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Synthesis, structural, and magnetic characterisation of magnesium-doped lithium ferrite of composition Li0.5Fe2.5O4
PERGAMON
Solid State Communications 120 (2001) 171±175
www.elsevier.com/locate/ssc
Synthesis, structural, and magnetic characterisation of magnesiumdoped lithium ferrite of composition Li0.5Fe2.5O4 Hisham M. Widatallah a,b, Clive Johnson a, Frank Berry a,*, Marek Pekala c a
Department of Chemistry, The Open University, Walton Hall,Milton Keynes MK7 6AA, UK b Department of Physics, University of Khartoum, P.O. Box 321, Khartoum 11115, Sudan c Department of Chemistry, University of Warsaw, Al. Zwirki i Wigury 101, PL-02-089 Warsaw, Poland Received 12 April 2001; accepted 4 July 2001 by R. Phillips
Abstract Spinel-related magnesium-doped Li0.5Fe2.5O4 has been synthesised by heating magnesium-substituted corundum-related a-Fe2O3 with Li2CO3 at 6008C which is ca. 6008C lower than the temperature normally used to prepare metal-doped Li0.5Fe2.5O4 by conventional sintering methods. Rietveld structure re®nement of the X-ray powder diffraction data shows that the Mg 21 ions substitute for Fe 31 ions on the tetrahedral sites whilst the Li 1 ions are located on the octahedral sites. The 57Fe MoÈssbauer spectrum and magnetic measurements are consistent with this structural model. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 61.10.2i; 61.18.Fs; 61.66.Fn; 67.80.Jd Keywords: A. Magnetic properties; C. X-ray diffraction; A. Lithium ferrite
1. Introduction The lithium ferrite, Li0.5Fe2.5O4, adopts an inverse spinelrelated structure and has attractive technological properties. Its high Curie temperature, high saturation magnetisation and hysteresis loop properties offer performance advantages over other spinel structures traditionally used in the ®eld of microwave- and memory core-applications [1±4]. In Li0.5Fe2.5O4 all the Li 1 ions and three-®fth of all the Fe 31 ions occupy the octahedral B sites whilst the remaining Fe 31 ions occupy the tetrahedral A sites [1]. The magnetic moments on each of these sites are antiparallel and the dominant contribution to magnetisation comes from ions on the octahedral sites. Ionic substitution of Li0.5Fe2.5O4 can in¯uence the magnetic properties of the material [1,2]. For example, the magnetisation can be raised or lowered by substituting a nonmagnetic ion for Fe 31 on the tetrahedral A or octahedral B sites respectively. Metal-substituted lithium ferrites have generally been prepared by conventional * Corresponding author. Tel.: 144-1908-652-801; fax: 1441908-858-327. E-mail address: [email protected] (F. Berry).
sintering of oxides, precipitated precursors, or, hydrothermally processed precipitates at temperatures exceeding 10008C [3±5]. Other methods of preparation have involved self-propagating high temperature synthesis [6], or synthesis from melts [7]. The high temperatures used in all these techniques lower the magnetisation due to the precipitation of a-Fe2O3 or the formation of Fe3O4 [1]. Furthermore, the reduction of Fe 31 to Fe 21 due to the formation of Fe3O4 leads to an increased electrical conductivity which limits the use of the material in, for example, microwave applications where high resistivity and minimum dielectric loss are required. We report here on the synthesis of magnesium-doped Li0.5Fe2.5O4 at 6008C from a mixture of magnesium-substituted a-Fe2O3 and Li2CO3 thus avoiding the drawbacks associated with high temperature synthesis. We also report on the structural and magnetic properties of the material. 2. Experimental Magnesium-substituted a-Fe2O3 was prepared hydrothermally as previously described [8]. A 7:1 molar mixture of magnesium-substituted a-Fe2O3 and Li2CO3 was ground
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00327-1
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H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
Fig. 1. X-ray powder diffraction patterns recorded from the mixture of magnesium-substituted a-Fe2O3 and Li2CO3 at (a) 258C; and following calcination at (b) 3008C; (c) 4008C; (d) 5008C; and (e) 6008C.
