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Mechanically induced cation redistribution in magnesium ferrite and its thermal stability
Solid State Ionics 141–142 Ž2001. 677–682 www.elsevier.comrlocaterssi
Mechanically induced cation redistribution in magnesium ferrite and its thermal stability ˇ V. Sepelak ´ a,) , D. Schultze b, F. Krumeich c , U. Steinike d, K.D. Becker e a
e
Institute of Geotechnics, SloÕak Academy of Sciences, WatsonoÕa 45, SK-04353 Kosice, ˇ SloÕak Republic b Federal Institute for Materials Research and Testing, D-12489 Berlin, Germany c Swiss Federal Institute of Technology Zurich, CH-8093 Zurich, Switzerland ¨ d Institute of Applied Chemistry, D-12484 Berlin, Germany Institute of Physical and Theoretical Chemistry, Technical UniÕersity Braunschweig, D-38106 Brunswick, Germany
Abstract The changes in magnesium ferrite ŽMgFe 2 O4 . caused by high-energy milling are investigated. Mechanical treatment reduces the average crystallite size of MgFe 2 O4 to the nanometer range and induces cation redistribution between tetrahedral and octahedral sites. The degree of inversion of the mechanically treated ferrite is compared with that of MgFe 2 O4 quenched from high temperatures. The range of thermal stability of mechanically induced metastable states is determined by studying the response of the metastable mechanically activated MgFe 2 O4 to changes in temperature. q 2001 Elsevier Science B.V. All rights reserved. MAT: MgFe 2 O4 Keywords: Mechanochemistry; Mechanical activation; High-energy milling; Nanocrystalline metastable material; Spinel ferrite; Thermal relaxation
1. Introduction Magnesium ferrite, MgFe 2 O4 , is a soft magnetic n-type semiconducting material w1x, which finds a number of applications in heterogeneous catalysis, adsorption, sensors, and in magnetic technologies. The structural formula of magnesium ferrite is usually written as ŽMg 1y x Fe x .wMg x Fe 2yx xO4 , where round and square brackets denote cation sites of
) Corresponding author. Tel.: q421-95-6323402; fax: q42195-6323402. ˇ E-mail address: [email protected] ŽV. Sepelak ´ ..
tetrahedral ŽA. and octahedral wBx coordination, respectively. x represents the so-called degree of inversion Ždefined as the fraction of the ŽA. sites occupied by Fe 3q cations.. The most remarkable effect in spinel ferrites is the strong dependence of properties on the state of chemical order, and in particular, on the cation distribution. It has been found that high-energy milling reduces the average crystallite size of the spinel ferrites to the nanometer range Ž; 10 nm. and induces cation redistribution between ŽA. and wBx sites w2–15x. Mechanical treatment of the spinel ferrites was found to be a useful activation method leading to an enhanced reactivity of nanoscale powders w16,17x. Spin canting, defined as a lack of full align-
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 7 7 7 - 9
678
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
ment of the spins in an applied magnetic field, has been proposed for the nano-sized, mechanically treated spinel ferrites w18–20x. Since the mechanically treated spinel ferrites are metastable with respect to structural and compositional changes under temperature, pressure, and environmental conditions in which they are utilised, there is great concern for understanding and controlling their limitations. This experimental work focuses on the study of both the mechanically induced disorder in MgFe 2 O4 and the structural response of this metastable solid to changes in temperature.
