.50
A B1 B2 B3 B4
0.269(8) 0.365(7) 0.365(7) 0.365(7) 0.365(7)
0.002 0.009 0.006 0.033 0.033
48.9(1) 52.2(1) 50.6 49.2 46.5(1)
0.616(3) 0.411(2) 0.354(3) 0.383(8) 0.155(3)
56.0 20.4 13.2 7.7 2.7
0.56
A B1 B2 B3 B4
0.30(1) 0.34(1) 0.34(1) 0.34(1) 0.34(1)
−0.02(2) 0.009 0.006 0.033 0.033
48.6 52.2 50.6 49.2 46.2
0.65(4) 0.41(4) 0.29(3) 0.30(6) 0.27(11)
55.0 19.9 13.6 8.4 3.1
0.55
Sample
Site
FeCo2 O4 (17, q)
A B1 B2 B3 B4
FeCo2 O4 (6, q)
FeCo2 O4 (s.c)
δ (mm s−1 )
A is relative to Fe3+ on A sites; B1, B2, B3 and B4 to Fe3+ ions on B sites with 4, 3, 2 and 1 Fe NN neighbours on A sites, respectively.
for λ = 0.77. Refinements for λ equal to 0.86 and 0.70 were also carried out in order to check if local minima of χ 2 could be found for λ values far from the 0.73–0.81 range. The χ 2 for λ = 0.77 was the lowest. The estimated parameters for this refinement (Table 3) are in good agreement with those reported by Sawatzky et al. [12,13]. Considering the small differences in χ 2 corresponding to the fittings with 0.76 λ 0.78 and the standard deviations of the estimated site occupation factors (Table 2), λ = 0.77 ± 0.01 which better describes the Mössbauer data is in very good agreement with the inversion degree estimated from powder XRD data (Table 2). The peaks in the spectrum of CoFe2 O4 annealed at 570 K are significantly broader (Fig. 4b). If domains with slightly different concentrations of Fe and Co are present, the hyperfine parameters of Fe3+ on the same crystallographic sites may present a distribution of slightly different values corresponding to each of the NN neighbours arrangements. A number of magnetic splittings larger than that analysed for the homogeneous CoFe2 O4 annealed at 1170 K and quenched is therefore expected and the Mössbauer spectrum in Fig. 4b agrees with the chemical heterogeneity of CoFe2 O4 annealed at 570 K suggested by
powder XRD data and the similar average grain sizes in the temperature range 570–1170 K observed by SEM. 3.5. Mössbauer data of the FeCo2 O4 samples The Mössbauer spectra of the FeCo2 O4 samples annealed at 1170 K are very similar. Typical spectra are shown in Fig. 5. The RT spectra show broad peaks (Fig. 5a). The XRD data suggests that these samples are single homogeneous phases. Furthermore, Mössbauer peaks are much better defined and narrow at 78 K (Fig. 5b). As the magnetic ordering temperature, Tord , of FeCo2 O4 is around 320 K [5,8], peak broadening in the RT Mössbauer spectra should be attributed to magnetic relaxation due to the proximity of Tord , rather than to chemical inhomogeneities. Since magnetic relaxation does not allow an accurate analysis of the relative fractions of Fe3+ in A and B sites from RT spectra, only the 78 K Mössbauer spectra were analysed. The refinements were performed considering the same model used for the CoFe2 O4 samples, described above. In this case the first fitting was performed assuming λ = 0.50 in agreement with the site occupation factors estimated from powder XRD data (Table 2). The P (m) corresponding to this
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Fig. 5. Mössbauer spectra taken at 298 K and 78 K, of the FeCo2 O4 samples annealed at 1170 K for 6 h and quenched ((a) and (b) taken at 298 K and 78 K, respectively) and annealed at 570 K ((c) and (d) taken at 298 K and 78 K, respectively).
inversion degree are: P (6) = 0.016;
P (5) = 0.094;
P (4) = 0.234;
P (3) = 0.312;
P (2) = 0.234;
P (1) = 0.094;
P (0) = 0.016. If 50% of the total Fe are on B sites the relative areas of the sextets are: I (6 + 5 + 4) = 17.2%; I (2) = 11.7%;
I (3) = 15.6%;
I (1 + 0) = 5.5%.