in an agate mortar for 30 min and calcined in air at temperatures between 300 and 6008C for 12 h intervals. X-ray powder diffraction data were recorded with a Siemens D5000 diffractometer in re¯ection mode using CuKa radiation. Structural re®nement and simulation of patterns for speci®c structural models was performed using the Rietveld program winmprof [9]. Thermogravimetric analysis was performed with a Rheometric Scienti®c STA 1500 system. The mixture (10 mg) was heated in air in the temperature range 30±10008C at a rate of 108/min. 57Fe MoÈssbauer spectra were recorded at 298 K using a microprocessor controlled MoÈssbauer spectrometer with a 25 mCi Co/Rh source. Chemical isomer shift data are quoted relative to that of metallic iron at room temperature. The temperature dependence of the magnetisation was measured at a heating rate of 4 K/min using a Faraday balance in a magnetic ®eld of 1.5 T with a relative accuracy better than 1%. The temperature was stabilised within 0.5 K.
Heating the mixture for 12 h at 3008C and followed by quenching in air (Fig. 1b) failed to induce a reaction between the components. Heating the mixture at 400 and 5008C (12 h) (Fig. 1c and d) followed by quenching in air resulted in a decrease in the intensity of the X-ray powder diffraction peaks corresponding to Li2CO3 and magnesiumsubstituted a-Fe2O3 and a concomitant growth in the peaks corresponding to the inverse spinel-related Li0.5Fe2.5O4 phase. The result is endorsed by the TGA curve (Fig. 2) which showed a sharp mass loss in the mixture between 400 and 5008C due to the decomposition of Li2CO3. Further heating at 6008C (12 h) and subsequent quenching in air resulted (Fig. 1e) in the development of a single Li0.5Fe2.5O4 spinel-related phase. This temperature is ca. 6008C lower than that at which magnesium-doped Li0.5Fe2.5O4 has been formed by conventional sintering methods [4]. ICP analysis of the metal components showed a lithium content of 1.7% and a magnesium content of 1.9%.
3. Results and discussion
3.2. Rietveld structure re®nement of the X-ray powder diffraction pattern of magnesium-doped Li0.5Fe2.5O4
3.1. Calcination of the mixture of magnesium-substituted a -Fe2O3 and Li2CO3 The X-ray powder diffraction patterns recorded from the mixture following calcination at different temperatures are shown in Fig. 1.
The X-ray powder diffraction pattern recorded following the calcination at 6008C for 12 h (Fig. 1e) is similar to that of Li0.5Fe2.5O4 which has an inverse spinel-related structure [10,11]. The absence of superstructure lines shows that the Li 1 and Fe 31 ions are, as in disordered Li0.5Fe2.5O4, randomly distributed over the octahedral sites [11]. The
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
173
Fig. 2. TGA recorded from the mixture of magnesium-substituted a-Fe2O3 and Li2CO3.
