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ordered phase wherein the Li and Fe ions are ordered in

ferrite at low temperatures to take care of lithium

volatilization that occur during sintering at higher

materials stems from their importance in many applica-

magnetization [7,8]. The citrate gel method is a well

at 350 -C with a large increase in the particle size [11].On the other hand, in another report, nanoparticles of the

disordered h-phase of size 10 and 20 nm are obtained by300 and 500 -C,

Materials Letters 59 (2005)the 1 :3 ratio in the octahedral sites of the cubic spinel

structure whereas the h-phase is a disordered phase wherethe Li and Fe ions are randomly distributed in the

octahedral sites. During the usual ceramic method of

synthesis of the bulk ferrite, a slow cooling process from

above 755 -C yields the ordered phase whereas thedisordered phase can be obtained by quenching from high

temperatures. An orderdisorder phase transition takes

place in the temperature range 735755 -C.There have been few attempts to synthesize lithium

known method for the synthesis of ferrite nanoparticles,

where the decomposition of a metal-citrate gel at low-

temperatures forms the required ferrite [9,10]. There are

some recent reports on the synthesis of lithium ferrite

nanoparticles at low temperatures by the citrate method.

However, different results are obtained though the

syntheses were made under identical conditions [11,12].

It has been shown that the disordered h-phase of lithiumferrite, with a particle size of 10 nm, is formed at 200-C which is converted to the ordered a-phase on heating0.5 2.5 4 5 8

type ferrite of the general formula AB2O4. Because of its

high saturation magnetization, high Curie temperature and

square loop properties, this ferrite is widely used in

microwave communications and memory core applications

[1]. Unlike other spinel-type ferrites, lithium ferrite exists

in two different crystalline forms [2]. The a-phase is an

tions such as ferrofluids, magnetic carriers for site-

specific drug delivery, local hyperthermia and contrast

enhancement of magnetic resonance imaging [46].

Nanosized ferrite particles exhibit unusual magnetic

properties, which are not observed in the bulk, such as

single domain behavior, superparamagnetism and reducedLithium ferrite (Li Fe O or LiFe O ) is a spinel-1. Introduction temperatures and for improved microstructure and densi-

fication [3]. Interest in the study of nanosized ferriteLow-temperature synthesis of nano

by an autocombustion metho

Seema Verma, Jayshri Kar

Physical and Materials Chemistry Division, National Chem

Received 27 November

Available on

Abstract

Nanocrystalline lithium ferrite is synthesized by an autocombusti

as glycine as fuels. The ordered phase of lithium ferrite, in single pha

by this method at a low temperature of 200 -C.D 2005 Elsevier B.V. All rights reserved.

Keywords: Low-temperature synthesis; Combustion synthesis; Nanomateria0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2005.04.005

* Corresponding author. Tel.: +91 20 2589 3300; fax: +91 20 2589 3044.

E-mail address: joy@dalton.ncl.res.in (P.A. Joy).stalline powders of lithium ferrite

sing citric acid and glycine

e, Arti Patidar, P.A. Joy*

Laboratory, Dr. Homi Bhabha Road, Pune 411008, India

accepted 7 April 2005

May 2005

ethod from the corresponding metal nitrates using citric acid as well

rm, with particle size of 3236 nm and large coercivity, is obtained

gnetic materials; Lithium ferrite; LiFe5O8

2630 2633

www.elsevier.com/locate/matletdecomposition of the citrate gel atrespectively [12]. Our studies show that the nanoparticles

formed at low temperatures, from the decomposition of

citric acid were prepared. The metal nitrates and citric

the powder obtained at 300 -C is extremely broad, indicating thatthe ferrite particles formed are extremely small. The average

is comparable to that reported by Sankaranarayanan et al. [11],

where some additional weak and broad features are observed (but

not indexed).

Fig. 2 shows the powder X-ray diffraction patterns of lithium

ferrite powders obtained by the citric acid (CA) and glycine (GA)

autocombustion methods. In both cases, the reflections are

relatively sharp. This is due to the autocombustion nature of

the reaction where the flame temperature can be very high. Both

patterns are identical and all the reflections are due to the ordered

phase of lithium ferrite. No additional reflections due to a-Fe2O3are observed indicating the formation of single phase material.

The cubic lattice parameter is calculated as 8.33 A, for both

samples, from least-squares refinement, and is comparable to the

reported value for the ordered phase of lithium ferrite [14]. For

the samples obtained by the citric acid and glycine routes, the

average crystallite sizes are obtained as 32 and 36 nm,

respectively.

The magnetization as a function of field strength, recorded at

room temperature, for the two different Li ferrite samples

synthesized by the citric acid and glycine autocombustion routes

is shown in Fig. 3. The saturation magnetization at 10 kOe is

obtained as 37.5 and 43.9 emu/g, respectively, for the samples

synthesized using citric acid and glycine. The corresponding

coercivities are obtained as 130 and 160 Oe. Very large values

Lettecrystallite size was calculated from the XRD line broadening

using the Scherrer relationship, d =0 .9 k /b cos u, where d is thecrystallite size in A, b is the half maximum line width of thepeak and k is the wavelength of X-rays [13]. The averagecrystallite size of the ferrite particles formed is obtained as 9 nm.

