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Structural evolution of ball-milled ZnFe2O4
Journal of Alloys and Compounds 339 (2002) 255–260
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Structural evolution of ball-milled ZnFe 2 O 4 a , ,1 b c ,2 H. Ehrhardt * , S.J. Campbell , M. Hofmann a
¨ Nanotechnologie, Forschungszentrum Karlsruhe GmbH, Postfach 3640, D-76021 Karlsruhe, Germany Institut f ur b School of Physics, University College, University of New South Wales, ADFA, Canberra, ACT 2600, Australia c ISIS, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0 QX, UK Received 20 August 2001; accepted 28 November 2001
Abstract Nanostructured zinc ferrite produced by milling in both low-energy and high-energy ball mills has been investigated by X-ray ¨ diffraction and Mossbauer effect spectroscopy. The lattice parameter of the milled products remains essentially unchanged from that of equilibrium ZnFe 2 O 4 with the steady-state average particle size found to decrease to d518(2) nm on low energy milling compared with ¨ d58(1) nm on high energy milling. The room temperature Mossbauer spectra of the milled materials have been analysed using two doublets, one of which is considered to be associated primarily with the octahedral lattice sites. Spectral broadening is observed with decreasing particle size, particularly below d|10 nm, for which the effects of magnetic hyperfine splitting become evident. The mean inversion parameter of nanostructured ZnFe 2 O 4 is found to increase to c|0.75 for particle sizes of d|8 nm reflecting the systematic evolution of zinc ferrite from its normal spinel structure towards an inverse spinel structure on mechanical treatment as observed ¨ previously. The other factors which contribute to the Mossbauer spectra of nanostructured ZnFe 2 O 4 (d|8–70 nm) are discussed. 2002 Elsevier Science B.V. All rights reserved. ¨ effect; Nanostructured ZnFe 2 O 4 Keywords: Mechanical milling; X-Ray diffraction; Mossbauer
1. Introduction Spinel ferrites are used in a wide range of technological applications [1]. This stems mainly from their useful catalytic properties as well as their interesting magnetic characteristics. Zinc ferrite is of particular interest as a result of the important role that it plays as an absorbent in the hot gas desulphurisation process [2–4] and its use in providing scope for mixed ferrites of purpose designed ˇ ´ et al. [3] magnetic parameters [5]. For example, Sepelak investigated the effect of mechanical activation on the reactivity of ZnFe 2 O 4 in the reaction with H 2 S, with Ahmed et al. [4] having recently explored the structural
*Corresponding author. Tel.: 161-2-6268-8011; fax: 161-2-62688786. E-mail address: [email protected] (H. Ehrhardt). 1 Present address: School of Physics, University College, University of New South Wales, ADFA, Canberra, ACT 2600, Australia. 2 Former address: BENSC, Hahn-Meitner-Institut, D-14109 Berlin, Germany.
changes occurring in ZnFe 2 O 4 during calcination at high temperature or regeneration in oxidant atmosphere. The spinel crystal structure is an approximately closepacked face-centered cubic array of anions with holes partly filled by the cations. The oxide spinel can generally be described by the formula (A)[B 2 ]O 4 where A and B denote divalent and trivalent cations, respectively. In the case of a normal spinel structure, all of the A atoms are tetrahedrally coordinated while the B atoms are octahedrally coordinated by oxygen atoms. In the case of an inverse spinel structure, the A atoms occupy half of the B sites. However, as is well known, spinels are often found with other than purely normal or purely inverse distributions of cations [6]. The site occupation can be represented by (A 12c B c )[A c B 22c ]O 4 where c is the inversion parameter (0#c#1) with tetrahedral sites denoted by round brackets and octahedral sites by square brackets. ˇ ´ et al. [7], spinel ferrites provide As outlined by Sepelak suitable model systems for investigation of effects induced by mechanochemical activation. ZnFe 2 O 4 is of particular interest with changes in magnetic behaviour having been reported by several workers for nanoscale particles prepared by milling treatments or other means (e.g. Ref. [5,8–14]). Equilibrium ZnFe 2 O 4 is antiferromagnetic with
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )02011-4
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H. Ehrhardt et al. / Journal of Alloys and Compounds 339 (2002) 255 – 260
´ temperature of T N |10.5 K (e.g. Ref. [15]) whereas a Neel magnetic behaviours characteristic of ferrimagnetic-like or ferromagnetic-like interactions with ordering temperatures above room temperature have been reported for nanoscale ¨ ZnFe 2 O 4 material (e.g. Ref. [5,8–15]). Mossbauer spectroscopy has been highly effective in providing insight to the microscopic changes that occur both in ZnFe 2 O 4 subjected to milling (e.g. Refs. [7,10,14]) and in ZnFe 2 O 4 produced by mechanochemical synthesis (e.g. Refs. [9,16]). Given that most studies have so far been undertaken using high-energy mills, we have carried out a systematic study of ZnFe 2 O 4 milled in both low-energy mode (LEM) in air and high-energy mode (HEM) in argon. The use of a low-energy mill designed for smallscale research purposes [17,18] rather than industrial-scale applications, is especially useful in the present investigation in helping to ensure that all processes which take place on milling ZnFe 2 O 4 can be followed. Our investigation of the microstructures of these two sets of ballmilled ZnFe 2 O 4 materials by X-ray diffraction and ¨ Mossbauer spectroscopy has enabled the lattice parameter and the inversion parameter for nanostructured ZnFe 2 O 4 particles of average grain sizes from d|27(3) nm to d| ¨ 8(1) nm to be determined. The changes in Mossbauer hyperfine parameters for these nanostructured samples have also been characterised systematically.
ments using FULLPROF [22]. The room temperature ¨ transmission Mossbauer experiments were carried out using a conventional microcomputer-controlled spectrometer in constant acceleration mode and a 57 CoRh ] source. The velocity scale was calibrated relative to natural a-Fe.