Rietveld structure re®nement of the X-ray powder diffraction data (Table 1, Fig. 3) based on a model in which Mg 21 substituted for Fe 31 on the tetrahedral sites resulted in a formula for the material of Li0.41Fe2.41Mg0.17O4 in which the Mg:Fe ratio is similar to that of the parent a-Fe1.89Mg0.17O3 phase. The present model is in agreement with that suggested previously [4] for magnesium-doped Li0.5Fe2.5O4 in which Mg 21 was envisaged as substituting for Fe 31 on the tetrahedral sites. The lattice parameter of Ê is slightly larger than that of pure Li0.5Fe2.5O4, 8.3411(2) A Ê [2] and re¯ects the larger ionic radius of Mg 21 8.3311(8) A as compared to Fe 31. The data were not amenable to satisfactory ®tting to a model involving no lithium atoms. 3.3. MoÈssbauer spectra and magnetic measurements The 57Fe MoÈssbauer spectrum recorded at 298 K from magnesium-doped Li0.5Fe2.5O4 prepared at 6008C (Fig. 4,
Table 2) was best ®tted to two sextets corresponding to Fe 31 in tetrahedral A sites and octahedral B sites. The chemical isomer shift- and quadrupole splitting-data for both components of the MoÈssbauer spectrum agree with those previously reported for Li0.5Fe2.5O4 [6,11]. However, the values for the magnetic hyper®ne ®eld on both sites are slightly smaller than those previously described for Li0.5Fe2.5O4 (50.0 T for the A site and 50.5 T for the B site). This may be attributed to the presence of the nonmagnetic Mg 21 ions in the vicinity of Fe 31 ions which decreases the FeA ±O±FeB exchange interaction. Assuming no signi®cant change in recoil free fraction on doping, the decrease in the spectral area of the tetrahedral A site sextet from 40% in pure Li0.5Fe2.5O4 to 33% in the magnesiumdoped variant is further evidence that Mg 21 ions partially substitute for Fe 31 ions at the tetrahedral A sites as indicated by the re®nement of the X-ray powder diffraction data. The magnetisation of the magnesium-doped Li0.5Fe2.5O4
Table 1 Re®ned atomic parameters from powder X-ray diffraction for magnesium-doped Li0.5Fe2.5O4. (% Li (analysis): 1.7, % Li (XRD): 1.4(1), % Mg (analysis): 1.9, % Mg (XRD): 2.0(1); unit cell: Fd-3m; a 8:3411 2; Constraints: Biso: Fe1 Li; Fe2 Mg; Occupancy: Fe1 12 1 Mg=2; Li 4 2 Mg=2; Fe2 8 2 Mg: Rfactors: Rexp 9:5; Rwp 14:2; R1 3:7% Rwp and Rexp have their normal signi®cance and relate to regions of the pro®le at which Bragg peaks contribute; R1 is the residual based on observed and calculated integrated intensities. Atom
Position
x/a
y/b
z/c
Ê 2) Biso (A
Occupancy
Fe1 Li Fe2 Mg O
16d 16d 8a 8a 32e
0.5 0.5 0.125 0.125 0.2555(2)
0.5 0.5 0.125 0.125 0.2555(2)
0.5 0.5 0.125 0.125 0.2555(2)
0.84(4) 0.84(4) 0.21(3) 0.21(3) 0.52(5)
12.69(2) 3.31(2) 6.63(5) 1.37(5) 32
174
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
Fig. 3. Observed, calculated and difference X-ray powder diffraction patterns recorded from magnesium -doped Li0.5Fe2.5O4.
Fig. 4. 57Fe MoÈssbauer spectrum recorded from the magnesium-doped Li0.5Fe2.5O4.
Table 2 57 Fe MoÈssbauer parameters for magnesium-doped Li0.5Fe2.5O4 at 298 K
Sextet A Sextet B
d ^ 0:01 (mm/s)
D ^ 0:01 (mm/s)
Heff ^ 0:3 (T)
G ^ 0:01 (mm/s)
Area ^ 3 (%)
0.18 0.39
0.03 0.02
49.5 49.9
0.43 0.42
33 67
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
175
Fig. 5. Temperature dependence of magnetisation of magnesium-doped Li0.5Fe2.5O4 (open squares) and Li0.5Fe2.5O4 (solid squares).
