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Nanosize ferrites obtained by ball milling: Crystal structure, cation distribution, size-strain analysis and Raman investigations
Solid State Sciences 8 (2006) 908–915 www.elsevier.com/locate/ssscie
Nanosize ferrites obtained by ball milling: Crystal structure, cation distribution, size-strain analysis and Raman investigations Zeljka Cvejic a,∗ , Srdjan Rakic a , Aleksandar Kremenovic b,c , Bratislav Antic c , Cedomir Jovalekic d , Philippe Colomban e a Institute of Physics, Faculty of Natural Sciences, University of Novi Sad, Trg D. Obradovica 6, 21000 Novi Sad, Serbia and Montenegro b Laboratory for Crystallography, Faculty of Mining and Geology, University of Belgrade, POB 162, 11001 Belgrade, Serbia and Montenegro c Condensed Matter Physics Laboratory, The “Vinca” Institute, POB 522, 11001 Belgrade, Serbia and Montenegro d Center for Multidisciplinary Studies, University of Belgrade, Kneza Viseslava 1, 11000 Belgrade, Serbia and Montenegro e LADIR, UMR 7075 CNRS, and Université Pierre and Marie Curie, 94230 Thiais, France
Received 3 January 2006; accepted 22 February 2006 Available online 11 May 2006
Abstract Spinels samples Fe2.85 Y0.15 O4 (S1) and Fe2.55 In0.45 O4 (S2), such as Fe3 O4 + γ -Fe2 O3 (S3) were obtained by ball milling. TEM micrograph as well as XRPD line broadening analyses show nanosize nature of the ferrite powders. Cation distribution, found by Rietveld refinement of the site occupancies, indicate Y3+ ions presence at 16d sites in Fe2.85 Y0.15 O4 , and a random In3+ ions distribution on both 8a and 16d sites in ¯ From the XRPD line broadening analysis crystallite size and strain values were determined. The crystallite Fe2.55 In0.45 O4 (space group Fd3m). size and strain anisotropy is significant, especially for Fe2.85 Y0.15 O4 and Fe2.55 In0.45 O4 . This result is discussed regarding the influence of the cation substitutions. The Raman signature confirms the spinel structure and the homogeneity of the particles. © 2006 Elsevier SAS. All rights reserved. Keywords: Spinel ferrites; Mechanochemical treatment; Rietveld’s analysis; Raman spectra
1. Introduction The interest for magnetic materials on nanoscale permanently increases because they posses unique magnetic, chemical and mechanical properties in comparison with their bulk counterparts. Iron oxides in nanosize form are significant both from scientific point of view and for technological applications, such as: medicine, including magnetic drug delivery [1] and contrast enhancement in magnetic resonance imaging (MRI) [2], ferrofluids [3], magnetic recording media [4], nanocomposite production [4], etc. It is well known that the physical and chemical properties of nanosize materials strongly depend on the preparation conditions. Consequently, different methods for preparation of nanosize powders were described in the literature. Especially impor* Corresponding author. Tel.: +381 21 455 318; fax: +381 21 459 367.
E-mail address: [email protected] (Z. Cvejic). 1293-2558/$ – see front matter © 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2006.02.041
tant is the possibility to control particle size and particle size distribution during synthesis. In the last decade, mechanochemistry was frequently used to produce different nanosize compounds. Depending on the system and the applied conditions, solid state reactions could be done at room temperature or at reduced temperatures, because the mechanochemical treatment already induced structural changes. One of the goals of the present work is mechanochemical synthesis of new nanosize ferrite samples: Y and In substituted magnetite (Fe3−x Inx O4 and Fe3−x Yx O4 ) as well as mechanochemically induced phase transformation in well known phase hematite (α-Fe2 O3 ). Bulk spinel samples Fe3−x Inx O4 have been synthesized for xmax = 0.30 [5,6]. To our best knowledge, spinels Fe3−x Yx O4 have not been synthesized, yet. Crystal structure of Fe2 YO4 (x = 1) has been refined in the space group (SG) P-1 [7] and R-3m [7]. There are many reports on phase transformations in hematite during the ball milling under different milling and atmospheres [8–12]. In several papers, formation of nonstoichiometric spinel phase Fe3−δ O4 , i.e., α-Fe2 O3 → Fe3−δ O4 has been reported.
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Hofmann et al. [8,9] found partial transformation of hematite to Fe3−δ O4 with δ = 0.15(1) after wet-milling for 72 h. However, with prolongated milling, part of cation deficient magnetite was transformed into hematite (δ = 0.21(1) for milling of 144 h). Some of the authors also suggested the existence of unreacted α-Fe2 O3 in amorphous-like form and small fraction of γ -Fe2 O3 [8,9]. Lee et al. [10] are analyzed the phase transformation of α-Fe2 O3 → γ -Fe2 O3 during high-energy ball milling. Zdujic et al. reported that the results of ball-milling of α-Fe2 O3 strongly depend on milling conditions. They observed both in air and oxygen atmosphere that α-Fe2 O3 has been completely transformed to Fe3 O4 , and for prolonged milling to the Fe1−x O phase [11]. Magnetite, Fe3 O4 , adapts crystallizes in spinel structure type ¯ Iron ions occupy both tetrahedral with space group (SG) Fd3m. ¯ ¯ A (8a, 43m) and octahedral B (16d, 3m) sites, while oxygens are localized in the 32e (3m) position. It is an inverse spinel with metal conductivity at room temperature. By cooling below TV = 122 K, electrical conductivity decreases by two orders of magnitude. At TV = 122 K a metal to insulator transition occurs (Verwey transition), with changes of crystal symmetry from cubic to orthorhombic [13,14]. The crystal structure of maghemite γ -Fe2 O3 is close by related to the crystal structure of Fe3 O4 with vacancies presented at cation sites. The basic structure of γ -Fe2 O3 is cubic (SG P43 32), whereas the ordered distribution of the cation vacancies results in symmetry reduction, P43 32 → P41 21 2 [15]. Structure of α-Fe2 O3 corresponds ¯ to SG R3c. Raman spectroscopy measurements on magnetite have been reported in several papers [16–20]. There are some discrepancies in observed number and positions of Raman modes. The investigated samples crystallize in spinel structure type, with 56 atoms in the unit cell and only 14 atoms in the asymmetric unit. As a result 42 vibrational modes are expected. Group theory predicts the following modes [21]: A1g + Eg + T1g + 3T2g + 2A2u + 2Eu + 5T1u + 2T2u . The T1g , A2u , Eu and T2u are silent. Therefore, there are five Raman-active modes (A1g + Eg + 3T2g ) and five infrared-active modes (5T1u ). The presence of an inversion center in the space ¯ implies mutual exclusion of Raman and infrared group Fd3m activities for the same vibration modes. Only the sites with ¯ symmetries 43m and 3m occupied, e.g., by Fe3+ and O2− in magnetite contribute to Raman activity [16]. Two main goals of this work are to confirm nanoparticle character of the sample and to define the influence of partial substitutions of iron by yttrium and indium atoms in Fe3 O4 on its structure and microstructure. To analyze it, Ramanspectroscopic, TEM and XRPD measurements were done, crystal structures were refined and X-ray line broadening due to size-strain effect was investigated. 2. Experimental 2.1. Samples preparation Mixtures of crystalline powders of In2 O3 and Fe2 O3 (Y2 O3 and Fe2 O3 ) were used as starting material to produce Fe2.55 -
