Journal of Crystallization Process and Technology
 Vol. 2  No. 4 (2012) , Article ID: 23431 , 4 pages DOI:10.4236/jcpt.2012.24022

Rietveld Refinement of Nanocrystalline LiFeO2 Synthesized by Sol-Gel Method and Its Structural and Magnetic Properties

K. Vijaya Kumar1, A. Sangeetha2, A. T. Raghavender3, Z. Skoko4, G. Nanda Kumar5

1Department of Physics, Jawaharlal Nehru Technological University Hyderabad College of Engineering, Nachupally (Kondagattu), Karimnagar-Dist., A. P., India; 2Department of Physics, Indur Institute of Engineering and Technology, Siddipet, Medak-Dist., A. P., India; 3Nanomagnetism Laboratory, Department of Physics and Astronomy, Seoul National University, Seoul, South Korea; 4Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia; 5Department of Geo-Physics, Osmania University, Hyderabad, A. P., India.

Email: kvkphd@gmail.com

Received May 10th, 2012; revised June 13th, 2012; accepted July 1st, 2012

Keywords: Nanoparticles; LiFeO2; Structural and Magnetic Properties

ABSTRACT

Nanocrystalline lithium iron oxide LiFeO2 was synthesized using sol-gel method. Rietveld analysis was performed to confirm the different phases associated in the formation of LiFeO2. Quantitative Rietveld refinement revealed that sample contains: 39.9 wt% of cubic α-LiFeO2 phase, 58.5 wt% of monoclinic β-LiFeO2 and tetragonal 1.7 wt% of γ-LiFeO2. The nanocrystalline nature of the prepared samples was confirmed by SEM analysis. The magnetic properties of LiFeO2 showed ferromagnetic property at room temperature.

1. Introduction

Lithium iron oxide was found to be the most promising and very interesting materials due to their potential applications as a cathode for rechargeable lithium batteries and also due to low cost and toxicity [1-3]. Lithium iron oxide LiFeO2 has NaCl type cubic closed packed (ccp) crystal structure with Li+ and Fe3+ being distributed in octahedral sites. LiFeO2 crystallizes in different polymorphic modifications (α, β, γ) based on the synthesis techniques and preparation conditions [4-6]. The unit cell of α-LiFeO2 has cubic distorted form with space group Fm3m. In α-LiFeO2 structure, Li+ and Fe3+ ions occupy randomly the octahedral sites [7]. γ-LiFeO2 structure is tetragonal cation disordered. Li+ and Fe3+ in the octahedral sites transform from cubic structure (Fm3m) to tetragonal (I4/m) structure [8]. In the case of monoclinic β-LiFeO2 the cation ordering was detected [5,8]. It was observed that synthesis of LiFeO2 is a difficult task as several phases are associated during preparation process. Earlier LiFeO2 have been synthesized by different techniques such as hydrothermal [6,9], citrate precursor method [10], solid state reaction [11], ion exchange reaction [12] etc. and observed different polymorphic phases and improved structural and electrical properties. In this paper we made an attempt to synthesize nanocrystalline LiFeO2 using sol-gel method. To clearly understand the structural formation and the corresponding phases of LiFeO2 and the underlying magnetic properties we have carried the present work.

2. Experimental

The LiFeO2 nanoparticles have been synthesized by sol-gel method [13]. The AR grade citric acid (C6H8O7·H2O), ferric nitrate (Fe(NO3)3·9H2O) and lithium nitrate (LiNO3) (≥99%) were used as starting materials. The entire synthesis procedure is described elsewhere [13]. The as prepared powder samples were sintered at 500˚C for 5 h.

Crystallographic structure of LiFeO2 nanopowder was measured using Philips PW 3020 Bragg-Brentano diffractometer using Cu Kα radiation (wave length λ = 1.54 Å). The morphology of powder was observed using scanning electron microscopy (SEM) from Carl Zeiss. Room temperature magnetization was measured using ADE magnetics DMS 4 Vibrating Sample Magnetometer (VSM).