and pure Li0.5Fe2.5O4 decreased monotonically with increasing temperature as shown in Fig. 5. The magnetisation of the magnesium-doped Li0.5Fe2.5O4 was found to be higher than that of pure Li0.5Fe2.5O4 over the whole temperature range investigated. This is consistent with the Mg 21 ions substituting for Fe 31 ions at the tetrahedral A sites since, due to the antiferromagnetic coupling between the A and B sublattices, the substitution enhances the magnetisation imbalance between both sublattices, thus resulting in the higher magnetisation of the magnesiumdoped lithium ferrite. 4. Conclusion Spinel-related magnesium-doped Li0.5Fe2.5O4 has been prepared from magnesium-substituted corundum-related a-Fe2O3, and Li2CO3 at lower temperatures than those used in conventional sintering methods. Rietveld re®nement of the X-ray powder diffraction data suggests that the Mg 21 ions partially substitute for Fe 31 at tetrahedral A sites. The 57 Fe MoÈssbauer spectrum and magnetic measurements are consistent with this structural model. Acknowledgements We thank the Gordon Memorial College Trust Fund
(HMW) and the Polish Committee for Scienti®c Research (MP) for ®nancial support. References [1] P.D. Baba, G.M. Argentina, IEEE Trans. Microwave Theory Technol. 22 (1974) 654. [2] E. Bermejo, J. Chassaing, D. Bizot, M. Quarton, Mater. Sci. Engng B22 (1994) 73. [3] V. Berbenni, A. Mariniand, D. Capsoni, Z. Naturforsch. 53(a) (1993) 611. [4] J.S. Baijal, S. Phanjoubam, D. Kothari, Solid State Commun. 83 (1992) 679. [5] A. Dias, N.D.S. Mohallem, Moreira, Mater. Res. Bull. 33 (1998) 475. [6] M.V. Kuznetsov, Q.A. Pankhurst, I.P. Parkin, J. Phys. D: Appl. Phys. 31 (1998) 2886. [7] A. Gonzalez Arias, A. del Gueto, J.M. Munoz, C. de Francisco, Matt. Lett. 33 (1998) 187. [8] F.J. Berry, C. Greaves, J. Macmanus, M. Mortimer, G. Oates, J. Solid State Chem. 130 (1997) 272. [9] A. Jouanneaux, ªWinMProfº, Int. Union Crystallogr. Newslett. 21 (1999) 13. [10] L. Fernandez-Barquin, M.V. Kuznetsov, Y.G. Morozov, Q.A. Pankhurst, I.P. Parkin, Int. J. Inorg. Mater. 1 (1993) 311. [11] M. Tabuchi, K. Ado, H. Kobayashi, I. Matsubara, H. Kagayama, M. Wakita, S. Tsutsui, S. Nasu, Y. Takeda, C. Masquelier, A. Hirano, R. Kanno, J. Solid State Chem. 141 (1998) 554.
Solid State Communications 120 (2001) 171±175
www.elsevier.com/locate/ssc
Synthesis, structural, and magnetic characterisation of magnesiumdoped lithium ferrite of composition Li0.5Fe2.5O4 Hisham M. Widatallah a,b, Clive Johnson a, Frank Berry a,*, Marek Pekala c a
Department of Chemistry, The Open University, Walton Hall,Milton Keynes MK7 6AA, UK b Department of Physics, University of Khartoum, P.O. Box 321, Khartoum 11115, Sudan c Department of Chemistry, University of Warsaw, Al. Zwirki i Wigury 101, PL-02-089 Warsaw, Poland Received 12 April 2001; accepted 4 July 2001 by R. Phillips
Abstract Spinel-related magnesium-doped Li0.5Fe2.5O4 has been synthesised by heating magnesium-substituted corundum-related a-Fe2O3 with Li2CO3 at 6008C which is ca. 6008C lower than the temperature normally used to prepare metal-doped Li0.5Fe2.5O4 by conventional sintering methods. Rietveld structure re®nement of the X-ray powder diffraction data shows that the Mg 21 ions substitute for Fe 31 ions on the tetrahedral sites whilst the Li 1 ions are located on the octahedral sites. The 57Fe MoÈssbauer spectrum and magnetic measurements are consistent with this structural model. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 61.10.2i; 61.18.Fs; 61.66.Fn; 67.80.Jd Keywords: A. Magnetic properties; C. X-ray diffraction; A. Lithium ferrite
1. Introduction The lithium ferrite, Li0.5Fe2.5O4, adopts an inverse spinelrelated structure and has attractive technological properties. Its high Curie temperature, high saturation magnetisation and hysteresis loop properties offer performance advantages over other spinel structures traditionally used in the ®eld of microwave- and memory core-applications [1±4]. In Li0.5Fe2.5O4 all the Li 1 ions and three-®fth of all the Fe 31 ions occupy the octahedral B sites whilst the remaining Fe 31 ions occupy the tetrahedral A sites [1]. The magnetic moments on each of these sites are antiparallel and the dominant contribution to magnetisation comes from ions on the octahedral sites. Ionic substitution of Li0.5Fe2.5O4 can in¯uence the magnetic properties of the material [1,2]. For example, the magnetisation can be raised or lowered by substituting a nonmagnetic ion for Fe 31 on the tetrahedral A or octahedral B sites respectively. Metal-substituted lithium ferrites have generally been prepared by conventional * Corresponding author. Tel.: 144-1908-652-801; fax: 1441908-858-327. E-mail address: [email protected] (F. Berry).
