Crystallographic and Mössbauer studies of Li 0.5 Fe 2.5 O 4 prepared by hightemperature thermal decomposition and sol-gel methodsSung Wook Hyun and Chul Sung Kim

Citation: Journal of Applied Physics 101, 09M513 (2007); doi: 10.1063/1.2712524 View online: http://dx.doi.org/10.1063/1.2712524 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/101/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Size-dependent magnetic properties of ordered Li 0.5 Fe 2.5 O 4 prepared by the sol-gel method J. Appl. Phys. 99, 08M917 (2006); 10.1063/1.2169468 Structural and magnetic properties of MSr 2 Y 1.5 Ce 0.5 Cu 2 O z (M-1222) compounds with M = Fe and Co J. Appl. Phys. 95, 6690 (2004); 10.1063/1.1688256 Mössbauer study of the magnetism and structure of amorphous and nanocrystalline Fe 81x Ni x Zr 7 B 12 (x=10–40) alloys J. Appl. Phys. 94, 638 (2003); 10.1063/1.1578701 Mössbauer spectroscopic and x-ray diffraction studies of structural and magnetic properties of heat-treated ( Ni0.5 Zn 0.5 ) Fe 2 O 4 nanoparticles J. Appl. Phys. 93, 7492 (2003); 10.1063/1.1540146 Magnetic and structural properties of ultrafine CoFe 1.9 RE 0.1 O 4 ( RE = Gd,Nd ) powders grown by using asol-gel method J. Appl. Phys. 91, 7607 (2002); 10.1063/1.1452214

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Crystallographic and Mössbauer studies of Li0.5Fe2.5O4 prepared by hightemperature thermal decomposition and sol-gel methods

Sung Wook Hyun and Chul Sung Kima�

Department of Physics, Kookmin University, Seoul 136-702, Korea

�Presented on 9 January 2007; received 31 October 2006; accepted 21 December 2006;published online 3 May 2007�

Li0.5Fe2.5O4 powders were prepared by high temperature thermal decomposition �HTTD� andsol-gel methods. The sample prepared by HTTD method �SA� has space group of Fd3m. Thesamples annealed at 700 °C �SB� and quenched at 1000 °C �SC� prepared by sol-gel method havespace groups of P4332 and Fd3m, respectively. The saturation magnetizations �Ms� for the sampleprepared by HTTD method �SA� at room temperature is 55 emu/g and those for the samplesannealed at 700 °C �SB� and quenched at 1000 °C �SC� prepared by sol-gel method are 59 and62 emu/g, respectively. In contrast, the coercivity �Hc� values of the each sample are 4.1, 93.7, and9.1 Oe, respectively. Mössbauer spectra of each sample have been obtained from 4.2 to 700 K. Thevalence state of Fe ions for the tetrahedral �A� and octahedral �B� sites is Fe3+. Mössbauer spectrumof the sample prepared by HTTD method shows superparamagnetic behavior at room temperature,while in case of sol-gel method, Mössbauer spectra show ferrimagnetic state of six line having thehyperfine field �Hf� values of 518 kOe for the A sites and 536 kOe for the B sites. © 2007 AmericanInstitute of Physics. �DOI: 10.1063/1.2712524�

I. INTRODUCTION

Lithium ferrites Li0.5Fe2.5O4 have been studied for cath-ode materials in rechargeable lithium batteries as well aslow-cost substitutes to garnet materials �Y3Fe5O12� in micro-wave frequency applications.1–3 The magnetic properties offerrite nanoparticles have long been of scientific and techno-logical interest which can be used in catalysts, high densitymagnetic storage, and biosensors.4,5 A decrease in particlesize may not only affect various magnetic structures but alsobreak down structural ordering related to cation occupancyin crystal. The vacancy ordering in �-Fe2O3 �space groupP4332� are well known to disappear when the particle size isreduced below 20 nm.6 In addition, ferrite nanoparticlesshow unusual magnetic properties which are not observed inthe bulk material such as single domain behavior andsuperparamagnetism.7–9 It has been reported that Li0.5Fe2.5O4

prepared by solid state reaction has ordered structure below735 °C and disordered structure around 1000 °C.10,11 Disor-dered structure is generally inverse spinel structure withspace group Fd3m and ordered structure corresponds tospace group P4332. In this work, we have studied the crys-tallographic and magnetic properties of the lithium ferritenanoparticles prepared by high temperature thermal decom-position �HTTD� �Refs 12 and 13� and sol-gel methods. Theproperties of lithium ferrite magnetic nanoparticles charac-terized by Mössbauer spectroscopy were compared with theanalyzed results by x-ray diffraction �XRD� and vibratingsample magnetometer �VSM�.