3. Results and discussion
2. Experimental
Examples of the set of X-ray diffraction patterns obtained for all of the milled samples are shown in Fig. 1: coarse-grained material (Fig. 1a); 120 h milled LEM sample (Fig. 1b) and the 12 h milled HEM sample (Fig. 1c). All samples were found to display the reflections of cubic spinels, essentially matching these of ZnFe 2 O 4 (Franklinite, JCPDS 22-1012) with the lattice parameter ˚ This for the starting material found to be a58.449(2) A. value is slightly larger (|0.1%) than the JCPDS value of ˚ but comparable to values of a58.450 A ˚ [4] a 0 58.4411 A ˚ [23] reported recently. The LEM samples and a58.4599 A ˚ were found to have the same value of a58.451(4) A (independent of milling time) compared with the slightly ˚ for the HEM samples. This larger value of a58.462(5) A increase in lattice parameter (|0.1%) may be associated with the dissolution of Ar in the samples during milling in the HEM mode. The inhomogeneous strain was found to increase slightly from |0.4% after LEM milling for 40 h to 0.8% on milling for 120 h. Reflecting the higher energy
Commercial coarse-grained powders of ZnFe 2 O 4 (Alfa Aesar) were milled in both a low-energy magnetic mill (LEM) [17,18] and a high-energy SPEX 8000 shaker mill (HEM) [19,20]. The LEM samples (initial charge |10 g) were wet milled with 5 ml water in air in a sealed stainless steel mill with four |22 mm steel balls with a ball-topowder weight ratio of |20:1. Small amounts of powder (|2–3 g) were extracted from the vial after different milling times (40, 60, 80, 120 h). For the HEM samples, 5 g charges were sealed in a steel vial under an Ar atmosphere with two 12.7 mm diameter and four 6.4 mm steel balls, yielding a ball-to-powder weight ratio of 3.5:1. The samples were then milled for different times (2, 4, 6, 12 h). There was no evidence of contamination from the milling media as monitored by energy-dispersive X-ray spectroscopy (EDX). The samples were characterised by X-ray diffraction using a Siemens Rigaku u –2u diffractometer equipped with a Cu tube. The (volume-weighted) average grain sizes were determined from Scherrer’s formula for peak broadening, particularly from the full width at half maximum (FWHM) of the (311) diffraction line. The inhomogeneous strain was estimated from the (220) and (440) reflections assuming a Gaussian strain broadening [21]. Lattice parameter and inversion parameter were determined in the standard way based on Rietveld refine-
Fig. 1. X-Ray diffraction patterns of ZnFe 2 O 4 after different milling times t m in the low-energy mill (LEM) and the high-energy mill (HEM).
H. Ehrhardt et al. / Journal of Alloys and Compounds 339 (2002) 255 – 260
level of the HEM environment of the SPEX mill compared with the LEM mill, ZnFe 2 O 4 milled for only 2 h was found to exhibit strain of |1%, remaining essentially unchanged around this strain level with increasing milling time to 12 h. The average grain sizes of the milled samples are depicted in Fig. 2 as functions of milling time t m for ZnFe 2 O 4 milled in both the LEM and HEM modes. Consistent with the energy levels of the two mills, an average particle size of d518(2) nm was obtained after milling for 120 h in the LEM compared with a reduced value of d58(1) nm after 12 h in the HEM. These latter results are similar to those obtained on milling ZnFe 2 O 4 for 20 h in a high-energy planetary ball mill (d|11 nm; Fritsch Pulverisette mill [8]). Fig. 3 shows the values of the mean inversion parameter c determined for the set of samples at different milling times. While the inversion parameter reaches a value of c|0.3 after milling for 120 h in the LEM mode, this value is found to already have reached c|0.4 after milling for only 2 h in the HEM mode, increasing steadily to c|0.75 with prolonged treatment. ¨ The room temperature Mossbauer spectra of the samples are shown in Fig. 4. In good agreement with published values the spectrum of the equilibrium ZnFe 2 O 4 starting material is well fitted with one doublet of isomer shift IS|0.35 mm s 21 and quadrupole splitting QS|0.38 mm s 21 [10,14]. As shown clearly by the LEM (Fig. 4a) and HEM spectra (Fig. 4b) of milled ZnFe 2 O 4 , the central resonance feature becomes increasingly broad with extended milling while retaining an overall doublet-like quality. This spectral broadening is likely to be associated with several effects induced by the milling. These include: increased fractions of disordered and interfacial regions in
Fig. 2. Average grain-sizes d as a function of milling time t m for the LEM samples (open symbols: upper scale) and the HEM samples (closed symbols: lower scale).