sintering of oxides, precipitated precursors, or, hydrothermally processed precipitates at temperatures exceeding 10008C [3±5]. Other methods of preparation have involved self-propagating high temperature synthesis [6], or synthesis from melts [7]. The high temperatures used in all these techniques lower the magnetisation due to the precipitation of a-Fe2O3 or the formation of Fe3O4 [1]. Furthermore, the reduction of Fe 31 to Fe 21 due to the formation of Fe3O4 leads to an increased electrical conductivity which limits the use of the material in, for example, microwave applications where high resistivity and minimum dielectric loss are required. We report here on the synthesis of magnesium-doped Li0.5Fe2.5O4 at 6008C from a mixture of magnesium-substituted a-Fe2O3 and Li2CO3 thus avoiding the drawbacks associated with high temperature synthesis. We also report on the structural and magnetic properties of the material. 2. Experimental Magnesium-substituted a-Fe2O3 was prepared hydrothermally as previously described [8]. A 7:1 molar mixture of magnesium-substituted a-Fe2O3 and Li2CO3 was ground
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00327-1
172
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
Fig. 1. X-ray powder diffraction patterns recorded from the mixture of magnesium-substituted a-Fe2O3 and Li2CO3 at (a) 258C; and following calcination at (b) 3008C; (c) 4008C; (d) 5008C; and (e) 6008C.
in an agate mortar for 30 min and calcined in air at temperatures between 300 and 6008C for 12 h intervals. X-ray powder diffraction data were recorded with a Siemens D5000 diffractometer in re¯ection mode using CuKa radiation. Structural re®nement and simulation of patterns for speci®c structural models was performed using the Rietveld program winmprof [9]. Thermogravimetric analysis was performed with a Rheometric Scienti®c STA 1500 system. The mixture (10 mg) was heated in air in the temperature range 30±10008C at a rate of 108/min. 57Fe MoÈssbauer spectra were recorded at 298 K using a microprocessor controlled MoÈssbauer spectrometer with a 25 mCi Co/Rh source. Chemical isomer shift data are quoted relative to that of metallic iron at room temperature. The temperature dependence of the magnetisation was measured at a heating rate of 4 K/min using a Faraday balance in a magnetic ®eld of 1.5 T with a relative accuracy better than 1%. The temperature was stabilised within 0.5 K.
Heating the mixture for 12 h at 3008C and followed by quenching in air (Fig. 1b) failed to induce a reaction between the components. Heating the mixture at 400 and 5008C (12 h) (Fig. 1c and d) followed by quenching in air resulted in a decrease in the intensity of the X-ray powder diffraction peaks corresponding to Li2CO3 and magnesiumsubstituted a-Fe2O3 and a concomitant growth in the peaks corresponding to the inverse spinel-related Li0.5Fe2.5O4 phase. The result is endorsed by the TGA curve (Fig. 2) which showed a sharp mass loss in the mixture between 400 and 5008C due to the decomposition of Li2CO3. Further heating at 6008C (12 h) and subsequent quenching in air resulted (Fig. 1e) in the development of a single Li0.5Fe2.5O4 spinel-related phase. This temperature is ca. 6008C lower than that at which magnesium-doped Li0.5Fe2.5O4 has been formed by conventional sintering methods [4]. ICP analysis of the metal components showed a lithium content of 1.7% and a magnesium content of 1.9%.