2. Experimental Polycrystalline MgFe 2 O4 was prepared by the conventional ceramic method Žfurther referred to as the non-activated sample.. The non-activated sample Ž2 g. was ground for various times in a planetary ball mill EI 2 = 150 Žproduct of the Institute of Solid State Chemistry, Novosibirsk. at room temperature. The ceramic-covered grinding chamber Ž150 cm3 in volume. and balls made of a-Al 2 O 3 with diameter of 3–5 mm were used. The ball-to-powder weight ratio was 50:1. Grinding experiments were performed in air at 750 rpm. Mossbauer measurements were carried out at 6.4 ¨ K under an external magnetic field of 5.5 T applied perpendicular to the g-ray direction. A 57CorRh g-ray source was used. The velocity scale was calibrated relative to 57 Fe in Rh. ‘Recoil’ spectral analysis software w21x was used for the quantitative evaluation of the Mossbauer spectra. ¨ The morphology of the powder and the size of individual crystallites were studied by means of transmission electron microscopy ŽTEM. ŽModel CM30ST, Philips, Eindhoven.. X-ray diffraction ŽXRD. patterns were collected using a URD 6 powder diffractometer ŽSeifert-FPM, Freiberg. with FeK a radiation. An XRK-A Paar camera ŽA Paar, Graz. was employed for in situ XRD in the temperature range between 300 K and 1100 K. Samples were heated in air at a heating rate of 2 K miny1 . Thermogravimetric ŽTG. measurements were performed using a Setaram Tag 24 thermobalance. Sam-
ples Ž130 mg. were thermally treated in open platinum crucibles in air. A permanent magnet was stacked on top of the horizontal furnace assembly of the TG analyser. The apparent weight of the sample in the magnetic field of the permanent magnet was recorded as a function of temperature. When the sample passed through the Neel ´ temperature ŽTN ., its apparent weight increased rapidly due to loss of spontaneous magnetisation. The Neel ´ temperature was arbitrarily defined by the intersection of the maximum slope of the thermogram with the upper base line, which corresponded to the true sample weight. Reproducibility of the measured Neel ´ temperature was "0.5 K. The sample was annealed at four consecutive temperatures Ž1130 K, 1180 K, 1230 K, 1280 K. without removing it from the TG analyser. The sample was held at the given annealing temperature for 2 h, cooled to 470 K at a cooling rate of 10 K miny1 , and then heated to the further annealing temperature. The magnetic disorder–order transition of the sample in the magnetic field of the permanent magnet was monitored during the cooling stage.
3. Results and discussion The mechanically induced evolution of MgFe 2 O4 submitted to the high-energy milling process was followed by 57 Fe Mossbauer spectroscopy. The low¨ temperature Mossbauer spectra taken in an applied ¨ magnetic field ŽFig. 1. clearly indicate that the mechanical treatment of MgFe 2 O4 is accompanied by the decrease of the concentration of iron cations on ŽA. sites. The degree of inversion, calculated from the subspectral area ratio, decreases with increasing milling time from x s 0.904Ž1. Žfor the non-activated sample. to x s 0.756Ž1. for MgFe 2 O4 mechanically activated for 30 min. The values of the average hyperfine magnetic fields Ž HA s 50.97Ž8. T, H B s 53.54Ž2. T., the isomer shifts Ž ISA s 0.24Ž8. mm sy1 , ISB s 0.35Ž3. mm sy1 ., and the degree of inversion for the non-activated sample are in agreement with previously published data w22,23x. Dark field TEM image of the milled MgFe 2 O4 ŽFig. 2. shows bright contrasts indicating many small crystals. Their average size was estimated Žfrom TEM. to be about 10 nm.
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
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Fig. 1. Mossbauer spectrum of the non-activated MgFe 2 O4 Ža. and of the mechanically activated MgFe 2 O4 Žb.. Spectra taken at 6.4 K in an ¨ external magnetic field of 5.5 T applied perpendicular to the g-ray direction.
By far, the most extensive contributions to our present knowledge on cation redistribution in unpro-
Fig. 2. Dark field image showing the nanoscale structure of mechanically activated MgFe 2 O4 .
cessed spinels stem from measurements performed at room temperature on samples quenched from the high temperatures of interest. Even though we are aware that it is not feasible to formally transfer the processes establishing the equilibrium distribution of cations, taking place during ‘thermal activation’, to the case of mechanical activation, we compare the value of the cation inversion parameter of mechanically treated MgFe 2 O4 with that of the quenched samples. A very detailed study of the equilibrium cation distribution in MgFe 2 O4 has been undertaken by O’Neill et al. w23x on quenched samples using XRD and Mossbauer spectroscopy. According to ¨ these authors, the cation distribution changes smoothly with quenching temperature. The inversion parameter decreases from 0.91 at 723 K to 0.72 at 1373 K. According to these data, the degree of
680
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
in the same sense. In the case of mechanical activation, this change of the cation distribution causes the
Fig. 3. Comparison of the cation distribution in quenched MgFe 2 O4 Žsquares. with that in both non-activated and mechanically activated MgFe 2 O4 Žcircles..
inversion of the non-activated MgFe 2 O4 Ž x s 0.904Ž1.. corresponds to a sample quenched from about 740 K. The value of x s 0.756Ž1., characterising the degree of disorder in the milled MgFe 2 O4 , corresponds to samples quenched from about 1170 K ŽFig. 3.. Thus, both the milling time and the temperature lead to the alteration of the cation distribution
Fig. 4. X-ray diffraction patterns of the mechanically activated MgFe 2 O4 taken during the thermal treatment in the XRK-A Paar camera. Diffraction lines of MgFe 2 O4 are denoted by Miller indices. The sample was measured on a platinum support.