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Fig. 3. Mean inversion parameter c as a function of milling time t m for the LEM samples (open symbols: upper scale) and the HEM samples (closed symbols: lower scale).
nanostructured ZnFe 2 O 4 as the grain size decreases (Fig. 2); changes in tetrahedral and octahedral bond lengths and angles (e.g. Ref. [19]) as well as changes in the site distributions of Fe 31 and Zn 21 ions (Fig. 3, see also Refs. [14,24]) and the onset of magnetic interactions (e.g. Ref. [15]). As shown by Fig. 4, good fits were obtained for the spectra of all of the milled samples using two doublets (doublets M and N) and lorentzian lineshapes. The need to introduce this second doublet (doublet N) in order to model the spectra is also drawn out by Fig. 5 which reflects the spectra of ZnFe 2 O 4 milled for 120 h (Fig. 5a) and for 6 h (Fig. 5b) and their derivatives. The misfit to the milled spectrum using a single doublet is particularly evident in the differentiated data. In agreement with previous work [10], there is no evidence in either the X-ray ¨ diffraction patterns or the Mossbauer spectra for decomposition of ZnFe 2 O 4 to ZnO and Fe 2 O 3 on milling. Fig. 6 shows the set of IS (Fig. 6a) and QS (Fig. 6b) values obtained for doublet M and doublet N as a function of the average grain size. While the IS values for both doublets remain essentially constant for all samples, the QS values increase significantly for an average grain size below d|10 nm. Given that doublet M characterises the behaviour of the crystalline fraction present in nanostructured ZnFe 2 O 4 , the IS and QS values would be expected to remain essentially unchanged with decreasing particle size, albeit with decreasing subspectral area. The increases in QS values for both doublets are considered to indicate the onset of an additional source of broadening in these samples with d#10 nm. This increase in broadening with decrease in grain size is also shown by the increase in width of the resonance feature overall (Fig. 6c). In a recent neutron diffraction investigation of nanostructured
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¨ Fig. 4. Room temperature Mossbauer spectra of ZnFe 2 O 4 after milling in (a) the low-energy mode and (b) the high-energy mode for the milling times t m indicated. The fits to the spectra are discussed in the text.
¨ Fig. 5. Room temperature Mossbauer spectra of (a) the 120 h LEM sample and (b) the 6 h HEM sample fitted with both one doublet (broken line) and two doublets (full line). The lower plots show the differentiated spectral data and the derivatives of the fits, respectively.
H. Ehrhardt et al. / Journal of Alloys and Compounds 339 (2002) 255 – 260
¨ Fig. 6. The variation of Mossbauer hyperfine parameters with average particle sizes for milled ZnFe 2 O 4 as discussed in the text (open symbols: LEM; closed symbols: HEM) (a) IS values for doublet M (squares) and N (triangles); (b) QS values for doublet M (circles) and N (triangles); (c) full width of the resonance feature at half height; (d) fractional area AM of doublet M.
ZnFe 2 O 4 with d|9 nm [15], additional magnetic scattering was observed in the milled sample up to |450 K. The ¨ increase in broadening of the Mossbauer spectra for d#10 nm (Fig. 6c) is considered to indicate the presence of magnetic hyperfine splitting and / or relaxation effects at room temperature. In our spectral fits using two doublets, this overall increase in broadening causes the apparent increase in the QS values for both doublets at low d values (Fig. 6b). While a more complete spectral analysis would require the addition of a further subspectral component to ¨ represent magnetic effects, the resolution of the Mossbauer spectra does not allow individual contributions to the broadening to be determined. Indeed, in the case of high temperature measurements of mechanically activated ZnFe 2 O 4 , the spectra were fitted on the basis of distributions of quadrupole splittings [7]. Nonetheless the present analysis serves to highlight the systematic onset of magnetic hyperfine splitting with decreasing particle size in nanostructured ZnFe 2 O 4 . Fig. 6d shows the variation of the fractional area AM of subspectrum M with particle size. While spectral resolution limits detailed quantitative analysis, the data indicate a