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In0.45 O4 (S2) as well as (Fe2.85 Y0.15 O4 (S1)). Mechanochemical treatment was performed in a planetary ball mill (Fritsch Pulferisette 5). A hardened-steel vial of 500 cm3 volume, filled with hardened-steel balls of a diameter of 13.4 mm, was used as milling medium. The mass of the powder was 10 g and the balls-to-powder mass ratio was 20 : 1. The milling was done in air without any additives. The angular velocity of the supporting disc and vial was 32.2 and 40.3 rad/s, respectively. The intensity of milling corresponded to an acceleration of about 10 times the gravitational acceleration. After selected milling times, the mill was stopped and a small amount of the powder was removed from the vial for further characterization. The same mechanochemical treatment was applied to α-Fe2 O3 . 2.2. X-ray data collection For the XRPD data collection a Philips PW1710 automated X-ray powder diffractometer was used. The diffractometer was equipped with a Cu-tube, secondary beam graphite monochromator and Xe-filled proportional counter. The generator was set-up on 40 kV and 32 mA. Divergence and receiving slits were 1◦ and 0.1 mm, respectively. Data for the Rietveld refinement were collected in a step scan mode between 15 and 135◦ 2θ at every 0.03◦ 2θ and counting time 14 s/step. 2.3. TEM measurement Transmission electron microscopy (TEM) measurement for sample Fe2.85 Y0.15 O4 was performed on a Philips M400 equipment, with magnifications up to 310 000. 2.4. Raman measurements Two different instruments were used. A high-resolution “XY” spectrograph (Dilor, Lille, France) equipped with a double monochromator filter and a back-illuminated, liquid nitrogen-cooled, 2000 × 256 pixels CCD detector (Spex, JobinYvon–Horiba Company) as well as high sensitivity multichannel notch-filtered INFINITY spectrograph (Jobin-Yvon–Horiba SAS, Longjumeau, France) equipped with a Peltier cooled CCD matrix were used to record Raman spectra between 10 (XY instrument)/150 (INFINITY instrument) and 2000 cm−1 , using 514.5 and 647.1/532 and 632 nm exciting lines (Ar+ – Kr+ /YAG, and He–Ne lasers), respectively. Backscattering illumination and collection of the scattered light were made through an Olympus confocal microscope (long focus Olympus ×10 or ×50 objective, total magnification ×500). Because of the strong coupling between black materials and the laser light, careful attention was paid to the used power of illumination. Examination was made on small particle aggregates, a procedure preferentially used for black material. With the XY instrument (2.5 mW) a scanning mirror was added to move the laser spot on a ∼100 µm line to decrease the light induced heating. The used powers of illumination ranged between 0.01 and 3 mW (laser spot ∼(<20 µm2 )/0.5) and 5 mW (scanned ∼2 µm2 spot) with the high sensitivity INFINITY/high resolution (XY) instruments.
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2.5. Chemical analysis The elemental analyses in investigated samples, Fe3−x Inx O4 (x = 0.45) and Fe3−x Yx O4 (x = 0.15) were performed by inductively coupled plasma optical emission spectroscopy (Spectroflame ICP, 2.5 KW, 27 MHz). 2.6. Size-strain analysis procedure The X-ray line-broadenings were analyzed by Fullprof [22]. In the Fullprof program, X-ray line broadenings were studied through refinement of the TCH-pV (in this case most reliable peak-shape function) function parameters and refinement of the multipolar functions, i.e., symmetrized cubic harmonics defined in [23,24]. Details of the applied models can be found elsewhere [22]. 3. Results and discussion 3.1. Mechanochemical treatment Magnetite, Fe3 O4 , has been subject of extensive fundamental and applied research in pure form, in substituted form and embedded in nonmagnetic matrix, e.g., [4,25,26]. Ocamura et al. [5] and Okudera et al. [6] reported synthesis of indium substituted magnetite, Fe3−x Inx O4 (0 x 0.30). These samples with spinel structure were obtained by classical ceramic methods, up to xmax = 0.30. To replace more Fe3+ by In3+ and replace Fe3+ by Y3+ in Fe3 O4 , we used mechanochemical method. The stoichiometric mixture of compounds In2 O3 and α-Fe2 O3 (Y2 O3 and α-Fe2 O3 ) were milled. The subject of milling was also hematite, α-Fe2 O3 . The milling process was conducted under air atmosphere and under the same conditions for all starting powders. To follow synthesis processing, a small quantity of milled products, after selected milling time, has been taken and analysed by X-ray powder diffraction (XRPD) method. Fig. 1, shows XRPD pattern of mixture Fe2 O3 –In2 O3 , after selected milling times. All reflections on the diffraction pattern after 20 h milling, are indexed on the spinel space group ¯ Similarly, in the second mixture Y2 O3 /Fe2 O3 , such as in Fd3m. the α-Fe2 O3 , a spinel phase was formed after milling of 20 h. A contamination with Fe was noticed, through a small intensity reflection at 2θ ≈ 44.54◦ . Residuum of hematite was not noticed in any diffraction pattern recorded after milling of 20 h. ICP element analysis was performed for Fe2.85 Y0.15 O4 (S1) and Fe2.55 In0.45 O4 (S2). Found cation ratio corresponds to the one given in terms of x in formula units, within experimental error. 3.2. TEM particle size/shape determination In order to see the differences between crystallites and particles in the investigated samples we analyzed the particle shape and size from the TEM micrograph. TEM micrograph for sample Fe2.85 Y0.15 O4 (S1) is shown at Fig. 2. It seems that ferrite particles are more or less isometric, i.e., spherical. Most of particles are with diameter around 17 nm.
Fig. 1. XRD pattern of a Fe2 O3 –In2 O3 mixture, after selected milling times.
Fig. 2. TEM micrograph of Fe2.85 Y0.15 O4 .