Fig. 5. Thermogravimetric curves Žtop. and the derivative thermogravimetric curves Žbottom. for the mechanically activated MgFe 2 O4 after consecutive annealing at 1130 K Ža., 1180 K Žb., 1230 K Žc., and 1280 K Žd..
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
transition of the system into a metastable activated state with a higher amount of the accumulated energy. In situ high-temperature XRD analysis revealed that from the 300 K to 600 K range, the shape of the diffraction patterns of mechanically treated MgFe 2 O4 remains the same, i.e. the range of the thermal stability of the mechanically induced defects in the structure of milled MgFe 2 O4 extends up to 600 K. However, at temperatures over 600 K, a gradual recrystallisation of the nanoscale MgFe 2 O4 powders takes place. This is manifested by a gradual narrowing of diffraction lines and by an increase of their intensities, as shown in Fig. 4. Redistribution of the integral intensities of diffraction lines provides evidence that the crystallite growth of the milled powders is accompanied by a change of the cation distribution. Thermogravimetric measurements of the high-energy milled MgFe 2 O4 were undertaken in order to detect how the mechanically induced cation distribution relaxes towards its equilibrium configuration. As shown in Fig. 5, the structural response of the mechanically activated MgFe 2 O4 to changes in temperature is accompanied by an increase in the Neel ´ temperature Žfrom TN s 632 K to TN s 648 K.. The Neel ´ temperature provides a readily observed, highly sensitive measure of cation distribution in MgFe 2 O4 w24x. According to O’Neill et al.
Fig. 6. Neel ´ temperature, TN , vs. degree of inversion, x, for the mechanically activated MgFe 2 O4 after annealing at four different temperatures in the TG analyser Žcircles.. The data of O’Neill et al. w23x are indicated by the solid line.
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w23x the Neel ´ temperature of MgFe 2 O4 increases linearly with increasing degree of inversion Žsee Fig. 6.. According to these data, the values of TN s 632 K, TN s 636 K, TN s 643 K, and T N s 648 K, characterising the magnetic disorder–order transition in the milled sample after annealing at four different temperatures ŽFig. 5., correspond to samples with the degree of inversion of x s 0.870Ž8., x s 0.879Ž1., x s 0.893Ž4., and x s 0.903Ž7., respectively ŽFig. 6.. This indicates that the cation distribution in MgFe 2 O4 resulting from the mechanical treatment is reversible, i.e. during the annealing process, the mechanically induced cation distribution Ž x s 0.756Ž1.. relaxes towards its equilibrium configuration Ž x s 0.903Ž7... The Neel ´ temperature of the mechanically activated MgFe 2 O4 , after annealing at 1280 K, is close to the non-activated one ŽTN s 648 K.. Thus, on heating, the milled MgFe 2 O4 has returned to a state that is similar to the initial one, and its properties are gradually restored.
4. Conclusions Mechanical treatment of MgFe 2 O4 leads to the formation of the nanoscale structure with the crystallite size of about 10 nm. The metastable nanostructural state of the milled MgFe 2 O4 is characterised by a reduced concentration of iron cations on ŽA. sites. The degree of inversion decreases with increasing milling time from 0.904Ž1., characteristic of the non-activated sample, to 0.756Ž1. for activated ferrite. The degree of inversion of the activated MgFe 2 O4 corresponds to a sample quenched from about 1170 K. The range of the thermal stability of the mechanically induced defects in the structure of milled MgFe 2 O4 extends up to 600 K. At temperatures over 600 K, a gradual recrystallisation of the nanoscale MgFe 2 O4 powders takes place, and the mechanically induced cation distribution relaxes toward its equilibrium configuration Ž x s 0.903Ž7... The latter is sensitively monitored by the Neel ´ temperature measurements. The Neel ´ temperature of the mechanically activated MgFe 2 O4 after annealing at 1280 K is close to the non-activated one ŽTN s 648 K..