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clear decrease in the spectral area of component M with decrease in particle size. This is consistent with the expected decrease in the spectral contribution associated with nanocrystalline ZnFe 2 O 4 . Correspondingly, the spectral contribution due to the surface area of nanograins and interfacial regions between the nanograins would be expected to increase with decrease in particle size (see Ref. ¨ [25] for discussion of the Mossbauer spectrum of nanocrystalline Fe). As noted above, the IS and QS values for doublet M (Fig. 4) show general agreement with the values for the equilibrium ZnFe 2 O 4 [IS50.348(1) mm s 21 ; QS50.343(1) mm s 21 ]. This latter doublet is identified with Fe 31 cations on octahedral occupation sites in equilibrium ZnFe 2 O 4 [23,26] and, in the present series of milled samples can therefore be identified with the nanograins which exhibit predominantly a normal spinel structure. As shown in Fig. 6a the IS values for doublet N remain essentially unchanged for all particle sizes. However, as noted above there are many factors, which are likely to contribute to this component (disordered and interfacial regions; changes in tetrahedral and octahedral bond lengths and angles; changes in site distributions; magnetic interactions) with the limited spectral resolution preventing separation of the various contributions and detailed quantitative analysis. As indicated above, the systematic changes which take place on extended LEM and HEM milling are reflected by the changes in d values as in Fig. 2. The systematic nature of the processes occurring on milling is also reflected by the monotonic increase in inversion parameter with decrease in particle size. While mechanically induced increases in inversion parameter are well known for milled ZnFe 2 O 4 (e.g. Refs. [3,12,16,27,28]), the present findings provide clear evidence of the systematic changes produced in nanostructured ZnFe 2 O 4 when subjected to extended milling. The mean inversion parameter increases from c|0 for d|70 nm to c|0.75 for d|8 nm. The latter value is similar to that obtained c|0.67 [12] and c|0.8 [5] for ZnFe 2 O 4 mechanically activated in a high-energy planetary mill. These changes indicate a systematic transformation from the normal spinel structure of equilibrium ZnFe 2 O 4 towards a random or inverse spinel-type structure on milling. Related systematic trends are also evident in ¨ the Mossbauer spectra (Fig. 4). It should be noted that even with an inversion parameter up to c|0.75, the Fe 31 ions remain predominantly on the octahedral sites, consistent with the association of doublet M with an Fe 31 ion octahedral environment as discussed above. Given this systematic redistribution of Zn 21 and Fe 31 ions induced by milling, it appears likely that the magnetic hyperfine splitting discerned in the samples for d#10 nm, is linked with the occurrence of interactions of the type (Fe 31 )–O 22 –[Fe 31 ] (e.g. Refs. [14,24]). Neutron diffraction measurements on the present samples will enable a more quantitative link between inversion parameter and the magnetic behaviour of nanostructured ZnFe 2 O 4 to be
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established [29]. In particular, interest is focused on the extent to which these well-characterised changes in occupancies of the octahedral and tetrahedral sites reflect the magnetic phases predicted for a spinel system as functions of the concentrations of the magnetic ions. Depending on the concentrations of the magnetic ions in these different sites, transitions from antiferromagnetism to ferrimagnetism via a spin glass region can be expected [30].
[7] [8]
[9] [10]
4. Conclusions ZnFe 2 O 4 has been milled in both low-energy modes (0–120 h) and high-energy modes (0–12 h) leading to nanostructured ZnFe 2 O 4 with mean particle sizes down to d|18 nm and d|8 nm, respectively. The inversion parameter is found to increase systematically with decreasing particle sizes in agreement with earlier works, reflecting a gradual transformation from the normal spinel of the starting material, to increased fraction of Fe 31 ions on the tetrahedral sites. Magnetic hyperfine splitting is observed at room temperature for nanoscale particles of d,|10 nm, consistent with the occurrence of (Fe 31 )–O 22 –[Fe 31 ] interactions in nanostructured ZnFe 2 O 4 .
[11] [12] [13]
[14] [15]
[16] [17] [18] [19] [20] [21]
Acknowledgements S.J.C. acknowledges renewal of an Alexander von Humboldt Research Fellowship while at the Johannes ¨ in Mainz, Germany. We thank Dr. Gutenberg Universitat W.A. Kaczmarek for helpful discussions on milling techniques.
[22] [23] [24] [25] [26]
References [1] P.I. Slick, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, Vol. 2, North Holland, 1980, Chapter 3. [2] S.K. Gangwal, J.M. Stogner, S.M. Harkins, S.J. Bossart, Environ. Prog. 8 (1989) 26. ˇ ´ K. Jancke, J. Richter-Mendau, U. Steinike, D.-Chr. [3] V. Sepelak, Uecker, A.Yu. Rogachev, Kona 12 (1994) 87. [4] M. Ahmed, L. Alonso, J.M. Palacios, C. Cilleruelo, J.C. Abanades, Solid State Ionics 138 (2000) 51. [5] H.H. Hamdeh, J.C. Ho, S.A. Oliver, R.J. Willey, G. Oliveri, G. Busca, J. Appl. Phys. 81 (1997) 1851. ¨ [6] R.E. Vandenberghe, E. De Grave, Mossbauer effect studies of oxidic