The XRD pattern of this sample is identical to that reported by

Dey et al. [12], who have reported that the disordered phase of

lithium ferrite is formed at this temperature. However, a closer

examination of the pattern indicated the presence of a very weak

reflection which corresponds to the most intense reflection fromacid (glycine) solutions were mixed in the metal to citric

acid (glycine) molar ratio of 1 :1. The solutions were

slowly evaporated on a laboratory hot plate kept at 200

-C. A thick viscous gel that formed underwent combus-tion immediately producing a powder. In another case, the

metal nitratescitric acid solution was slowly evaporated

on a water bath to form a viscous gel. Further drying was

carried out in an oven at 110 -C to remove the adsorbedwater completely. During this process, the gel swells into

a fluffy mass, which eventually breaks into little fakes.

The dried gel was decomposed in a furnace at 300 -C andthe powder formed was kept at the same temperature for

4 h and was further annealed at 600 -C for 4 h. The as-decomposed and annealed samples were characterized for

phase purity and crystallinity by powder X-ray diffraction

measurements (Philips, PW-1730) with CuKa radiationusing Ni filter. Magnetic measurements were carried out

using a PAR EG & G 4500 vibrating sample magneto-

meter (VSM).

3. Results and discussion

Fig. 1 shows the powder X-ray diffraction patterns of lithium

ferrite powders synthesized by the usual decomposition of the

citrate gel and annealed at 300 and 600 -C. The XRD pattern ofthe dried citrate gel, is actually Fe3O4 with minor

amounts of a-Fe2O3 as an impurity phase and thispowder on annealing at 600 -C forms the ordered phaseof lithium ferrite but with the a-Fe2O3 impurities stillpresent. However, we have synthesized the pure a-phaseof lithium ferrite by a simple autocombustion of the

metal nitrate/citric acid mixture at a low temperature of

200 -C and identical results are obtained when glycine isused as the fuel instead of citric acid.

2. Experimental

Lithium ferrite was synthesized from AR grade

chemicals by the following steps. Aqueous solutions of

metal nitrates in the required molar ratio, glycine and

S. Verma et al. / Materialsa-Fe2O3. Moreover, the cubic lattice parameter is obtained as8.36 A, which is larger than that of h-LiFe5O8 (8.33 A) [14] andis closer to that of Fe3O4 (8.38 A). When the powder sample is

heated to 600 -C, the pattern obtained corresponds to that of a-LiFe5O8, with a lattice parameter of 8.33 A, and the crystallite

size is increased to 40 nm. However, still the reflections due to a-Fe2O3 are clearly seen in the pattern, with increased intensities.

Approximately 710% of a-Fe2O3 is found in the sampleobtained at 300 and 600 -C, indicating that the a-Fe2O3 phaseformed initially is not converted to lithium ferrite on treatment at

600 -C. The increase in intensities of the peaks of the two phasesis due to the increased particle sizes of the ferrite and the a-Fe2O3 phases. The XRD pattern of the sample annealed at 600 -C

(a)

(b)

Fig. 1. Powder X-ray diffraction patterns of lithium ferrite synthesized by

the citrate gel method at 300 -C (a) and after annealing at 600 -C (b).

rs 59 (2005) 26302633 2631of the coercivity is obtained due to the single domain size limit

of smaller particles when compared to the reported small value

temperature of 200 -C in a single step. Usually this phase is

Fig. 2. Powder X-ray diffraction patterns of lithium ferrite synthesized by

the autocombustion method using citric acid (CA) and glycine (GA).

S. Verma et al. / Materials Lette2632of the coercivity (

[5] M.J. Shinkai, Biosci. Bioeng. 94 (2002) 606.

[6] M. Zahn, J. Nanopart. Res. 3 (2001) 73.

[7] D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8 (1996) 1770.

[8] R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359.

[9] C. Marcilly, P. Courty, B. Delmon, J. Am. Ceram. Soc. 53 (1970) 56.

[10] N.S. Gajbhiye, S. Prasad, G. Balaji, IEEE Trans. Magn. 35 (1999)

2155.

[11] V.K. Sankaranarayanan, O. Prakash, R.P. Pant, M. Islam, J. Magn.

Magn. Mater. 252 (2002) 7.

[12] S. Dey, A. Roy, D. Das, J. Ghose, J. Magn. Magn. Mater. 270 (2004)

224.

[13] B.D. Cullity, Elements of X-Ray Diffraction, 2nd edR, Addison-

Wesley, Reading, 1978, p. 99.

[14] P.B. Braun, Nature 170 (1952) 1123.

S. Verma et al. / Materials Letters 59 (2005) 26302633 2633

Low-temperature synthesis of nanocrystalline powders of lithium ferrite by an autocombustion method using citric acid and glycineIntroductionExperimentalResults and discussionConclusionsAcknowledgementsReferences