3.3. Structure refinement procedure The collected XRPD data of S1, S2 and S3 samples were used to refine their structural and microstructural parameters. The refinement was performed with the program Fullprof, which enables to refine simultaneously both, the structural and microstructural parameters, such as: lattice parameters, atomic coordinates, site-occupancies, thermal parameters, and microstructural parameters, i.e., crystallite size and strain. The samples Fe2.85 Y0.15 O4 (S1) and Fe2.55 In0.45 O4 (S2) were refined both in space group Fd3m assuming a spinel type structure with Fe and Me (Me–Y, In) atoms in the special Wyckoff positions 8a and 16d and O in the 32e. Starting model for
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the determination of the cation distributions was based on site preferences for cation sites in a spinel structure. Iron ions have no preferences and could occupy both sites (8a and 16d). Most of the In3+ ions occupy tetrahedrally-coordinated sites [5,6]. In all known compounds, Y3+ has been found to be octahedrally coordinated [27]. A deviation from stoichiometry was allowed during refinement of occupation numbers. To identify possible crystalline phases in S3 sample and to refine its structures, several structure models were tested and Rietveld refinements have been run. To choose an appropriate structure model, we have considered the literature data on the formation of multiphase samples during the mechanochemical treatment of hematite α-Fe2 O3 and observed X-ray diffraction pattern. Two structural models for refinement of S3 sample were used: (i) Fe3−x O4 and (ii) Fe3−x O4 + γ -Fe2 O3 . Crystal structure refinements based on a two-phase model, Fe3 O4 + γ -Fe2 O3 (x = 0), gave better results. From the Rietveld’s refinement procedure, an amount of about 14% of the γ -Fe2 O3 was obtained. For the instrumental broadening correction, standard specimen was used [28]. XRPD pattern of the standard was fitted by convolution to the experimental TCH-pV (Thompson Cox Hastings–pseudo Voight) function (U = 0.0106; V = −0.01435; W = 0.00722; X = 0.04731; Y = 0.04721). The background intensity of each pattern was refined using linear interpolation between selected points. Initially, the position of the peaks was corrected by successive refinements of zero-shift. The obtained profiles were fitted by the TCH-pV function. Two asymmetry parameters were sufficient for successful refinement (Asy_1 and Asy_3). Size parameters K00 , K41 , K61 , K81 , as well as strain parameters S400 , S200 , Lσ (Lσ —Lorentzian anisotropic strain mixing parameter) were refined simultaneously. Refinements continued till convergence was reached with a value of the agreement factor, χ 2 (χ 2 —goodness of fit) close to 1 (varies between 1.30 and 1.74), which is the measure of how well the fitted model accounts for the data. Refined crystal structure parameters are given in Table 1. Fig. 3 shows good agreement between experimental data and structure model for S2 sample. 3.4. Cation distribution and bond valence sum The cation distribution in 8a (A) and 16d (B) crystallographic sites for spinels Fe2.55 In0.45 O4 and Fe2.85 Y0.15 O4 was investigated through the refinement of the occupation numbers (N). Partially inverse cation distribution was obtained in Fe2.55 In0.45 O4 . Namely, refined values of occupation numbers denote a random In3+ distribution, Table 1. Results on cation distribution reported by Okudera et al. [6] and Ocamura et al. [5] on bulk Fe3−x Inx O4 (x 0.30) are in good agreement, and show preference of In3+ for the tetrahedral A-sites. Furthermore, for x = 0.1 In3+ exclusively occupy the A-sites [6]. By extrapolation of the concentration dependence of the occupation numbers found by Okudera [6] to a hypothetical bulk sample with x = 0.45, it was found that about 75% of In3+ should occupy the A-sites. However, in nanosize sample that
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we have synthesized only 33% of In3+ is located on A-sites. In spinel-type Fe2.85 Y0.15 O4 , Y3+ exclusively occupied the octahedral B-sites, Table 1. According literature data, Y3+ was not found in tetrahedral coordination [27]. There are numerous examples with metastable cation distribution, contrary to the one expected according to known cation site preference. For example, in spite of the fact, that Zn2+ prefers tetrahedral sites, in nanosize Zn-ferrite this cation occupy both, A- and B-sites [29], leading to drastic changes of its magnetic properties. In Fe2.55 In0.45 O4 indium occupy both cation sites (half of both A- and B-sites are occupied by In3+ ), without preference to any site (Table 1), contrary to results found in bulk samples [5,6]. In Fe2.85 Y0.15 O4 spinel, Y3+ occupies exclusively octahedral B-sites, as expected according to known preference. The Y3+ is a large ion that can not occupy small tetrahedral sites. Cation–anion bond lengths for the first coordination spheres were calculated from Fullprof program and given in Table 1. The bond valence sums (BVS) for the cations were calculated using the parameterization for the Fe–O, Y–O, In–O bond given by Brown and Altermatt [30,31]. Taking into account the occupation parameters obtained from Rietveld’s analysis, the total valence unit for cations in tetrahedral position are 2.52, 2.52 and 2.61 v.u., and in octahedral position are 2.75, 2.85 and 2.76 v.u. for Fe3 O4 , Fe2.85 Y0.15 O4 , Fe2.55 In0.45 O4 , respectively. According with obtained results, it can be assumed that part of Fe2+ ions are localized at tetrahedral sites what differs from cation distribution of ideally inverse spinel structure. 3.5. Analysis of size-strain results As it can be seen from Table 1, the average mixing strain values increase in the following order: Fe3 O4 < Fe2.85 Y0.15 O4 < Fe2.55 In0.45 O4 . This fact could be explained by the influence of Y3+ (≈5%) and In3+ (≈15%) concentration on the crystal lattice strain. Crystallite size dimensions increase in following order Fe2.85 Y0.15 O4 < Fe3 O4 < Fe2.55 In0.45 O4 what can be explained thought the cation radii influence on the crystallite size. One should bear in mind that all three specimens were treated mechanochemically under the same conditions. The values for the crystallite size (Table 1) are smaller than those for particle size (Section 3.2). Hence, we can conclude that on average, each particle consists of few crystallites. In order to explain the X-ray line broadening anisotropy we are discussing influence of yttrium and indium cation concentration and ionic radius on the specimen microstructure. The X-ray line broadening anisotropy due to crystallite size effect is significant for Fe2.85 Y0.15 O4 (65 ± 15 Å) and Fe2.55 In0.45 O4 (176 ± 39 Å). The yttrium cation is the biggest. Therefore, the anisotropy increases with cation radius increase. In spite the fact that the indium and iron ionic radii are close, the indium content is higher than yttrium in investigated sample. Therefore, the anisotropies of these two samples are close. The size anisotropy for Fe3 O4 (114 ± 9 Å) is small but can not be neglected. The X-ray line broadening anisotropy due to strain effect is small in all investigated samples (see Table 1).