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¨ spinels, in: G.J. Long, F. Grandjean (Eds.), Mossbauer Spectroscopy—Applied to Inorganic Chemistry, Vol. 3, Plenum Press, New York, 1989, Chapter 3. ˇ ´ S. Wissmann, K.D. Becker, J. Mater. Sci. 33 (1998) V. Sepelak, 2845. C.N. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chat´ topadhyay, H. Guerault, J.-M. Greneche, J. Phys. Condens. Matter. 12 (2000) 7795. G.F. Goya, H.R. Rechenberg, J. Magn. Magn. Mater. 196–197 (1999) 191. J.Z. Jiang, P. Wynn, S. Morup, T. Okada, F.J. Berry, NanoStruct. Mater. 12 (1999) 737. T. Sato, K. Haneda, M. Seki, T. Iijima, Appl. Phys. A 50 (1990) 13. ˇ ´ K. Tkacova, ´ ˇ ´ A.I. Rykov, Cryst. Res. Technol. 28 (1993) V. Sepelak, 53. M.R. Anantharaman, S. Jagatheesan, K.A. Malini, S. Sindhu, A. Narayanasamy, C.N. Chinnasamy, J.P. Jacobs, S. Reijne, K. Seshan, R.H.H. Smits, H.H. Brongersma, J. Magn. Magn. Mater. 189 (1998) 83. C.N. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chattopadhyay, Mater. Sci. Eng. A 304–306 (2001) 983. ¨ W. Schafer, W. Kockelmann, A. Kirfel, W. Potzel, F.J. Burghart, G.M. Kalvius, A. Martin, W.A. Kaczmarek, S.J. Campbell, Mater. Sci. Forum 321–324 (2000) 802. ˇ ´ U. Steinike, D.Chr. Uecker, S. Wibmann, K.D. Becker, V. Sepelak, J. Solid State Chem. 135 (1998) 52. A. Calka, A. Radlinski, Mater. Sci. Eng. A 134 (1991) 1350. A. Calka, J.I. Nikolov, NanoStruct. Mater. 6 (1995) 409. D. Basset, P. Matteazzi, F. Miani, Mater. Sci. Eng. A 174 (1994) 71. T.H. Courtney, D. Maurice, Scripta Mater. 34 (1996) 5. H.P. Klug, L.E. Alexander, in: X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd Edition, Wiley, 1974, Chapter 9. B.D. Cullity, Elements of X-Ray Diffraction, 2nd Edition, AddisonWesley, London, 1974. W. Schiessl, W. Potzel, H. Karzel, M. Steiner, G.M. Kalvius, Phys. Rev. B 53 (1996) 9143. ˇ ´ J. Solid State Chem. 146 (1999) P. Druska, U. Steinike, V. Sepelak, 13. U. Herr, J. Jing, R. Birringer, U. Gonser, H. Gleiter, Appl. Phys. Lett. 50 (1987) 472. W. Potzel, G.M. Kalvius, W. Schiessl, H. Karzel, M. Steiner, A. Kratzer, A. Martin, M.K. Krause, A. Schneider, I. Halevy, J. Gal, W. ¨ ¨ Schafer, G. Will, M. Hillberg, R. Wappling, D.W. Mitchell, T.P. Das, Hyperfine Interact. 97 (1996) 373. F.J. Burghart, W. Potzel, G.M. Kalvius, E. Schreiner, G. Grosse, ¨ D.R. Noakes, W. Schafer, W. Kockelmann, S.J. Campbell, W.A. Kaczmarek, A. Martin, M.K. Krause, Physica B 289–290 (2000) 286. ˇ ˇ ´ ˇ ´ V. Sepelak, ´ N. Stevulova, ´ V.V. Boldyrev, J. Solid State K. Tkacova, Chem. 123 (1996) 100. M. Hofmann, S.J. Campbell, H. Ehrhardt, J. Magn. Magn. Mater (2002) submitted. Ch.P. Poole Jr., H.A. Farach, Z. Phys. B Condens. Matter. 47 (1982) 55.
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Structural evolution of ball-milled ZnFe 2 O 4 a , ,1 b c ,2 H. Ehrhardt * , S.J. Campbell , M. Hofmann a
¨ Nanotechnologie, Forschungszentrum Karlsruhe GmbH, Postfach 3640, D-76021 Karlsruhe, Germany Institut f ur b School of Physics, University College, University of New South Wales, ADFA, Canberra, ACT 2600, Australia c ISIS, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0 QX, UK Received 20 August 2001; accepted 28 November 2001
Abstract Nanostructured zinc ferrite produced by milling in both low-energy and high-energy ball mills has been investigated by X-ray ¨ diffraction and Mossbauer effect spectroscopy. The lattice parameter of the milled products remains essentially unchanged from that of equilibrium ZnFe 2 O 4 with the steady-state average particle size found to decrease to d518(2) nm on low energy milling compared with ¨ d58(1) nm on high energy milling. The room temperature Mossbauer spectra of the milled materials have been analysed using two doublets, one of which is considered to be associated primarily with the octahedral lattice sites. Spectral broadening is observed with decreasing particle size, particularly below d|10 nm, for which the effects of magnetic hyperfine splitting become evident. The mean inversion parameter of nanostructured ZnFe 2 O 4 is found to increase to c|0.75 for particle sizes of d|8 nm reflecting the systematic evolution of zinc ferrite from its normal spinel structure towards an inverse spinel structure on mechanical treatment as observed ¨ previously. The other factors which contribute to the Mossbauer spectra of nanostructured ZnFe 2 O 4 (d|8–70 nm) are discussed. 2002 Elsevier Science B.V. All rights reserved. ¨ effect; Nanostructured ZnFe 2 O 4 Keywords: Mechanical milling; X-Ray diffraction; Mossbauer
1. Introduction Spinel ferrites are used in a wide range of technological applications [1]. This stems mainly from their useful catalytic properties as well as their interesting magnetic characteristics. Zinc ferrite is of particular interest as a result of the important role that it plays as an absorbent in the hot gas desulphurisation process [2–4] and its use in providing scope for mixed ferrites of purpose designed ˇ ´ et al. [3] magnetic parameters [5]. For example, Sepelak investigated the effect of mechanical activation on the reactivity of ZnFe 2 O 4 in the reaction with H 2 S, with Ahmed et al. [4] having recently explored the structural
*Corresponding author. Tel.: 161-2-6268-8011; fax: 161-2-62688786. E-mail address: [email protected] (H. Ehrhardt). 1 Present address: School of Physics, University College, University of New South Wales, ADFA, Canberra, ACT 2600, Australia. 2 Former address: BENSC, Hahn-Meitner-Institut, D-14109 Berlin, Germany.