3. Results and discussion
3.2. Rietveld structure re®nement of the X-ray powder diffraction pattern of magnesium-doped Li0.5Fe2.5O4
3.1. Calcination of the mixture of magnesium-substituted a -Fe2O3 and Li2CO3 The X-ray powder diffraction patterns recorded from the mixture following calcination at different temperatures are shown in Fig. 1.
The X-ray powder diffraction pattern recorded following the calcination at 6008C for 12 h (Fig. 1e) is similar to that of Li0.5Fe2.5O4 which has an inverse spinel-related structure [10,11]. The absence of superstructure lines shows that the Li 1 and Fe 31 ions are, as in disordered Li0.5Fe2.5O4, randomly distributed over the octahedral sites [11]. The
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
173
Fig. 2. TGA recorded from the mixture of magnesium-substituted a-Fe2O3 and Li2CO3.
Rietveld structure re®nement of the X-ray powder diffraction data (Table 1, Fig. 3) based on a model in which Mg 21 substituted for Fe 31 on the tetrahedral sites resulted in a formula for the material of Li0.41Fe2.41Mg0.17O4 in which the Mg:Fe ratio is similar to that of the parent a-Fe1.89Mg0.17O3 phase. The present model is in agreement with that suggested previously [4] for magnesium-doped Li0.5Fe2.5O4 in which Mg 21 was envisaged as substituting for Fe 31 on the tetrahedral sites. The lattice parameter of Ê is slightly larger than that of pure Li0.5Fe2.5O4, 8.3411(2) A Ê [2] and re¯ects the larger ionic radius of Mg 21 8.3311(8) A as compared to Fe 31. The data were not amenable to satisfactory ®tting to a model involving no lithium atoms. 3.3. MoÈssbauer spectra and magnetic measurements The 57Fe MoÈssbauer spectrum recorded at 298 K from magnesium-doped Li0.5Fe2.5O4 prepared at 6008C (Fig. 4,
Table 2) was best ®tted to two sextets corresponding to Fe 31 in tetrahedral A sites and octahedral B sites. The chemical isomer shift- and quadrupole splitting-data for both components of the MoÈssbauer spectrum agree with those previously reported for Li0.5Fe2.5O4 [6,11]. However, the values for the magnetic hyper®ne ®eld on both sites are slightly smaller than those previously described for Li0.5Fe2.5O4 (50.0 T for the A site and 50.5 T for the B site). This may be attributed to the presence of the nonmagnetic Mg 21 ions in the vicinity of Fe 31 ions which decreases the FeA ±O±FeB exchange interaction. Assuming no signi®cant change in recoil free fraction on doping, the decrease in the spectral area of the tetrahedral A site sextet from 40% in pure Li0.5Fe2.5O4 to 33% in the magnesiumdoped variant is further evidence that Mg 21 ions partially substitute for Fe 31 ions at the tetrahedral A sites as indicated by the re®nement of the X-ray powder diffraction data. The magnetisation of the magnesium-doped Li0.5Fe2.5O4
Table 1 Re®ned atomic parameters from powder X-ray diffraction for magnesium-doped Li0.5Fe2.5O4. (% Li (analysis): 1.7, % Li (XRD): 1.4(1), % Mg (analysis): 1.9, % Mg (XRD): 2.0(1); unit cell: Fd-3m; a 8:3411 2; Constraints: Biso: Fe1 Li; Fe2 Mg; Occupancy: Fe1 12 1 Mg=2; Li 4 2 Mg=2; Fe2 8 2 Mg: Rfactors: Rexp 9:5; Rwp 14:2; R1 3:7% Rwp and Rexp have their normal signi®cance and relate to regions of the pro®le at which Bragg peaks contribute; R1 is the residual based on observed and calculated integrated intensities. Atom
Position
x/a
y/b
z/c
Ê 2) Biso (A
Occupancy
Fe1 Li Fe2 Mg O
16d 16d 8a 8a 32e
0.5 0.5 0.125 0.125 0.2555(2)
0.5 0.5 0.125 0.125 0.2555(2)
0.5 0.5 0.125 0.125 0.2555(2)
0.84(4) 0.84(4) 0.21(3) 0.21(3) 0.52(5)
12.69(2) 3.31(2) 6.63(5) 1.37(5) 32
174
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
Fig. 3. Observed, calculated and difference X-ray powder diffraction patterns recorded from magnesium -doped Li0.5Fe2.5O4.