3. Results and Discussions

Figure 1 shows (a) experimental and (b) calculated X-ray diffraction patterns of nanocrystalline LiFeO2. Crystal structures of different phases present in the

(a)(b)

Figure 1. Rietveld analysis of nanocrystalline LiFeO2 (a) experimental and (b) calculated data.

sample were refined by the Rietveld method. The analysis started using the structural models from the Inorganic Crystal Structure Database (2011) as follows: card No. 174084-ICSD, 28366-ICSD, 28364-ICSD for LiFeO2 monoclinic, tetragonal and cubic structures. Rietveld structure refinement was performed with the program X’Pert High Score Plus (PANalytical 2009) using a pseudo-Voigt profile function and polynomial background model. During the Rietveld analysis, the following parameters were refined: profile parameters W and V, asymmetry parameter 1 and peak shape parameter 1, as well as the atomic fractional coordinates. Isotropic displacement parameters were assumed for all atoms. The preferred-orientation correction did not significantly improve the fit. Refinement converged with Rwp 9.8% which indicates a good reliability of the result. Refined unit-cell parameters for both phases were as follows:

1) Crystal structure—Monoclinic (β-Phase, 174084 ICSD), space group C12/c1 (15), lattice parameters a = 5.795(3) Å, b = 11.580 (5) Å, c = 5.157 (2) Å.

2) Crystal structure—Tetragonal (γ-Phase, 28366 ICSD), space group I4/m (87), lattice parameters a = 2.8926 (9) Å, c = 4.283 (2) Å.

3) Crystal structure—Cubic (α-Phase, 28364 ICSD), space group Fm3m (225), lattice parameters a = 4.1585 (8) Å.

Quantitative Rietveld refinement revealed that sample contains: 39.9 wt% of cubic α-LiFeO2 phase, 58.5 wt% of monoclinic β-LiFeO2 and tetragonal 1.7 wt% of γ- LiFeO2.

The morphology of nanocrystalline LiFeO2 as observed from SEM is shown in Figure 2. The morphology shows the un-even particle size distribution with an average particle size of 100 nm.

LiFeO2 powder showed spontaneous magnetization at room temperature as shown in Figure 3. The magnetization curve clearly shows the ferromagnetic behavior having the maximum magnetization value 0.2 emu/g and coercivity of 189 Oe. The magnetization curve does not seem to be saturated even with the maximum applied field of 20 kOe. The observed magnetization value is very small compared to LiFeO2 prepared using different techniques. The lower magnetization value in our nanocrystalline LiFeO2 may be probably due to the occurrence of different phases in the synthesized sample. Tabuchi et al. [8,14] showed that the individual α, β, γ LiFeO2 phases behave paramagnetic at low temperature and ferromagnetic at room temperature. Ferromagnetic impurities such as LiFe5O8 could influence the magnetic properties of LiFeO2. It was observed that LiFeO2 is antiferromagnetic below 90 - 280 K [15,16]. But in our samples we could not see any ferromagnetic impurities. The ferromagnetism observed for our samples may be due to the presence of different phases in our LiFeO2. For a single phase LiFeO2 it is expected to show higher magnetization value, where as for multiple phases in LiFeO2 could lower the magnetization values.

4. Conclusion

Nanocrystalline lithium iron oxide LiFeO2 was synthesized using sol-gel method. Rietveld analysis showed the different phases corresponding to α—cubic, β—monoclinic and γ—tetragonal polymorphic LiFeO2 phases. SEM analysis showed the nanocrystallinity with particle size of 100 nm. The synthesized sample showed ferromagnetic property at room temperature.

Figure 2. SEM image of nanocrystalline LiFeO2.

Figure 3. M-H curve of nanocrystalline LiFeO2, measured at 300 K. Inset shows the expanded field curve.

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