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Table 1 Crystal data and corresponding agreement factors for investigated specimens Crystal system: Face centered cubic
¯ (227) Space group: Fd3m
Composition Lattice parameter a (Å) Cation-anion distance d (Å) d(M8a –O) d(M16d –O) Temperature factors Ueq (Å2 ) U8a U16d U32e Occupation parameters N N (Fe)8a N (Y)8a N (In)8a N (Fe)16d N (Y)16d N (In)16d N (O)32e Profile parameters U X Y Asy_1 Asy_3 Cubic harmonic function parameters for crystallite size K00 K41 K61 K81 Cubic harmonic function parameters for crystallite strain S400 S220 Lσ Agreement factors cRp (%) cRwp (%) RB (%) χ2 D Average apparent size (Å) Average mixing strain ×103
Fe3 O4 8.3942(3)
Fe2.85 Y0.15 O4 8.4205(5)
Fe2.55 In0.45 O4 8.4829(3)
1.8808(1) × 4 2.0626(1) × 6
1.8544(1) × 4 2.0872(1) × 6
1.8748(1) × 4 2.0989(1) × 6
0.0102(4) 0.0093(2) 0.0129(9)
0.0118(5) 0.0071(2) 0.0175(11)
0.0093(3) 0.0120(8) 0.0152(8)
0.250(0) – – 0.500(0) – – 1.0000(1)
0.250(0) – – 0.471(1) 0.029(1) – 1.0000(1)
0.2124(1) – 0.0376(1) 0.4250(1) – 0.0750(1) 1.0000(1)
0.05(2) 0.45(2) 0.46(1) −0.44(9) 0.86(20)
1.01(6) 0.06(4) 0.83(1) −0.03(4) 0.08(2)
0.16(2) 0.88(3) 0.29(1) −0.02(3) 0.07(7)
0.65(7) −0.48(20) 0.14(15) −1.37(13)
0.51(12) −2.33(34) 0.76(26) −4.24(20)
0.28(6) −0.96(16) 0.69(12) −0.74(10)
0.025(29) −0.023(66) 0.16(8)
0.336(70) −1.42(20) 0.05(5)
0.015(3) −0.064(3) 0.17(11)
9.74 12.70 3.50 1.65 1.67 114(9) 3.6(1)
19.70 21.40 5.37 1.42 1.43 68(12) 4.8(4)
12.80 15.50 2.39 1.23 1.24 184(25) 6.8(1)
3.6. Raman spectra. Structural stability of ferrite based nanoparticles under laser illumination Fig. 4 shows the Raman signatures of Fe2.55 In0.45 O4 (S2) and Fe2.85 Y0.15 O4 (S1) samples for different wavelengths and powers of illumination. In Fig. 5 comparison is made with the Raman signature of pure Fe3 O4 + γ -Fe2 O3 (S3). For pure Fe3 O4 a narrow signature (Full Width at Half height FWHH < 40 cm−1 , lower than for the Y derivative) is clearly observed, as for Y derivative. According to the presence of maghemite (14% from the XRPD measurement) a shoulder is additionally observed at ∼710 cm−1 . Very low powers of illumination are needed to avoid the transformation of magnetite-like into the hematite-like structure with its very characteristic ca. 220–280 cm−1 doublet (associated to the corundum stacking) and second order ca. 1220 cm−1 peak [32,33]. The ∼10 cm−1 downshift observed versus γ -Fe2 O3 spectrum is consistent with the substitution of some Fe by heavier In or Y. With the illu-
mination power below 0.2 mW, the typical Raman signatures of the magnetite structure (strong ca. 660–700 cm−1 peak and weak bumps at ca. 300 and 540 cm−1 [34]) are obtained. However, all magnetite Raman signatures show such strong symmetrical stretching mode, located at about the same wavenumber whatever the metal at the center of the vibrational unit, because only oxygen atoms move in a stretching mode. For this reason the Raman discrimination between Fe3 O4 magnetite and γ -Fe2 O3 maghemite is not straightforward. According to the literature, maghemite Raman signature consists of a strong ca. 670 cm−1 peak, as magnetite, but with a shoulder on the higher wavenumber side and additional bumps at ca. 400–450 cm−1 . Furthermore, magnetite transforms under heating, first into maghemite at ∼200 ◦ C and then into hematite at ∼400 ◦ C [31]. By comparison with nicest and narrowest magnetite spectra in the literature, as that of [32], our Fe2.85 Y0.145 O4 spectrum exhibits a very narrow 670 cm−1 peak (FWHH ∼ 45 cm−1 ). The Fe2.55 In0.45 O4 spectrum shows a broader Raman signature and
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Fig. 3. Comparison of observed (circles) and calculated (solid line) intensities for Fe2.55 In0.45 O4 . The difference pattern appears below. The vertical bars, at the bottom, indicate reflection positions.
Fig. 4. Raman spectra for the Fe2.55 In0.45 O4 and Fe2.85 Y0.15 O4 samples.
some peak asymmetry. Band broadening is intrinsic to many nanophased oxides [35–38] because the small particle size hinders the phonon propagation and hence induces a Brillouin zone folding which makes all the phonons Raman active. Very similar feature is provoked by disordered substitution of atoms by heaver ones or by vacancies. Thus, the observed broadening in In derivative can be due to a (i) nanophasic character, (ii) mixture of magnetite-like and maghemite-like Fe–In spinel or (iii) In/Fe substitution (or the sum or the three contributions!). The
results of XRPD (Rietveld) analysis indicated absence of significant quantity (less then about 1% of a phase presence could not be accurately measured by XRPD) of maghemite-like Fe– In phase. Therefore, broadening of the Raman bands could be provoked by nanophasic character, disordered substitution of Fe and In or by presence of vacancies. The spectra recorded with green and red lines are very similar; whatever the penetration depth of the light is certainly different for both exciting lines. This indicates that there is no
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Acknowledgements The Serbian Ministry of Science and Environmental Protection has financially supported this work. We thank to Professor Volker Kahlenberg for very useful suggestions. Also, we thank to Professor Natasa Bibic to allow us to use TEM equipment and to M.Sc. Milovan Stoiljkovic for performing ICP analysis. References
Fig. 5. Raman spectra for the Fe3 O4 + γ -Fe2 O3 sample.
difference between the surface layers (“skin”) and the center of the studied grain aggregates. The second order feature (related to the resonance character of the Raman spectrum under green excitation) peaks at ∼1400 cm−1 is as expected for pure magnetite-like materials. Additional doublet at ∼1320 and 1610 cm−1 is, however, observed. This signature is typical for carbon traces [35,36]. 4. Conclusion Nanosize ferrite powders Fe2.85 Y0.15 O4 (S1), Fe2.55 In0.45 O4 (S2) and Fe3 O4 + γ -Fe2 O3 (S3) were synthesized by ball milling. The starting oxides were milled under the same conditions (the ball to mass ratio, the rotational speed and atmosphere) in order to study change in structure and microstructure influenced by partial cation substitutions in magnetite. The single phase samples S1 and S2 were refined in the space group Fd3m and spinel type structure. In the sample S3 two phases were found 86% of magnetite Fe3 O4 and 14% maghemite γ -Fe2 O3 . Cation distribution found by refinement of occupation numbers show a Y3+ ions preference to 16d sites in S2 and a random In3+ ions distribution on both 8a and 16d sites in S1. X-ray line broadening analysis indicated that the average mixing strain increases in the following order: Fe3 O4 < Fe2.85 Y0.15 O4 < Fe2.55 In0.45 O4 while the crystallite size increases in the following order Fe2.85 Y0.15 O4 < Fe3 O4 < Fe2.55 In0.45 O4 . There are significant anisotropy of X-ray line broadening in S1 and S2. TEM image show formation of nanosize powders. Raman spectra confirm presence of magnetite-like structure for all three phases. For S3 additional bands ascribed to maghemite were noticed. The broadening observed for In derivative is assigned to: nanophasic character of sample, presence of vacancies or the disordered Fe/In substitution which break the phonon propagation and induces some Brillouin zone folding.