changes occurring in ZnFe 2 O 4 during calcination at high temperature or regeneration in oxidant atmosphere. The spinel crystal structure is an approximately closepacked face-centered cubic array of anions with holes partly filled by the cations. The oxide spinel can generally be described by the formula (A)[B 2 ]O 4 where A and B denote divalent and trivalent cations, respectively. In the case of a normal spinel structure, all of the A atoms are tetrahedrally coordinated while the B atoms are octahedrally coordinated by oxygen atoms. In the case of an inverse spinel structure, the A atoms occupy half of the B sites. However, as is well known, spinels are often found with other than purely normal or purely inverse distributions of cations [6]. The site occupation can be represented by (A 12c B c )[A c B 22c ]O 4 where c is the inversion parameter (0#c#1) with tetrahedral sites denoted by round brackets and octahedral sites by square brackets. ˇ ´ et al. [7], spinel ferrites provide As outlined by Sepelak suitable model systems for investigation of effects induced by mechanochemical activation. ZnFe 2 O 4 is of particular interest with changes in magnetic behaviour having been reported by several workers for nanoscale particles prepared by milling treatments or other means (e.g. Ref. [5,8–14]). Equilibrium ZnFe 2 O 4 is antiferromagnetic with
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )02011-4
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H. Ehrhardt et al. / Journal of Alloys and Compounds 339 (2002) 255 – 260
´ temperature of T N |10.5 K (e.g. Ref. [15]) whereas a Neel magnetic behaviours characteristic of ferrimagnetic-like or ferromagnetic-like interactions with ordering temperatures above room temperature have been reported for nanoscale ¨ ZnFe 2 O 4 material (e.g. Ref. [5,8–15]). Mossbauer spectroscopy has been highly effective in providing insight to the microscopic changes that occur both in ZnFe 2 O 4 subjected to milling (e.g. Refs. [7,10,14]) and in ZnFe 2 O 4 produced by mechanochemical synthesis (e.g. Refs. [9,16]). Given that most studies have so far been undertaken using high-energy mills, we have carried out a systematic study of ZnFe 2 O 4 milled in both low-energy mode (LEM) in air and high-energy mode (HEM) in argon. The use of a low-energy mill designed for smallscale research purposes [17,18] rather than industrial-scale applications, is especially useful in the present investigation in helping to ensure that all processes which take place on milling ZnFe 2 O 4 can be followed. Our investigation of the microstructures of these two sets of ballmilled ZnFe 2 O 4 materials by X-ray diffraction and ¨ Mossbauer spectroscopy has enabled the lattice parameter and the inversion parameter for nanostructured ZnFe 2 O 4 particles of average grain sizes from d|27(3) nm to d| ¨ 8(1) nm to be determined. The changes in Mossbauer hyperfine parameters for these nanostructured samples have also been characterised systematically.
ments using FULLPROF [22]. The room temperature ¨ transmission Mossbauer experiments were carried out using a conventional microcomputer-controlled spectrometer in constant acceleration mode and a 57 CoRh ] source. The velocity scale was calibrated relative to natural a-Fe.
3. Results and discussion
2. Experimental
Examples of the set of X-ray diffraction patterns obtained for all of the milled samples are shown in Fig. 1: coarse-grained material (Fig. 1a); 120 h milled LEM sample (Fig. 1b) and the 12 h milled HEM sample (Fig. 1c). All samples were found to display the reflections of cubic spinels, essentially matching these of ZnFe 2 O 4 (Franklinite, JCPDS 22-1012) with the lattice parameter ˚ This for the starting material found to be a58.449(2) A. value is slightly larger (|0.1%) than the JCPDS value of ˚ but comparable to values of a58.450 A ˚ [4] a 0 58.4411 A ˚ [23] reported recently. The LEM samples and a58.4599 A ˚ were found to have the same value of a58.451(4) A (independent of milling time) compared with the slightly ˚ for the HEM samples. This larger value of a58.462(5) A increase in lattice parameter (|0.1%) may be associated with the dissolution of Ar in the samples during milling in the HEM mode. The inhomogeneous strain was found to increase slightly from |0.4% after LEM milling for 40 h to 0.8% on milling for 120 h. Reflecting the higher energy
Commercial coarse-grained powders of ZnFe 2 O 4 (Alfa Aesar) were milled in both a low-energy magnetic mill (LEM) [17,18] and a high-energy SPEX 8000 shaker mill (HEM) [19,20]. The LEM samples (initial charge |10 g) were wet milled with 5 ml water in air in a sealed stainless steel mill with four |22 mm steel balls with a ball-topowder weight ratio of |20:1. Small amounts of powder (|2–3 g) were extracted from the vial after different milling times (40, 60, 80, 120 h). For the HEM samples, 5 g charges were sealed in a steel vial under an Ar atmosphere with two 12.7 mm diameter and four 6.4 mm steel balls, yielding a ball-to-powder weight ratio of 3.5:1. The samples were then milled for different times (2, 4, 6, 12 h). There was no evidence of contamination from the milling media as monitored by energy-dispersive X-ray spectroscopy (EDX). The samples were characterised by X-ray diffraction using a Siemens Rigaku u –2u diffractometer equipped with a Cu tube. The (volume-weighted) average grain sizes were determined from Scherrer’s formula for peak broadening, particularly from the full width at half maximum (FWHM) of the (311) diffraction line. The inhomogeneous strain was estimated from the (220) and (440) reflections assuming a Gaussian strain broadening [21]. Lattice parameter and inversion parameter were determined in the standard way based on Rietveld refine-
Fig. 1. X-Ray diffraction patterns of ZnFe 2 O 4 after different milling times t m in the low-energy mill (LEM) and the high-energy mill (HEM).