II. EXPERIMENT

The lithium ferrite, Li0.5Fe2.5O4, samples were preparedby HTTD and sol-gel methods. Lithium acetylacetonate�Li�CH3COCHvC�O– �CH3��, iron �III� acetylacetonate�Fe�CH3COCHvC�O– �CH3�3�, lithium acetate dihydrate�Li�CH3CO2� ·2H2O�, and iron �III� nitrate nonahydrate�Fe�NO3�3 ·9H2O� were used as starting materials to prepare

a�Author to whom correspondence should be addressed; FAX: �82-2-910-4752�5170�; electronic mail: cskim@phys.kookmin.ac.kr

FIG. 1. X-ray diffraction patterns of the samples �a� SA prepared by HTTDmethod, �b� SB annealed at 700 °C, and �c� SC quenched at 1000 °C pre-pared by sol-gel method.

JOURNAL OF APPLIED PHYSICS 101, 09M513 �2007�

0021-8979/2007/101�9�/09M513/3/$23.00 © 2007 American Institute of Physics101, 09M513-1

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Li0.5Fe2.5O4 nanoparticles. Lithium acetylacetonate and iron�III� acetylacetonate were mixed in phenyl ether with 1,2-hexadecanediol for synthesizing Li0.5Fe2.5O4 sample SA.The reaction with surfactants at high temperature success-fully leads to form the lithium ferrite nanoparticles becausethe reaction allows lithium ferrite nanoparticles to be easilyisolated during the chemical reaction between by-productsand the ether solvent.12

Lithium acetate dihydrate and iron �III� nitrate nonahy-drate were mixed in the correct ratio of cation and dissolvedin methanol and distilled water. The solution was refluxed at60 °C for 30 min and then it was dried at 120 °C in an ovenfor 24 h. The samples were calcined at 400 °C for the com-plete thermal decomposition of organic compounds. After-wards, the sample SB was ground and annealed at 700 °Cfor 6 h in air to yield lithium ferrite nanoparticles. The othersample SC was ground and quenched at 1000 °C to form thedisordered spinel structure �space group Fd3m�.

The synthesized samples were characterized for phasepurity and crystallinity by XRD measurements using Cu K�radiation ��=1.5406 Å�. Mössbauer spectrometer of theelectromechanical type with a 50 mCi 57Co source in Rhmatrix was used in the constant-acceleration mode.14 Thespectrometer was calibrated by collecting the Mössbauerspectra of a standard �-Fe foil at room temperature. To pro-duce a uniform thickness over the area of the Mössbauerabsorber, each sample was clamped between two berylliumdisks of 0.005 in. thick and 1 in. in diameter. The low tem-perature was obtained using a model SHI-850 closed-cyclerefrigerator system �Janis Research Company� with a RDK-205D Cold Head �Sumitomo Heavy Industries, Ltd.�, and thetemperature controller was a model 332 manufactured byLake Shore Cryotronics, Inc. The Mössbauer parameters

were obtained by a least-squares fitting program assumingLorentzian line shapes. The magnetizations were measuredby using VSM.

III. RESULTS AND DISCUSSION

XRD was used to confirm the crystal structure and crys-tallographic parameters of all samples SA, SB, and SC, asshown in Fig. 1. An analysis of XRD patterns by Rietveldrefinement15 method using FULLPROF program showed thatthe SA and SB samples have a disordered cubic spinel struc-ture �space group Fd3m� with different lattice constants a0

=8.394±0.01 and 8.335±0.01 Å, respectively. It may be

TABLE I. The saturation magnetization �Ms� and the coercivity �Hc� of allthe samples. The samples were prepared by HTTD method �SA� and sol-gelmethod annealed at 700 °C �SB�, quenched at 1000 °C �SC�.

SamplesMs

�emu/g�Hc

�Oe�Particle size

�nm�

SA 55.7 4.1 6SB 59.7 93.7 40SC 61.6 9.1 46

FIG. 2. Hysteresis loops of the samples SA, SB, and SC measured usingVSM with maximum applied field 5 kOe at room temperature.

FIG. 3. Magnetization curves of the samples SB and SC at various tempera-tures ranging from 300 to 1000 K measured with 100 Oe external appliedfield under zero field cooled. Insert: SA at various temperatures rangingfrom 30 to 300 K measured with 100 Oe external applied field under zerofield cooled.

FIG. 4. Mössbauer spectra of the samples SA, SB, and SC at varioustemperatures.