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Nanosize ferrites obtained by ball milling: Crystal structure, cation distribution, size-strain analysis and Raman investigations Zeljka Cvejic a,∗ , Srdjan Rakic a , Aleksandar Kremenovic b,c , Bratislav Antic c , Cedomir Jovalekic d , Philippe Colomban e a Institute of Physics, Faculty of Natural Sciences, University of Novi Sad, Trg D. Obradovica 6, 21000 Novi Sad, Serbia and Montenegro b Laboratory for Crystallography, Faculty of Mining and Geology, University of Belgrade, POB 162, 11001 Belgrade, Serbia and Montenegro c Condensed Matter Physics Laboratory, The “Vinca” Institute, POB 522, 11001 Belgrade, Serbia and Montenegro d Center for Multidisciplinary Studies, University of Belgrade, Kneza Viseslava 1, 11000 Belgrade, Serbia and Montenegro e LADIR, UMR 7075 CNRS, and Université Pierre and Marie Curie, 94230 Thiais, France
Received 3 January 2006; accepted 22 February 2006 Available online 11 May 2006
Abstract Spinels samples Fe2.85 Y0.15 O4 (S1) and Fe2.55 In0.45 O4 (S2), such as Fe3 O4 + γ -Fe2 O3 (S3) were obtained by ball milling. TEM micrograph as well as XRPD line broadening analyses show nanosize nature of the ferrite powders. Cation distribution, found by Rietveld refinement of the site occupancies, indicate Y3+ ions presence at 16d sites in Fe2.85 Y0.15 O4 , and a random In3+ ions distribution on both 8a and 16d sites in ¯ From the XRPD line broadening analysis crystallite size and strain values were determined. The crystallite Fe2.55 In0.45 O4 (space group Fd3m). size and strain anisotropy is significant, especially for Fe2.85 Y0.15 O4 and Fe2.55 In0.45 O4 . This result is discussed regarding the influence of the cation substitutions. The Raman signature confirms the spinel structure and the homogeneity of the particles. © 2006 Elsevier SAS. All rights reserved. Keywords: Spinel ferrites; Mechanochemical treatment; Rietveld’s analysis; Raman spectra
1. Introduction The interest for magnetic materials on nanoscale permanently increases because they posses unique magnetic, chemical and mechanical properties in comparison with their bulk counterparts. Iron oxides in nanosize form are significant both from scientific point of view and for technological applications, such as: medicine, including magnetic drug delivery [1] and contrast enhancement in magnetic resonance imaging (MRI) [2], ferrofluids [3], magnetic recording media [4], nanocomposite production [4], etc. It is well known that the physical and chemical properties of nanosize materials strongly depend on the preparation conditions. Consequently, different methods for preparation of nanosize powders were described in the literature. Especially impor* Corresponding author. Tel.: +381 21 455 318; fax: +381 21 459 367.
E-mail address: [email protected] (Z. Cvejic). 1293-2558/$ – see front matter © 2006 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2006.02.041
tant is the possibility to control particle size and particle size distribution during synthesis. In the last decade, mechanochemistry was frequently used to produce different nanosize compounds. Depending on the system and the applied conditions, solid state reactions could be done at room temperature or at reduced temperatures, because the mechanochemical treatment already induced structural changes. One of the goals of the present work is mechanochemical synthesis of new nanosize ferrite samples: Y and In substituted magnetite (Fe3−x Inx O4 and Fe3−x Yx O4 ) as well as mechanochemically induced phase transformation in well known phase hematite (α-Fe2 O3 ). Bulk spinel samples Fe3−x Inx O4 have been synthesized for xmax = 0.30 [5,6]. To our best knowledge, spinels Fe3−x Yx O4 have not been synthesized, yet. Crystal structure of Fe2 YO4 (x = 1) has been refined in the space group (SG) P-1 [7] and R-3m [7]. There are many reports on phase transformations in hematite during the ball milling under different milling and atmospheres [8–12]. In several papers, formation of nonstoichiometric spinel phase Fe3−δ O4 , i.e., α-Fe2 O3 → Fe3−δ O4 has been reported.
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Hofmann et al. [8,9] found partial transformation of hematite to Fe3−δ O4 with δ = 0.15(1) after wet-milling for 72 h. However, with prolongated milling, part of cation deficient magnetite was transformed into hematite (δ = 0.21(1) for milling of 144 h). Some of the authors also suggested the existence of unreacted α-Fe2 O3 in amorphous-like form and small fraction of γ -Fe2 O3 [8,9]. Lee et al. [10] are analyzed the phase transformation of α-Fe2 O3 → γ -Fe2 O3 during high-energy ball milling. Zdujic et al. reported that the results of ball-milling of α-Fe2 O3 strongly depend on milling conditions. They observed both in air and oxygen atmosphere that α-Fe2 O3 has been completely transformed to Fe3 O4 , and for prolonged milling to the Fe1−x O phase [11]. Magnetite, Fe3 O4 , adapts crystallizes in spinel structure type ¯ Iron ions occupy both tetrahedral with space group (SG) Fd3m. ¯ ¯ A (8a, 43m) and octahedral B (16d, 3m) sites, while oxygens are localized in the 32e (3m) position. It is an inverse spinel with metal conductivity at room temperature. By cooling below TV = 122 K, electrical conductivity decreases by two orders of magnitude. At TV = 122 K a metal to insulator transition occurs (Verwey transition), with changes of crystal symmetry from cubic to orthorhombic [13,14]. The crystal structure of maghemite γ -Fe2 O3 is close by related to the crystal structure of Fe3 O4 with vacancies presented at cation sites. The basic structure of γ -Fe2 O3 is cubic (SG P43 32), whereas the ordered distribution of the cation vacancies results in symmetry reduction, P43 32 → P41 21 2 [15]. Structure of α-Fe2 O3 corresponds ¯ to SG R3c. Raman spectroscopy measurements on magnetite have been reported in several papers [16–20]. There are some discrepancies in observed number and positions of Raman modes. The investigated samples crystallize in spinel structure type, with 56 atoms in the unit cell and only 14 atoms in the asymmetric unit. As a result 42 vibrational modes are expected. Group theory predicts the following modes [21]: A1g + Eg + T1g + 3T2g + 2A2u + 2Eu + 5T1u + 2T2u . The T1g , A2u , Eu and T2u are silent. Therefore, there are five Raman-active modes (A1g + Eg + 3T2g ) and five infrared-active modes (5T1u ). The presence of an inversion center in the space ¯ implies mutual exclusion of Raman and infrared group Fd3m activities for the same vibration modes. Only the sites with ¯ symmetries 43m and 3m occupied, e.g., by Fe3+ and O2− in magnetite contribute to Raman activity [16]. Two main goals of this work are to confirm nanoparticle character of the sample and to define the influence of partial substitutions of iron by yttrium and indium atoms in Fe3 O4 on its structure and microstructure. To analyze it, Ramanspectroscopic, TEM and XRPD measurements were done, crystal structures were refined and X-ray line broadening due to size-strain effect was investigated. 2. Experimental 2.1. Samples preparation Mixtures of crystalline powders of In2 O3 and Fe2 O3 (Y2 O3 and Fe2 O3 ) were used as starting material to produce Fe2.55 -