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Acknowledgements This work was supported by the Alexander von Humboldt Foundation and Slovak Grant Agency for Science ŽGrant 2r7040r20.. References w1x R.J. Willey, P. Noirclerc, G. Busca, Chem. Eng. Commun. 123 Ž1993. 1. w2x V.V. Boldyrev, Reactivity of Solids: Past, Present and Future, Blackwell, Oxford, 1996. ˇ w3x V. Sepelak, ´ K. Tkacova, ´ˇ ´ V.V. Boldyrev, S. Wißmann, K.D. Becker, Physica B 234–236 Ž1997. 617. w4x T.M. Clark, B.J. Evans, IEEE Trans. Magn. 33 Ž1997. 3745. w5x H.H. Hamdeh, J.C. Ho, S.A. Oliver, R.J. Willey, G. Oliver, G. Busca, J. Appl. Phys. 81 Ž1997. 1851. ˇ w6x V. Sepelak, ´ U. Steinike, D.Chr. Uecker, S. Wißmann, K.D. Becker, J. Solid State Chem. 135 Ž1998. 52. w7x M. Millot, S. Begin-Colin, P. Perriat, G. Le Caer, ¨ J. Solid State Chem. 139 Ž1998. 66. w8x D.J. Fatemi, V.G. Harris, V.M. Browning, J.P. Kirkland, J. Appl. Phys. 83 Ž1998. 6867. w9x D. Arcos, N. Rangavittal, M. Vazquez, M. Vallet-Regi, Mater. Sci. Forum 269–272 Ž1998. 87. ˇ w10x P. Druska, U. Steinike, V. Sepelak, ´ J. Solid State Chem. 146 Ž1999. 13.
w11x J.Z. Jiang, G.F. Goya, H.R. Rechenberg, J. Phys.: Condens. Matter 11 Ž1999. 4063. w12x D.J. Fatemi, V.G. Harris, M.X. Chen, S.K. Malik, W.B. Yelon, G.J. Long, A. Mohan, J. Appl. Phys. 85 Ž1999. 5172. ˇ w13x V. Sepelak, ´ K.D. Becker, J. Metastable Nanocryst. Mater. 8 Ž2000. 332. ˇ w14x V. Sepelak, ´ D. Baabe, F.J. Litterst, K.D. Becker, Hyperfine Interact. 126 Ž2000. 143. ˇ w15x V. Sepelak, ´ D. Baabe, F.J. Litterst, K.D. Becker, J. Appl. Phys. 88 Ž2000. 5884. ˇ ˇ w16x K. Tkacova, ´ˇ ´ V. Sepelak, ´ N. Stevulova, ´ V.V. Boldyrev, J. Solid State Chem. 123 Ž1996. 100. ˇ w17x V. Sepelak, ´ U. Steinike, D.C. Uecker, R. Trettin, S. Wiß mann, K.D. Becker, Solid State Ionics 101–103 Ž1997. 1343. w18x D. Lin, A.C. Nunes, C.F. Majkrzak, A.E. Berkowitz, J. Magn. Magn. Mater. 145 Ž1995. 343. w19x R.H. Kodama, A.E. Berkowitz, E.J. McNiff, S. Foner, Phys. Rev. Lett. 77 Ž1996. 394. w20x M.P. Morales, C.J. Serna, F. Bødker, S. Mørup, J. Phys.: Condens. Matter 9 Ž1997. 5461. w21x K. Lagarec, D.G. Rancourt, Recoil—Mossbauer Spectral ¨ Analysis Software for Windows, Version 1.02,1998, ŽUniversity of Ottawa, Ottawa.. w22x E. De Grave, C. Dauwe, A. Govaert, J. De Sitter, Phys. Status Solidi B 73 Ž1976. 527. w23x H.St.C. O’Neill, H. Annersten, D. Virgo, Am. Mineral. 77 Ž1992. 725. w24x D.S. Walters, G.P. Wirtz, J. Am. Ceram. Soc. 54 Ž1971. 563.