Again, contributions with relative areas lower than 5% were considered indistinguishable from those, with higher probability, differing by 1 or 2 Fe3+ NN neighbours. The Mössbauer spectra were therefore fitted with 5 magnetic splittings, A for the Fe3+ in A sites and B1, B2, B3 and B4. It should be noticed that due to the different stoichiometry and λ, the sextets referred to B1, B2, B3 and B4 in the case of FeCo2 O4 do not correspond to the same NN neighbour configurations as in the case of CoFe2 O4 . Refinements corresponding to λ values varying by 0.02 from λ = 0.57 down to λ = 0.37 were performed and, near the values corresponding to the lowest χ 2 , the refinements were performed for λ values differing by only 0.01. The minimum value of the final χ 2 was obtained for λ = 0.56 in the case of FeCo2 O4 annealed for 6 h and quenched, λ = 0.50 in the case of FeCo2 O4 annealed for 17 h and λ = 0.55 for FeCo2 O4 annealed for 6 h and slowly cooled.
The estimated parameters corresponding to these fittings are summarized in Table 4. In the case of the FeCo2 O4 samples, differences in the final χ 2 are less dependent on the λ value considered for the fitting of the spectra than in the case of CoFe2 O4 . λ values estimated from the FeCo2 O4 Mössbauer data by the above procedure are therefore expected to be less accurate than those obtained for CoFe2 O4 and it would not be surprising that agreement with the λ values deduced from the Rietveld refinement might be worse for the FeCo2 O4 samples. In fact, for FeCo2 O4 (17, q) the difference between the λ values deduced by both techniques is 0.04 (Tables 2 and 4). It should be noticed however that, although the site occupation factors are the same, within experimental error, for the (17, q) and the (s.c.) samples, the most probable values estimated for these parameters, as well as those calculated for rt and ro (Table 2), indicate a larger occupation of the A sites by Co2+ and of B sites by Fe3+ in the (17, q) sample in agreement with the lower λ deduced from Mössbauer data. In the RT Mössbauer spectrum for the FeCo2 O4 sample annealed at 570 K (Fig. 5c), a central quadrupole doublet overlaps a broad distribution of magnetic sextets. This result is consistent with the presence of two spinel phases as observed in the powder X-ray diffractogram. The spinel with higher Fe3+ concentration should order magnetically above 320 K, Tord of FeCo2 O4 . As Fe3+ is replaced by Co3+ , Tord is expected to decrease down to ≈ 30–40 K, temperature range where Co3 O4 orders magnetically [5,8]. It is there-
T.A.S. Ferreira et al. / Solid State Sciences 5 (2003) 383–392
fore not surprising that Tord of the phase with higher Co concentration, in the FeCo2 O4 sample annealed at 570 K, is lower than RT. In the 78 K spectrum (Fig. 5d), a better defined magnetically ordered component is observed but a low-intensity paramagnetic contribution is still detected, certainly corresponding to the domains where the Co concentration is the highest. 4. Conclusion The cation distribution in CoFe2 O4 and FeCo2 O4 spinels was determined from powder XRD and Mössbauer data (Tables 2, 3 and 4). Together with SEM morphological analysis, these data have also allowed a better characterization of the chemical homogeneity of the prepared samples. For both FeCo2 O4 and CoFe2 O4 compositions, only phases with the spinel structure are observed immediately after the coprecipitation of the hydroxides. The XRD patterns of samples annealed at temperatures as low as 570 K are already well defined although diffraction peaks are broad and, in the case of FeCo2 O4 , splitted. Mössbauer and SEM data suggest that the broadening of the diffraction peaks of the CoFe2 O4 samples annealed at 570 K is due to chemical heterogeneities rather than small grain size. Microdomains slightly richer in Fe and others in Co seem to be formed as the precipitation of the hydroxides occurs in the boiling hydroxide solution of the metal salts used as starting materials. Annealing at temperatures as high as 770 K for 6 h is not enough to homogenize the sample. A single homogeneous phase may however be obtained by annealing for the same time at 1170 K. In CoFe2 O4 a random cation distribution corresponds to λ = 0.67. Considering the analyses of Mössbauer (Table 3) and powder XRD data (Table 2), an inversion degree λ ≈ 0.76 ± 0.02 is deduced in the present study for CoFe2 O4 annealed at 1170 K and quenched. This value is similar to those reported for CoFe2 O4 quenched from 1520 K, λ ≈ 0.76 [12,13], and 1073 K, λ ≈ 0.78 [9,10]. All these experimental data agree with λ ≈ 0.8 predicted for 1473 K by the thermodynamic model described in [3] confirming the stronger preference of Co2+ for octahedral coordination as compared to Fe3+ . This preference is also consistent with the higher values of λ at lower equilibrium temperatures (λ ≈ 0.80 at 870 K [11] and 0.93 for slow cooled samples [12,13]), which clearly indicate that the inverse cation distribution is progressively favoured as the entropy term becomes less important with decreasing temperature [1,2,4]. λ should still decrease as the temperature increases above 1073 K. However, considering λ ≈ 0.78, 0.76 reported for 1070 K [9,10] and 1520 K [12,13], respectively, and λ ≈ 0.76 obtained in this study for 1170 K, it seems that in the range 1073–1520 K either the cooling rates are not fast enough to quench the cation distribution or the variation of λ with temperature is lower than the experimental uncertainties in the analysis of Mössbauer or XRD data.