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In0.45 O4 (S2) as well as (Fe2.85 Y0.15 O4 (S1)). Mechanochemical treatment was performed in a planetary ball mill (Fritsch Pulferisette 5). A hardened-steel vial of 500 cm3 volume, filled with hardened-steel balls of a diameter of 13.4 mm, was used as milling medium. The mass of the powder was 10 g and the balls-to-powder mass ratio was 20 : 1. The milling was done in air without any additives. The angular velocity of the supporting disc and vial was 32.2 and 40.3 rad/s, respectively. The intensity of milling corresponded to an acceleration of about 10 times the gravitational acceleration. After selected milling times, the mill was stopped and a small amount of the powder was removed from the vial for further characterization. The same mechanochemical treatment was applied to α-Fe2 O3 . 2.2. X-ray data collection For the XRPD data collection a Philips PW1710 automated X-ray powder diffractometer was used. The diffractometer was equipped with a Cu-tube, secondary beam graphite monochromator and Xe-filled proportional counter. The generator was set-up on 40 kV and 32 mA. Divergence and receiving slits were 1◦ and 0.1 mm, respectively. Data for the Rietveld refinement were collected in a step scan mode between 15 and 135◦ 2θ at every 0.03◦ 2θ and counting time 14 s/step. 2.3. TEM measurement Transmission electron microscopy (TEM) measurement for sample Fe2.85 Y0.15 O4 was performed on a Philips M400 equipment, with magnifications up to 310 000. 2.4. Raman measurements Two different instruments were used. A high-resolution “XY” spectrograph (Dilor, Lille, France) equipped with a double monochromator filter and a back-illuminated, liquid nitrogen-cooled, 2000 × 256 pixels CCD detector (Spex, JobinYvon–Horiba Company) as well as high sensitivity multichannel notch-filtered INFINITY spectrograph (Jobin-Yvon–Horiba SAS, Longjumeau, France) equipped with a Peltier cooled CCD matrix were used to record Raman spectra between 10 (XY instrument)/150 (INFINITY instrument) and 2000 cm−1 , using 514.5 and 647.1/532 and 632 nm exciting lines (Ar+ – Kr+ /YAG, and He–Ne lasers), respectively. Backscattering illumination and collection of the scattered light were made through an Olympus confocal microscope (long focus Olympus ×10 or ×50 objective, total magnification ×500). Because of the strong coupling between black materials and the laser light, careful attention was paid to the used power of illumination. Examination was made on small particle aggregates, a procedure preferentially used for black material. With the XY instrument (2.5 mW) a scanning mirror was added to move the laser spot on a ∼100 µm line to decrease the light induced heating. The used powers of illumination ranged between 0.01 and 3 mW (laser spot ∼(<20 µm2 )/0.5) and 5 mW (scanned ∼2 µm2 spot) with the high sensitivity INFINITY/high resolution (XY) instruments.
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2.5. Chemical analysis The elemental analyses in investigated samples, Fe3−x Inx O4 (x = 0.45) and Fe3−x Yx O4 (x = 0.15) were performed by inductively coupled plasma optical emission spectroscopy (Spectroflame ICP, 2.5 KW, 27 MHz). 2.6. Size-strain analysis procedure The X-ray line-broadenings were analyzed by Fullprof [22]. In the Fullprof program, X-ray line broadenings were studied through refinement of the TCH-pV (in this case most reliable peak-shape function) function parameters and refinement of the multipolar functions, i.e., symmetrized cubic harmonics defined in [23,24]. Details of the applied models can be found elsewhere [22]. 3. Results and discussion 3.1. Mechanochemical treatment Magnetite, Fe3 O4 , has been subject of extensive fundamental and applied research in pure form, in substituted form and embedded in nonmagnetic matrix, e.g., [4,25,26]. Ocamura et al. [5] and Okudera et al. [6] reported synthesis of indium substituted magnetite, Fe3−x Inx O4 (0 x 0.30). These samples with spinel structure were obtained by classical ceramic methods, up to xmax = 0.30. To replace more Fe3+ by In3+ and replace Fe3+ by Y3+ in Fe3 O4 , we used mechanochemical method. The stoichiometric mixture of compounds In2 O3 and α-Fe2 O3 (Y2 O3 and α-Fe2 O3 ) were milled. The subject of milling was also hematite, α-Fe2 O3 . The milling process was conducted under air atmosphere and under the same conditions for all starting powders. To follow synthesis processing, a small quantity of milled products, after selected milling time, has been taken and analysed by X-ray powder diffraction (XRPD) method. Fig. 1, shows XRPD pattern of mixture Fe2 O3 –In2 O3 , after selected milling times. All reflections on the diffraction pattern after 20 h milling, are indexed on the spinel space group ¯ Similarly, in the second mixture Y2 O3 /Fe2 O3 , such as in Fd3m. the α-Fe2 O3 , a spinel phase was formed after milling of 20 h. A contamination with Fe was noticed, through a small intensity reflection at 2θ ≈ 44.54◦ . Residuum of hematite was not noticed in any diffraction pattern recorded after milling of 20 h. ICP element analysis was performed for Fe2.85 Y0.15 O4 (S1) and Fe2.55 In0.45 O4 (S2). Found cation ratio corresponds to the one given in terms of x in formula units, within experimental error. 3.2. TEM particle size/shape determination In order to see the differences between crystallites and particles in the investigated samples we analyzed the particle shape and size from the TEM micrograph. TEM micrograph for sample Fe2.85 Y0.15 O4 (S1) is shown at Fig. 2. It seems that ferrite particles are more or less isometric, i.e., spherical. Most of particles are with diameter around 17 nm.
Fig. 1. XRD pattern of a Fe2 O3 –In2 O3 mixture, after selected milling times.
Fig. 2. TEM micrograph of Fe2.85 Y0.15 O4 .