Mechanically induced cation redistribution in magnesium ferrite and its thermal stability ˇ V. Sepelak ´ a,) , D. Schultze b, F. Krumeich c , U. Steinike d, K.D. Becker e a
e
Institute of Geotechnics, SloÕak Academy of Sciences, WatsonoÕa 45, SK-04353 Kosice, ˇ SloÕak Republic b Federal Institute for Materials Research and Testing, D-12489 Berlin, Germany c Swiss Federal Institute of Technology Zurich, CH-8093 Zurich, Switzerland ¨ d Institute of Applied Chemistry, D-12484 Berlin, Germany Institute of Physical and Theoretical Chemistry, Technical UniÕersity Braunschweig, D-38106 Brunswick, Germany
Abstract The changes in magnesium ferrite ŽMgFe 2 O4 . caused by high-energy milling are investigated. Mechanical treatment reduces the average crystallite size of MgFe 2 O4 to the nanometer range and induces cation redistribution between tetrahedral and octahedral sites. The degree of inversion of the mechanically treated ferrite is compared with that of MgFe 2 O4 quenched from high temperatures. The range of thermal stability of mechanically induced metastable states is determined by studying the response of the metastable mechanically activated MgFe 2 O4 to changes in temperature. q 2001 Elsevier Science B.V. All rights reserved. MAT: MgFe 2 O4 Keywords: Mechanochemistry; Mechanical activation; High-energy milling; Nanocrystalline metastable material; Spinel ferrite; Thermal relaxation
1. Introduction Magnesium ferrite, MgFe 2 O4 , is a soft magnetic n-type semiconducting material w1x, which finds a number of applications in heterogeneous catalysis, adsorption, sensors, and in magnetic technologies. The structural formula of magnesium ferrite is usually written as ŽMg 1y x Fe x .wMg x Fe 2yx xO4 , where round and square brackets denote cation sites of
) Corresponding author. Tel.: q421-95-6323402; fax: q42195-6323402. ˇ E-mail address: [email protected] ŽV. Sepelak ´ ..
tetrahedral ŽA. and octahedral wBx coordination, respectively. x represents the so-called degree of inversion Ždefined as the fraction of the ŽA. sites occupied by Fe 3q cations.. The most remarkable effect in spinel ferrites is the strong dependence of properties on the state of chemical order, and in particular, on the cation distribution. It has been found that high-energy milling reduces the average crystallite size of the spinel ferrites to the nanometer range Ž; 10 nm. and induces cation redistribution between ŽA. and wBx sites w2–15x. Mechanical treatment of the spinel ferrites was found to be a useful activation method leading to an enhanced reactivity of nanoscale powders w16,17x. Spin canting, defined as a lack of full align-
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 7 7 7 - 9
678
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
ment of the spins in an applied magnetic field, has been proposed for the nano-sized, mechanically treated spinel ferrites w18–20x. Since the mechanically treated spinel ferrites are metastable with respect to structural and compositional changes under temperature, pressure, and environmental conditions in which they are utilised, there is great concern for understanding and controlling their limitations. This experimental work focuses on the study of both the mechanically induced disorder in MgFe 2 O4 and the structural response of this metastable solid to changes in temperature.