391
As far as the samples with global average composition 1Fe:2Co:4O are concerned, the coexistence of two spineltype phases with broad but resolved diffraction peaks in the temperature range 570–1070 K is clearly detected by powder XRD. The overall average Fe/Co ratios of the samples are still 1:2 as revealed by ICP and AAS analyses (Table 1). Both the unit cell parameters estimated from XRD data as well as Tord deduced from Mössbauer data suggest that the two spinel phases formed correspond to phases with distinct concentrations of Fe and Co, a phase richer in Fe with higher a and Tord , and a phase with higher Co concentration with lower a and Tord . Chemical heterogeneities within each of these two main phases may also be inferred from the large widths of the diffraction peaks and from the large range of Tord detected by the Mössbauer effect. The relative areas of the magnetically ordered subspectra increase from RT down to 78 K but, at this temperature, a small fraction of the domains with higher Co concentration is still paramagnetic. With the synthetic procedures used in this study, it was not possible to prepare single phase FeCo2 O4 at temperatures, equal or lower than 1070 K, as reported by Uzunova et al. [11]. Only the samples annealed at 1170 K were obtained as single homogeneous FeCo2 O4 phases. Our data are, therefore, in agreement with the results of Kawano et al. [24] and the Co–Fe–O phase diagram reported by Murray et al. [10] which shows that FeCo2 O4 is only stable in a narrow temperature range around 1170 K. Furthermore, the kinetics of decomposition of this phase seems to be slow since the single phase FeCo2 O4 formed at 1170 K does not decompose when it is cooled down slowly (at 3 ◦ C/min) or even, as reported by Kawano et al. [24], if it is kept at 610 K long enough to obtain a powder neutron diffraction pattern. The powder XRD (Table 2) and Mössbauer data (Table 4) of the samples annealed for 6 h indicate that the fraction of Fe3+ in A sites is ≈ 55 ± 1%, whether the sample is slowly cooled or quenched. Although no significant differences are detected between samples annealed for 6 and for 17 hours, the Mössbauer analysis as well as the interatomic distances deduced from XRD data suggest a lower λ value for the sample (17, q), closer to λ = 0.50 reported by Smith et al. [8] for FeCo2 O4 annealed at the same temperature. Low-spin Co3+ has a very strong crystal-field stabilization energy in octahedral coordination. All the Co3+ is therefore expected to be on B sites even at temperatures higher than those at which our samples were subjected. Random cation distribution may only be attained between Fe3+ and Co2+ and would therefore correspond to λ ≈ 0.5, i.e., 50% of the A sites occupied by Fe3+ . The results obtained point out that at 1170 K the distribution of Fe3+ and Co2+ is close to random. A slightly higher λ for the sample annealed for 6 h may be explained by the fact that 6 h is not long enough for FeCo2 O4 to reach equilibrium. λ at equilibrium should still be higher than 0.50, if the stronger B site preference of Co2+ relative to Fe3+ observed in CoFe2 O4 is also detected in FeCo2 O4 .
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Acknowledgements This work was supported by Ph.D. grant PRAXIS XXI/ BD/11541/97 and Project No. 435—PRAXIS/PCEX/QUI/ 83/96 from Fundação para a Ciência e Tecnologia.
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