Elsevier

Solid State Ionics

Volume 177, Issues 19–25, 15 October 2006, Pages 1865-1868
Solid State Ionics

Preparation of nanoscale MgFe2O4 via non-conventional mechanochemical route

https://doi.org/10.1016/j.ssi.2006.04.002Get rights and content

Abstract

The single-step synthesis of nanosized MgFe2O4 via mechanochemical processing of binary oxide precursors is reported. The mechanochemical route is followed by X-ray diffraction and 57Fe Mössbauer spectroscopy. The nanoscale nature of the mechanosynthesized material is evidenced by transmission electron microscopy. Quantitative information is provided on both ionic and spin configurations in nanosized MgFe2O4.

Introduction

Spinel ferrites of the type MFe2O4 (M is a divalent metal cation) are currently key materials for advancements in electronics, magnetic storage, ferrofluid technology, and many bioinspired applications (e.g., as drugs carriers for magnetically guided drug delivery and as contrast agents in magnetic resonance imaging) [1]. Conventional ceramic processing of ferrites requires a number of stages, including homogenization of the powder precursors, compaction of the reactants, and finally prolonged heat treatment at considerably elevated temperatures. One goal of modern ferrite research and development has been to identify simpler processing schemes that do not rely upon high temperature treatments for inducing solid-state reactions [2].

In this article, we will report on the single-step synthesis of MgFe2O4 via mechanochemical route at room temperature. Moreover, quantitative information is obtained on both the nonequilibrium cation distribution and the noncollinear spin arrangement in mechanosynthesized MgFe2O4. It should be emphasized that previous attempts to synthesize MgFe2O4 in a ball mill only led to the partial conversion to the ferrite [3]. To emphasize the site occupancy on the atomic level, the structural formula of MgFe2O4 spinel may be written as (Mg1−λFeλ)[MgλFe2−λ]O4, where parentheses and square brackets denote cation sites of tetrahedral (A) and octahedral [B] coordination, respectively, and where λ represents the degree of inversion defined as the fraction of the (A) sites occupied by Fe3+ cations. Pradhan et al. [4] have recently attempted to obtain information about the cation distribution in nanocrystalline mechanosynthesized MgFe2O4 using Rietveld analysis of the X-ray diffraction (XRD) data. However, the XRD technique loses much of its resolving power in nanoscale systems and does not allow a quantitative estimate of the degree of inversion in a disordered spinel.

Section snippets

Experimental

High-purity powders of α-Fe2O3 and MgO (Merck, Darmstadt) were used as precursors for the mechanosynthesis of MgFe2O4. Stoichiometric α-Fe2O3/MgO mixtures (10 g) were milled for various uninterrupted times (up to 12 h) in a Pulverisette 6 planetary ball mill (Fritsch, Idar-Oberstein) at room temperature. The grinding chamber (250 cm3 in volume) and balls (5 mm in diameter) were made of tungsten carbide. The ball-to-powder weight ratio was 20:1. Milling experiments were performed in air at

Results and discussion

The mechanically induced evolution of the α-Fe2O3/MgO mixture submitted to high-energy milling was followed by X-ray powder diffraction. XRD patterns of the samples milled for various times are shown in Fig. 1(a). The XRD pattern of the starting powder is characterized by sharp diffraction peaks corresponding to crystalline α-Fe2O3 (JCPDS PDF # 89-0599) and MgO (JCPDS PDF # 87-0653). During the early stages of milling (for milling times less than 2 h), XRD merely reveals a decrease of the

Conclusions

Nanosized MgFe2O4 with an average crystallite size of about 9 nm has been synthesized by high-energy milling of the α-Fe2O3/MgO mixture at room temperature. The mechanochemical route used represents a one-step procedure that does not require a subsequent firing step. The results obtained support a previously proposed model of a core/shell structure of ferrite nanoparticles consisting of an ordered particle core surrounded by a disordered surface shell. The near-surface layers of MgFe2O4

Acknowledgments

The work was supported by the Deutsche Forschungsgemeinschaft (DFG). One of the authors (V. Šepelák) thanks the DFG for supporting his work at the Center for Solid State Chemistry and New Materials, University of Hannover, within a Mercator Visiting Professorship. Partial support by the Grant Agency of the Ministry of Education of the Slovak Republic and of the Slovak Academy of Sciences (Grant 2/5146/25) and by the Alexander von Humboldt Foundation is gratefully acknowledged.

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1

On leave from the Slovak Academy of Sciences, Watsonova 45, SK-04353 Košice, Slovakia.

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