2. Experimental Polycrystalline MgFe 2 O4 was prepared by the conventional ceramic method Žfurther referred to as the non-activated sample.. The non-activated sample Ž2 g. was ground for various times in a planetary ball mill EI 2 = 150 Žproduct of the Institute of Solid State Chemistry, Novosibirsk. at room temperature. The ceramic-covered grinding chamber Ž150 cm3 in volume. and balls made of a-Al 2 O 3 with diameter of 3–5 mm were used. The ball-to-powder weight ratio was 50:1. Grinding experiments were performed in air at 750 rpm. Mossbauer measurements were carried out at 6.4 ¨ K under an external magnetic field of 5.5 T applied perpendicular to the g-ray direction. A 57CorRh g-ray source was used. The velocity scale was calibrated relative to 57 Fe in Rh. ‘Recoil’ spectral analysis software w21x was used for the quantitative evaluation of the Mossbauer spectra. ¨ The morphology of the powder and the size of individual crystallites were studied by means of transmission electron microscopy ŽTEM. ŽModel CM30ST, Philips, Eindhoven.. X-ray diffraction ŽXRD. patterns were collected using a URD 6 powder diffractometer ŽSeifert-FPM, Freiberg. with FeK a radiation. An XRK-A Paar camera ŽA Paar, Graz. was employed for in situ XRD in the temperature range between 300 K and 1100 K. Samples were heated in air at a heating rate of 2 K miny1 . Thermogravimetric ŽTG. measurements were performed using a Setaram Tag 24 thermobalance. Sam-
ples Ž130 mg. were thermally treated in open platinum crucibles in air. A permanent magnet was stacked on top of the horizontal furnace assembly of the TG analyser. The apparent weight of the sample in the magnetic field of the permanent magnet was recorded as a function of temperature. When the sample passed through the Neel ´ temperature ŽTN ., its apparent weight increased rapidly due to loss of spontaneous magnetisation. The Neel ´ temperature was arbitrarily defined by the intersection of the maximum slope of the thermogram with the upper base line, which corresponded to the true sample weight. Reproducibility of the measured Neel ´ temperature was "0.5 K. The sample was annealed at four consecutive temperatures Ž1130 K, 1180 K, 1230 K, 1280 K. without removing it from the TG analyser. The sample was held at the given annealing temperature for 2 h, cooled to 470 K at a cooling rate of 10 K miny1 , and then heated to the further annealing temperature. The magnetic disorder–order transition of the sample in the magnetic field of the permanent magnet was monitored during the cooling stage.
3. Results and discussion The mechanically induced evolution of MgFe 2 O4 submitted to the high-energy milling process was followed by 57 Fe Mossbauer spectroscopy. The low¨ temperature Mossbauer spectra taken in an applied ¨ magnetic field ŽFig. 1. clearly indicate that the mechanical treatment of MgFe 2 O4 is accompanied by the decrease of the concentration of iron cations on ŽA. sites. The degree of inversion, calculated from the subspectral area ratio, decreases with increasing milling time from x s 0.904Ž1. Žfor the non-activated sample. to x s 0.756Ž1. for MgFe 2 O4 mechanically activated for 30 min. The values of the average hyperfine magnetic fields Ž HA s 50.97Ž8. T, H B s 53.54Ž2. T., the isomer shifts Ž ISA s 0.24Ž8. mm sy1 , ISB s 0.35Ž3. mm sy1 ., and the degree of inversion for the non-activated sample are in agreement with previously published data w22,23x. Dark field TEM image of the milled MgFe 2 O4 ŽFig. 2. shows bright contrasts indicating many small crystals. Their average size was estimated Žfrom TEM. to be about 10 nm.
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
679
Fig. 1. Mossbauer spectrum of the non-activated MgFe 2 O4 Ža. and of the mechanically activated MgFe 2 O4 Žb.. Spectra taken at 6.4 K in an ¨ external magnetic field of 5.5 T applied perpendicular to the g-ray direction.
By far, the most extensive contributions to our present knowledge on cation redistribution in unpro-
Fig. 2. Dark field image showing the nanoscale structure of mechanically activated MgFe 2 O4 .
cessed spinels stem from measurements performed at room temperature on samples quenched from the high temperatures of interest. Even though we are aware that it is not feasible to formally transfer the processes establishing the equilibrium distribution of cations, taking place during ‘thermal activation’, to the case of mechanical activation, we compare the value of the cation inversion parameter of mechanically treated MgFe 2 O4 with that of the quenched samples. A very detailed study of the equilibrium cation distribution in MgFe 2 O4 has been undertaken by O’Neill et al. w23x on quenched samples using XRD and Mossbauer spectroscopy. According to ¨ these authors, the cation distribution changes smoothly with quenching temperature. The inversion parameter decreases from 0.91 at 723 K to 0.72 at 1373 K. According to these data, the degree of
680
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
in the same sense. In the case of mechanical activation, this change of the cation distribution causes the
Fig. 3. Comparison of the cation distribution in quenched MgFe 2 O4 Žsquares. with that in both non-activated and mechanically activated MgFe 2 O4 Žcircles..
inversion of the non-activated MgFe 2 O4 Ž x s 0.904Ž1.. corresponds to a sample quenched from about 740 K. The value of x s 0.756Ž1., characterising the degree of disorder in the milled MgFe 2 O4 , corresponds to samples quenched from about 1170 K ŽFig. 3.. Thus, both the milling time and the temperature lead to the alteration of the cation distribution
Fig. 4. X-ray diffraction patterns of the mechanically activated MgFe 2 O4 taken during the thermal treatment in the XRK-A Paar camera. Diffraction lines of MgFe 2 O4 are denoted by Miller indices. The sample was measured on a platinum support.