3.3. Structure refinement procedure The collected XRPD data of S1, S2 and S3 samples were used to refine their structural and microstructural parameters. The refinement was performed with the program Fullprof, which enables to refine simultaneously both, the structural and microstructural parameters, such as: lattice parameters, atomic coordinates, site-occupancies, thermal parameters, and microstructural parameters, i.e., crystallite size and strain. The samples Fe2.85 Y0.15 O4 (S1) and Fe2.55 In0.45 O4 (S2) were refined both in space group Fd3m assuming a spinel type structure with Fe and Me (Me–Y, In) atoms in the special Wyckoff positions 8a and 16d and O in the 32e. Starting model for
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the determination of the cation distributions was based on site preferences for cation sites in a spinel structure. Iron ions have no preferences and could occupy both sites (8a and 16d). Most of the In3+ ions occupy tetrahedrally-coordinated sites [5,6]. In all known compounds, Y3+ has been found to be octahedrally coordinated [27]. A deviation from stoichiometry was allowed during refinement of occupation numbers. To identify possible crystalline phases in S3 sample and to refine its structures, several structure models were tested and Rietveld refinements have been run. To choose an appropriate structure model, we have considered the literature data on the formation of multiphase samples during the mechanochemical treatment of hematite α-Fe2 O3 and observed X-ray diffraction pattern. Two structural models for refinement of S3 sample were used: (i) Fe3−x O4 and (ii) Fe3−x O4 + γ -Fe2 O3 . Crystal structure refinements based on a two-phase model, Fe3 O4 + γ -Fe2 O3 (x = 0), gave better results. From the Rietveld’s refinement procedure, an amount of about 14% of the γ -Fe2 O3 was obtained. For the instrumental broadening correction, standard specimen was used [28]. XRPD pattern of the standard was fitted by convolution to the experimental TCH-pV (Thompson Cox Hastings–pseudo Voight) function (U = 0.0106; V = −0.01435; W = 0.00722; X = 0.04731; Y = 0.04721). The background intensity of each pattern was refined using linear interpolation between selected points. Initially, the position of the peaks was corrected by successive refinements of zero-shift. The obtained profiles were fitted by the TCH-pV function. Two asymmetry parameters were sufficient for successful refinement (Asy_1 and Asy_3). Size parameters K00 , K41 , K61 , K81 , as well as strain parameters S400 , S200 , Lσ (Lσ —Lorentzian anisotropic strain mixing parameter) were refined simultaneously. Refinements continued till convergence was reached with a value of the agreement factor, χ 2 (χ 2 —goodness of fit) close to 1 (varies between 1.30 and 1.74), which is the measure of how well the fitted model accounts for the data. Refined crystal structure parameters are given in Table 1. Fig. 3 shows good agreement between experimental data and structure model for S2 sample. 3.4. Cation distribution and bond valence sum The cation distribution in 8a (A) and 16d (B) crystallographic sites for spinels Fe2.55 In0.45 O4 and Fe2.85 Y0.15 O4 was investigated through the refinement of the occupation numbers (N). Partially inverse cation distribution was obtained in Fe2.55 In0.45 O4 . Namely, refined values of occupation numbers denote a random In3+ distribution, Table 1. Results on cation distribution reported by Okudera et al. [6] and Ocamura et al. [5] on bulk Fe3−x Inx O4 (x 0.30) are in good agreement, and show preference of In3+ for the tetrahedral A-sites. Furthermore, for x = 0.1 In3+ exclusively occupy the A-sites [6]. By extrapolation of the concentration dependence of the occupation numbers found by Okudera [6] to a hypothetical bulk sample with x = 0.45, it was found that about 75% of In3+ should occupy the A-sites. However, in nanosize sample that
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we have synthesized only 33% of In3+ is located on A-sites. In spinel-type Fe2.85 Y0.15 O4 , Y3+ exclusively occupied the octahedral B-sites, Table 1. According literature data, Y3+ was not found in tetrahedral coordination [27]. There are numerous examples with metastable cation distribution, contrary to the one expected according to known cation site preference. For example, in spite of the fact, that Zn2+ prefers tetrahedral sites, in nanosize Zn-ferrite this cation occupy both, A- and B-sites [29], leading to drastic changes of its magnetic properties. In Fe2.55 In0.45 O4 indium occupy both cation sites (half of both A- and B-sites are occupied by In3+ ), without preference to any site (Table 1), contrary to results found in bulk samples [5,6]. In Fe2.85 Y0.15 O4 spinel, Y3+ occupies exclusively octahedral B-sites, as expected according to known preference. The Y3+ is a large ion that can not occupy small tetrahedral sites. Cation–anion bond lengths for the first coordination spheres were calculated from Fullprof program and given in Table 1. The bond valence sums (BVS) for the cations were calculated using the parameterization for the Fe–O, Y–O, In–O bond given by Brown and Altermatt [30,31]. Taking into account the occupation parameters obtained from Rietveld’s analysis, the total valence unit for cations in tetrahedral position are 2.52, 2.52 and 2.61 v.u., and in octahedral position are 2.75, 2.85 and 2.76 v.u. for Fe3 O4 , Fe2.85 Y0.15 O4 , Fe2.55 In0.45 O4 , respectively. According with obtained results, it can be assumed that part of Fe2+ ions are localized at tetrahedral sites what differs from cation distribution of ideally inverse spinel structure. 3.5. Analysis of size-strain results As it can be seen from Table 1, the average mixing strain values increase in the following order: Fe3 O4 < Fe2.85 Y0.15 O4 < Fe2.55 In0.45 O4 . This fact could be explained by the influence of Y3+ (≈5%) and In3+ (≈15%) concentration on the crystal lattice strain. Crystallite size dimensions increase in following order Fe2.85 Y0.15 O4 < Fe3 O4 < Fe2.55 In0.45 O4 what can be explained thought the cation radii influence on the crystallite size. One should bear in mind that all three specimens were treated mechanochemically under the same conditions. The values for the crystallite size (Table 1) are smaller than those for particle size (Section 3.2). Hence, we can conclude that on average, each particle consists of few crystallites. In order to explain the X-ray line broadening anisotropy we are discussing influence of yttrium and indium cation concentration and ionic radius on the specimen microstructure. The X-ray line broadening anisotropy due to crystallite size effect is significant for Fe2.85 Y0.15 O4 (65 ± 15 Å) and Fe2.55 In0.45 O4 (176 ± 39 Å). The yttrium cation is the biggest. Therefore, the anisotropy increases with cation radius increase. In spite the fact that the indium and iron ionic radii are close, the indium content is higher than yttrium in investigated sample. Therefore, the anisotropies of these two samples are close. The size anisotropy for Fe3 O4 (114 ± 9 Å) is small but can not be neglected. The X-ray line broadening anisotropy due to strain effect is small in all investigated samples (see Table 1).