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level of the HEM environment of the SPEX mill compared with the LEM mill, ZnFe 2 O 4 milled for only 2 h was found to exhibit strain of |1%, remaining essentially unchanged around this strain level with increasing milling time to 12 h. The average grain sizes of the milled samples are depicted in Fig. 2 as functions of milling time t m for ZnFe 2 O 4 milled in both the LEM and HEM modes. Consistent with the energy levels of the two mills, an average particle size of d518(2) nm was obtained after milling for 120 h in the LEM compared with a reduced value of d58(1) nm after 12 h in the HEM. These latter results are similar to those obtained on milling ZnFe 2 O 4 for 20 h in a high-energy planetary ball mill (d|11 nm; Fritsch Pulverisette mill [8]). Fig. 3 shows the values of the mean inversion parameter c determined for the set of samples at different milling times. While the inversion parameter reaches a value of c|0.3 after milling for 120 h in the LEM mode, this value is found to already have reached c|0.4 after milling for only 2 h in the HEM mode, increasing steadily to c|0.75 with prolonged treatment. ¨ The room temperature Mossbauer spectra of the samples are shown in Fig. 4. In good agreement with published values the spectrum of the equilibrium ZnFe 2 O 4 starting material is well fitted with one doublet of isomer shift IS|0.35 mm s 21 and quadrupole splitting QS|0.38 mm s 21 [10,14]. As shown clearly by the LEM (Fig. 4a) and HEM spectra (Fig. 4b) of milled ZnFe 2 O 4 , the central resonance feature becomes increasingly broad with extended milling while retaining an overall doublet-like quality. This spectral broadening is likely to be associated with several effects induced by the milling. These include: increased fractions of disordered and interfacial regions in
Fig. 2. Average grain-sizes d as a function of milling time t m for the LEM samples (open symbols: upper scale) and the HEM samples (closed symbols: lower scale).
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Fig. 3. Mean inversion parameter c as a function of milling time t m for the LEM samples (open symbols: upper scale) and the HEM samples (closed symbols: lower scale).
nanostructured ZnFe 2 O 4 as the grain size decreases (Fig. 2); changes in tetrahedral and octahedral bond lengths and angles (e.g. Ref. [19]) as well as changes in the site distributions of Fe 31 and Zn 21 ions (Fig. 3, see also Refs. [14,24]) and the onset of magnetic interactions (e.g. Ref. [15]). As shown by Fig. 4, good fits were obtained for the spectra of all of the milled samples using two doublets (doublets M and N) and lorentzian lineshapes. The need to introduce this second doublet (doublet N) in order to model the spectra is also drawn out by Fig. 5 which reflects the spectra of ZnFe 2 O 4 milled for 120 h (Fig. 5a) and for 6 h (Fig. 5b) and their derivatives. The misfit to the milled spectrum using a single doublet is particularly evident in the differentiated data. In agreement with previous work [10], there is no evidence in either the X-ray ¨ diffraction patterns or the Mossbauer spectra for decomposition of ZnFe 2 O 4 to ZnO and Fe 2 O 3 on milling. Fig. 6 shows the set of IS (Fig. 6a) and QS (Fig. 6b) values obtained for doublet M and doublet N as a function of the average grain size. While the IS values for both doublets remain essentially constant for all samples, the QS values increase significantly for an average grain size below d|10 nm. Given that doublet M characterises the behaviour of the crystalline fraction present in nanostructured ZnFe 2 O 4 , the IS and QS values would be expected to remain essentially unchanged with decreasing particle size, albeit with decreasing subspectral area. The increases in QS values for both doublets are considered to indicate the onset of an additional source of broadening in these samples with d#10 nm. This increase in broadening with decrease in grain size is also shown by the increase in width of the resonance feature overall (Fig. 6c). In a recent neutron diffraction investigation of nanostructured
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¨ Fig. 4. Room temperature Mossbauer spectra of ZnFe 2 O 4 after milling in (a) the low-energy mode and (b) the high-energy mode for the milling times t m indicated. The fits to the spectra are discussed in the text.
¨ Fig. 5. Room temperature Mossbauer spectra of (a) the 120 h LEM sample and (b) the 6 h HEM sample fitted with both one doublet (broken line) and two doublets (full line). The lower plots show the differentiated spectral data and the derivatives of the fits, respectively.
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¨ Fig. 6. The variation of Mossbauer hyperfine parameters with average particle sizes for milled ZnFe 2 O 4 as discussed in the text (open symbols: LEM; closed symbols: HEM) (a) IS values for doublet M (squares) and N (triangles); (b) QS values for doublet M (circles) and N (triangles); (c) full width of the resonance feature at half height; (d) fractional area AM of doublet M.