Fig. 5. Thermogravimetric curves Žtop. and the derivative thermogravimetric curves Žbottom. for the mechanically activated MgFe 2 O4 after consecutive annealing at 1130 K Ža., 1180 K Žb., 1230 K Žc., and 1280 K Žd..
ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
transition of the system into a metastable activated state with a higher amount of the accumulated energy. In situ high-temperature XRD analysis revealed that from the 300 K to 600 K range, the shape of the diffraction patterns of mechanically treated MgFe 2 O4 remains the same, i.e. the range of the thermal stability of the mechanically induced defects in the structure of milled MgFe 2 O4 extends up to 600 K. However, at temperatures over 600 K, a gradual recrystallisation of the nanoscale MgFe 2 O4 powders takes place. This is manifested by a gradual narrowing of diffraction lines and by an increase of their intensities, as shown in Fig. 4. Redistribution of the integral intensities of diffraction lines provides evidence that the crystallite growth of the milled powders is accompanied by a change of the cation distribution. Thermogravimetric measurements of the high-energy milled MgFe 2 O4 were undertaken in order to detect how the mechanically induced cation distribution relaxes towards its equilibrium configuration. As shown in Fig. 5, the structural response of the mechanically activated MgFe 2 O4 to changes in temperature is accompanied by an increase in the Neel ´ temperature Žfrom TN s 632 K to TN s 648 K.. The Neel ´ temperature provides a readily observed, highly sensitive measure of cation distribution in MgFe 2 O4 w24x. According to O’Neill et al.
Fig. 6. Neel ´ temperature, TN , vs. degree of inversion, x, for the mechanically activated MgFe 2 O4 after annealing at four different temperatures in the TG analyser Žcircles.. The data of O’Neill et al. w23x are indicated by the solid line.
681
w23x the Neel ´ temperature of MgFe 2 O4 increases linearly with increasing degree of inversion Žsee Fig. 6.. According to these data, the values of TN s 632 K, TN s 636 K, TN s 643 K, and T N s 648 K, characterising the magnetic disorder–order transition in the milled sample after annealing at four different temperatures ŽFig. 5., correspond to samples with the degree of inversion of x s 0.870Ž8., x s 0.879Ž1., x s 0.893Ž4., and x s 0.903Ž7., respectively ŽFig. 6.. This indicates that the cation distribution in MgFe 2 O4 resulting from the mechanical treatment is reversible, i.e. during the annealing process, the mechanically induced cation distribution Ž x s 0.756Ž1.. relaxes towards its equilibrium configuration Ž x s 0.903Ž7... The Neel ´ temperature of the mechanically activated MgFe 2 O4 , after annealing at 1280 K, is close to the non-activated one ŽTN s 648 K.. Thus, on heating, the milled MgFe 2 O4 has returned to a state that is similar to the initial one, and its properties are gradually restored.
4. Conclusions Mechanical treatment of MgFe 2 O4 leads to the formation of the nanoscale structure with the crystallite size of about 10 nm. The metastable nanostructural state of the milled MgFe 2 O4 is characterised by a reduced concentration of iron cations on ŽA. sites. The degree of inversion decreases with increasing milling time from 0.904Ž1., characteristic of the non-activated sample, to 0.756Ž1. for activated ferrite. The degree of inversion of the activated MgFe 2 O4 corresponds to a sample quenched from about 1170 K. The range of the thermal stability of the mechanically induced defects in the structure of milled MgFe 2 O4 extends up to 600 K. At temperatures over 600 K, a gradual recrystallisation of the nanoscale MgFe 2 O4 powders takes place, and the mechanically induced cation distribution relaxes toward its equilibrium configuration Ž x s 0.903Ž7... The latter is sensitively monitored by the Neel ´ temperature measurements. The Neel ´ temperature of the mechanically activated MgFe 2 O4 after annealing at 1280 K is close to the non-activated one ŽTN s 648 K..
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ˇ ´ et al.r Solid State Ionics 141–142 (2001) 677–682 V. Sepelak
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