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Table 1 Crystal data and corresponding agreement factors for investigated specimens Crystal system: Face centered cubic
¯ (227) Space group: Fd3m
Composition Lattice parameter a (Å) Cation-anion distance d (Å) d(M8a –O) d(M16d –O) Temperature factors Ueq (Å2 ) U8a U16d U32e Occupation parameters N N (Fe)8a N (Y)8a N (In)8a N (Fe)16d N (Y)16d N (In)16d N (O)32e Profile parameters U X Y Asy_1 Asy_3 Cubic harmonic function parameters for crystallite size K00 K41 K61 K81 Cubic harmonic function parameters for crystallite strain S400 S220 Lσ Agreement factors cRp (%) cRwp (%) RB (%) χ2 D Average apparent size (Å) Average mixing strain ×103
Fe3 O4 8.3942(3)
Fe2.85 Y0.15 O4 8.4205(5)
Fe2.55 In0.45 O4 8.4829(3)
1.8808(1) × 4 2.0626(1) × 6
1.8544(1) × 4 2.0872(1) × 6
1.8748(1) × 4 2.0989(1) × 6
0.0102(4) 0.0093(2) 0.0129(9)
0.0118(5) 0.0071(2) 0.0175(11)
0.0093(3) 0.0120(8) 0.0152(8)
0.250(0) – – 0.500(0) – – 1.0000(1)
0.250(0) – – 0.471(1) 0.029(1) – 1.0000(1)
0.2124(1) – 0.0376(1) 0.4250(1) – 0.0750(1) 1.0000(1)
0.05(2) 0.45(2) 0.46(1) −0.44(9) 0.86(20)
1.01(6) 0.06(4) 0.83(1) −0.03(4) 0.08(2)
0.16(2) 0.88(3) 0.29(1) −0.02(3) 0.07(7)
0.65(7) −0.48(20) 0.14(15) −1.37(13)
0.51(12) −2.33(34) 0.76(26) −4.24(20)
0.28(6) −0.96(16) 0.69(12) −0.74(10)
0.025(29) −0.023(66) 0.16(8)
0.336(70) −1.42(20) 0.05(5)
0.015(3) −0.064(3) 0.17(11)
9.74 12.70 3.50 1.65 1.67 114(9) 3.6(1)
19.70 21.40 5.37 1.42 1.43 68(12) 4.8(4)
12.80 15.50 2.39 1.23 1.24 184(25) 6.8(1)
3.6. Raman spectra. Structural stability of ferrite based nanoparticles under laser illumination Fig. 4 shows the Raman signatures of Fe2.55 In0.45 O4 (S2) and Fe2.85 Y0.15 O4 (S1) samples for different wavelengths and powers of illumination. In Fig. 5 comparison is made with the Raman signature of pure Fe3 O4 + γ -Fe2 O3 (S3). For pure Fe3 O4 a narrow signature (Full Width at Half height FWHH < 40 cm−1 , lower than for the Y derivative) is clearly observed, as for Y derivative. According to the presence of maghemite (14% from the XRPD measurement) a shoulder is additionally observed at ∼710 cm−1 . Very low powers of illumination are needed to avoid the transformation of magnetite-like into the hematite-like structure with its very characteristic ca. 220–280 cm−1 doublet (associated to the corundum stacking) and second order ca. 1220 cm−1 peak [32,33]. The ∼10 cm−1 downshift observed versus γ -Fe2 O3 spectrum is consistent with the substitution of some Fe by heavier In or Y. With the illu-
mination power below 0.2 mW, the typical Raman signatures of the magnetite structure (strong ca. 660–700 cm−1 peak and weak bumps at ca. 300 and 540 cm−1 [34]) are obtained. However, all magnetite Raman signatures show such strong symmetrical stretching mode, located at about the same wavenumber whatever the metal at the center of the vibrational unit, because only oxygen atoms move in a stretching mode. For this reason the Raman discrimination between Fe3 O4 magnetite and γ -Fe2 O3 maghemite is not straightforward. According to the literature, maghemite Raman signature consists of a strong ca. 670 cm−1 peak, as magnetite, but with a shoulder on the higher wavenumber side and additional bumps at ca. 400–450 cm−1 . Furthermore, magnetite transforms under heating, first into maghemite at ∼200 ◦ C and then into hematite at ∼400 ◦ C [31]. By comparison with nicest and narrowest magnetite spectra in the literature, as that of [32], our Fe2.85 Y0.145 O4 spectrum exhibits a very narrow 670 cm−1 peak (FWHH ∼ 45 cm−1 ). The Fe2.55 In0.45 O4 spectrum shows a broader Raman signature and
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Fig. 3. Comparison of observed (circles) and calculated (solid line) intensities for Fe2.55 In0.45 O4 . The difference pattern appears below. The vertical bars, at the bottom, indicate reflection positions.
Fig. 4. Raman spectra for the Fe2.55 In0.45 O4 and Fe2.85 Y0.15 O4 samples.
some peak asymmetry. Band broadening is intrinsic to many nanophased oxides [35–38] because the small particle size hinders the phonon propagation and hence induces a Brillouin zone folding which makes all the phonons Raman active. Very similar feature is provoked by disordered substitution of atoms by heaver ones or by vacancies. Thus, the observed broadening in In derivative can be due to a (i) nanophasic character, (ii) mixture of magnetite-like and maghemite-like Fe–In spinel or (iii) In/Fe substitution (or the sum or the three contributions!). The
results of XRPD (Rietveld) analysis indicated absence of significant quantity (less then about 1% of a phase presence could not be accurately measured by XRPD) of maghemite-like Fe– In phase. Therefore, broadening of the Raman bands could be provoked by nanophasic character, disordered substitution of Fe and In or by presence of vacancies. The spectra recorded with green and red lines are very similar; whatever the penetration depth of the light is certainly different for both exciting lines. This indicates that there is no
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Acknowledgements The Serbian Ministry of Science and Environmental Protection has financially supported this work. We thank to Professor Volker Kahlenberg for very useful suggestions. Also, we thank to Professor Natasa Bibic to allow us to use TEM equipment and to M.Sc. Milovan Stoiljkovic for performing ICP analysis. References
Fig. 5. Raman spectra for the Fe3 O4 + γ -Fe2 O3 sample.
difference between the surface layers (“skin”) and the center of the studied grain aggregates. The second order feature (related to the resonance character of the Raman spectrum under green excitation) peaks at ∼1400 cm−1 is as expected for pure magnetite-like materials. Additional doublet at ∼1320 and 1610 cm−1 is, however, observed. This signature is typical for carbon traces [35,36]. 4. Conclusion Nanosize ferrite powders Fe2.85 Y0.15 O4 (S1), Fe2.55 In0.45 O4 (S2) and Fe3 O4 + γ -Fe2 O3 (S3) were synthesized by ball milling. The starting oxides were milled under the same conditions (the ball to mass ratio, the rotational speed and atmosphere) in order to study change in structure and microstructure influenced by partial cation substitutions in magnetite. The single phase samples S1 and S2 were refined in the space group Fd3m and spinel type structure. In the sample S3 two phases were found 86% of magnetite Fe3 O4 and 14% maghemite γ -Fe2 O3 . Cation distribution found by refinement of occupation numbers show a Y3+ ions preference to 16d sites in S2 and a random In3+ ions distribution on both 8a and 16d sites in S1. X-ray line broadening analysis indicated that the average mixing strain increases in the following order: Fe3 O4 < Fe2.85 Y0.15 O4 < Fe2.55 In0.45 O4 while the crystallite size increases in the following order Fe2.85 Y0.15 O4 < Fe3 O4 < Fe2.55 In0.45 O4 . There are significant anisotropy of X-ray line broadening in S1 and S2. TEM image show formation of nanosize powders. Raman spectra confirm presence of magnetite-like structure for all three phases. For S3 additional bands ascribed to maghemite were noticed. The broadening observed for In derivative is assigned to: nanophasic character of sample, presence of vacancies or the disordered Fe/In substitution which break the phonon propagation and induces some Brillouin zone folding.
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