09M513-2 S. Hyun and C. Kim J. Appl. Phys. 101, 09M513 �2007�

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caused by the small particle size and the interface structurewith a large volume fraction.16 The other sample SB has anordered cubic spinel structure �space group P4332� with a0

=8.330±0.01 Å. The sample SB has the superstructurepeaks, �110�, �210�, �211�, etc., arising from a 1:3 Li- toFe-ordered distribution on octahedral sites. In contrast, it dis-appeared in the sample SC, as shown in Fig. 1. The particlesizes calculated using Scherrer equation of the samples SA,SB, and SC, as shown in Table I, are 6, 40, and 46 nm,respectively.

Figure 2 shows the hysteresis loops of the samples SA,SB, and SC measured using VSM with maximum appliedfield 5 kOe at room temperature. The typical superparamag-netic behavior was observed in the sample SA. The satura-tion magnetization �Ms� and the coercivity �Hc� of the eachsample are summarized in Table I. Ms is 55.7 emu/g for thesample SA and increases up to 59.7 and 61.6 emu/g for thesamples SB and SC, respectively. However, Hc decreasesfrom 93.7 to 9.1 Oe, where the grain size of single phaseparticle increases by quenching the sample at 1000 °C. TheNéel temperatures �TN� of the samples SB and SC are foundto be the same values, 912±2 K from the zero field cooledcurves with 100 Oe applied field, as shown in Fig. 3. A weakkinklike anomaly was observed at around 500 K for thesample SC, and it nearly spread out for the sample SB. Thisanomaly have been disappeared when one applied externalfield over 1 kOe, which was saturation magnetic field in bothsamples.

The Mössbauer spectra of Li0.5Fe2.5O4 nanoparticleswere taken at various temperatures ranging from4.2 to 295 K for the sample SA, and the samples SB and SCwere measured in the temperature range from 4.2 to 700 K,to understand localized nearest neighbor effects on effectivefield, as shown in Fig. 4. The Mössbauer spectrum for thesample SA showed a superparamagnetic behavior at roomtemperature. Just seeing three figures in Figs. 4�a�–4�c�, onecan see single lines of Mössbauer spectrum in Fig. 4�a� at300 K. Reminding that the size of sample SA is smallenough to develop superparamagnetic nanoparticle, we care-fully conclude that it is originated from the superparamag-netic behaviors by size effects. Also, the diagnosis of thesuperparamagnetic behavior is shown in magnetizationcurve, as shown in Fig. 2. One can see a typical Langevintype of superparamagnetic curve nearly zero coercivity. Inaddition, it is clear that the sample has a pure lithium ferritecrystal phase by analysis of Mössbauer spectra at low tem-perature. The Mössbauer spectrum of the sample SA at 4.2 Kshows typical lithium ferrite phase. The Mössbauer spectraof the all samples were analyzed by a least-squares fitting totwo hyperfine sextets of the following model,�Fe�A�Li0.5Fe1.5�BO4, where the subscripts A and B denote thetetrahedral sites and octahedral sites, respectively.17 This isconsistent with the results obtained by Rietveld x-ray diffrac-tion refinement, too. The isomer shifts at room temperaturefor the A and B sites of the sample SA are found to be0.33±0.01 and 0.26±0.01 mm/s, and the samples SB and

SC are found to be �0.14�0.16� ±0.01 and �0.21�0.24�±0.01 mm/s, relative to the Fe metal, respectively, which areconsistent with the Fe3+ valence state. The ferric character ofthe Fe ions is also manifested by the magnitudes of the mag-netic hyperfine fields; the magnetic hyperfine fields oflithium ferrite nanoparticles are approximately510–518±5 kOe and 520–536±5 kOe at 4.2 K for the Aand B sites, respectively, and they are typical values for fer-ric ions.

IV. CONCLUSION

Single phase lithium ferrite, Li0.5Fe2.5O4, nanoparticleshave been obtained by high temperature thermal decomposi-tion and sol-gel methods. The quantitative phase analysis andstructural ordering mechanism have been studied by usingx-ray refinement and Mössbauer analysis. The crystal struc-ture has been changed from the ordered cubic spinel struc-ture �space group P4332� to the disordered cubic spinelstructure �space group Fd3m that has the same of 6 nmsample�, when it was quenched the sample at 1000 °C. Wecould explain the superparamagnetic behavior of 6 nmlithium ferrite nanoparticle by VSM and Mössbauer spec-troscopy. The samples SB and SC show similar magneticproperties with Mössbauer parameters and saturation magne-tizations. However, the coercivity value for the sample SB islarger than that of the sample SC as much about 85 Oe.Mössbauer spectroscopy with two magnetic components ofisomer shifts and magnetic hyperfine fields for the allsamples SA, SB, and SC demonstrated the Fe3+ valencestate.

ACKNOWLEDGMENTS

This paper was performed for the Hydrogen EnergyR&D Center, one of the 21st Century Frontier R&D Pro-gram, funded by the Ministry of Science and Technology ofKorea.

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