ZnFe 2 O 4 with d|9 nm [15], additional magnetic scattering was observed in the milled sample up to |450 K. The ¨ increase in broadening of the Mossbauer spectra for d#10 nm (Fig. 6c) is considered to indicate the presence of magnetic hyperfine splitting and / or relaxation effects at room temperature. In our spectral fits using two doublets, this overall increase in broadening causes the apparent increase in the QS values for both doublets at low d values (Fig. 6b). While a more complete spectral analysis would require the addition of a further subspectral component to ¨ represent magnetic effects, the resolution of the Mossbauer spectra does not allow individual contributions to the broadening to be determined. Indeed, in the case of high temperature measurements of mechanically activated ZnFe 2 O 4 , the spectra were fitted on the basis of distributions of quadrupole splittings [7]. Nonetheless the present analysis serves to highlight the systematic onset of magnetic hyperfine splitting with decreasing particle size in nanostructured ZnFe 2 O 4 . Fig. 6d shows the variation of the fractional area AM of subspectrum M with particle size. While spectral resolution limits detailed quantitative analysis, the data indicate a
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clear decrease in the spectral area of component M with decrease in particle size. This is consistent with the expected decrease in the spectral contribution associated with nanocrystalline ZnFe 2 O 4 . Correspondingly, the spectral contribution due to the surface area of nanograins and interfacial regions between the nanograins would be expected to increase with decrease in particle size (see Ref. ¨ [25] for discussion of the Mossbauer spectrum of nanocrystalline Fe). As noted above, the IS and QS values for doublet M (Fig. 4) show general agreement with the values for the equilibrium ZnFe 2 O 4 [IS50.348(1) mm s 21 ; QS50.343(1) mm s 21 ]. This latter doublet is identified with Fe 31 cations on octahedral occupation sites in equilibrium ZnFe 2 O 4 [23,26] and, in the present series of milled samples can therefore be identified with the nanograins which exhibit predominantly a normal spinel structure. As shown in Fig. 6a the IS values for doublet N remain essentially unchanged for all particle sizes. However, as noted above there are many factors, which are likely to contribute to this component (disordered and interfacial regions; changes in tetrahedral and octahedral bond lengths and angles; changes in site distributions; magnetic interactions) with the limited spectral resolution preventing separation of the various contributions and detailed quantitative analysis. As indicated above, the systematic changes which take place on extended LEM and HEM milling are reflected by the changes in d values as in Fig. 2. The systematic nature of the processes occurring on milling is also reflected by the monotonic increase in inversion parameter with decrease in particle size. While mechanically induced increases in inversion parameter are well known for milled ZnFe 2 O 4 (e.g. Refs. [3,12,16,27,28]), the present findings provide clear evidence of the systematic changes produced in nanostructured ZnFe 2 O 4 when subjected to extended milling. The mean inversion parameter increases from c|0 for d|70 nm to c|0.75 for d|8 nm. The latter value is similar to that obtained c|0.67 [12] and c|0.8 [5] for ZnFe 2 O 4 mechanically activated in a high-energy planetary mill. These changes indicate a systematic transformation from the normal spinel structure of equilibrium ZnFe 2 O 4 towards a random or inverse spinel-type structure on milling. Related systematic trends are also evident in ¨ the Mossbauer spectra (Fig. 4). It should be noted that even with an inversion parameter up to c|0.75, the Fe 31 ions remain predominantly on the octahedral sites, consistent with the association of doublet M with an Fe 31 ion octahedral environment as discussed above. Given this systematic redistribution of Zn 21 and Fe 31 ions induced by milling, it appears likely that the magnetic hyperfine splitting discerned in the samples for d#10 nm, is linked with the occurrence of interactions of the type (Fe 31 )–O 22 –[Fe 31 ] (e.g. Refs. [14,24]). Neutron diffraction measurements on the present samples will enable a more quantitative link between inversion parameter and the magnetic behaviour of nanostructured ZnFe 2 O 4 to be
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established [29]. In particular, interest is focused on the extent to which these well-characterised changes in occupancies of the octahedral and tetrahedral sites reflect the magnetic phases predicted for a spinel system as functions of the concentrations of the magnetic ions. Depending on the concentrations of the magnetic ions in these different sites, transitions from antiferromagnetism to ferrimagnetism via a spin glass region can be expected [30].
[7] [8]
[9] [10]
4. Conclusions ZnFe 2 O 4 has been milled in both low-energy modes (0–120 h) and high-energy modes (0–12 h) leading to nanostructured ZnFe 2 O 4 with mean particle sizes down to d|18 nm and d|8 nm, respectively. The inversion parameter is found to increase systematically with decreasing particle sizes in agreement with earlier works, reflecting a gradual transformation from the normal spinel of the starting material, to increased fraction of Fe 31 ions on the tetrahedral sites. Magnetic hyperfine splitting is observed at room temperature for nanoscale particles of d,|10 nm, consistent with the occurrence of (Fe 31 )–O 22 –[Fe 31 ] interactions in nanostructured ZnFe 2 O 4 .
[11] [12] [13]
[14] [15]
[16] [17] [18] [19] [20] [21]
Acknowledgements S.J.C. acknowledges renewal of an Alexander von Humboldt Research Fellowship while at the Johannes ¨ in Mainz, Germany. We thank Dr. Gutenberg Universitat W.A. Kaczmarek for helpful discussions on milling techniques.
[22] [23] [24] [25] [26]
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