Fig. 4. 57Fe MoÈssbauer spectrum recorded from the magnesium-doped Li0.5Fe2.5O4.
Table 2 57 Fe MoÈssbauer parameters for magnesium-doped Li0.5Fe2.5O4 at 298 K
Sextet A Sextet B
d ^ 0:01 (mm/s)
D ^ 0:01 (mm/s)
Heff ^ 0:3 (T)
G ^ 0:01 (mm/s)
Area ^ 3 (%)
0.18 0.39
0.03 0.02
49.5 49.9
0.43 0.42
33 67
H.M. Widatallah et al. / Solid State Communications 120 (2001) 171±175
175
Fig. 5. Temperature dependence of magnetisation of magnesium-doped Li0.5Fe2.5O4 (open squares) and Li0.5Fe2.5O4 (solid squares).
and pure Li0.5Fe2.5O4 decreased monotonically with increasing temperature as shown in Fig. 5. The magnetisation of the magnesium-doped Li0.5Fe2.5O4 was found to be higher than that of pure Li0.5Fe2.5O4 over the whole temperature range investigated. This is consistent with the Mg 21 ions substituting for Fe 31 ions at the tetrahedral A sites since, due to the antiferromagnetic coupling between the A and B sublattices, the substitution enhances the magnetisation imbalance between both sublattices, thus resulting in the higher magnetisation of the magnesiumdoped lithium ferrite. 4. Conclusion Spinel-related magnesium-doped Li0.5Fe2.5O4 has been prepared from magnesium-substituted corundum-related a-Fe2O3, and Li2CO3 at lower temperatures than those used in conventional sintering methods. Rietveld re®nement of the X-ray powder diffraction data suggests that the Mg 21 ions partially substitute for Fe 31 at tetrahedral A sites. The 57 Fe MoÈssbauer spectrum and magnetic measurements are consistent with this structural model. Acknowledgements We thank the Gordon Memorial College Trust Fund
(HMW) and the Polish Committee for Scienti®c Research (MP) for ®nancial support. References [1] P.D. Baba, G.M. Argentina, IEEE Trans. Microwave Theory Technol. 22 (1974) 654. [2] E. Bermejo, J. Chassaing, D. Bizot, M. Quarton, Mater. Sci. Engng B22 (1994) 73. [3] V. Berbenni, A. Mariniand, D. Capsoni, Z. Naturforsch. 53(a) (1993) 611. [4] J.S. Baijal, S. Phanjoubam, D. Kothari, Solid State Commun. 83 (1992) 679. [5] A. Dias, N.D.S. Mohallem, Moreira, Mater. Res. Bull. 33 (1998) 475. [6] M.V. Kuznetsov, Q.A. Pankhurst, I.P. Parkin, J. Phys. D: Appl. Phys. 31 (1998) 2886. [7] A. Gonzalez Arias, A. del Gueto, J.M. Munoz, C. de Francisco, Matt. Lett. 33 (1998) 187. [8] F.J. Berry, C. Greaves, J. Macmanus, M. Mortimer, G. Oates, J. Solid State Chem. 130 (1997) 272. [9] A. Jouanneaux, ªWinMProfº, Int. Union Crystallogr. Newslett. 21 (1999) 13. [10] L. Fernandez-Barquin, M.V. Kuznetsov, Y.G. Morozov, Q.A. Pankhurst, I.P. Parkin, Int. J. Inorg. Mater. 1 (1993) 311. [11] M. Tabuchi, K. Ado, H. Kobayashi, I. Matsubara, H. Kagayama, M. Wakita, S. Tsutsui, S. Nasu, Y. Takeda, C. Masquelier, A. Hirano, R. Kanno, J. Solid State Chem. 141 (1998) 554.