Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum

doi:10.1016/S0022-4596(03)00294-9

Journal of Solid State Chemistry

Volume 174, Issue 2, September 2003, Pages 424-430

https://doi.org/10.1016/S0022-4596(03)00294-9 Get rights and content

Abstract

A detailed Raman study on natural magnetite has been carried out. Raman spectra show four out of the five predicted Raman bands located at 668, 538, 306, and 193 rcm⁻¹. The location of the fifth, unobserved phonon mode, is inferred from spectra of other ferrites at 450–490 rcm⁻¹. Polarized experiments on an oriented single crystal provide a new interpretation of the Raman spectrum with the following assignment for symmetries of the observed modes: A_1g for 668 rcm⁻¹, E_g for 306 rcm⁻¹, and T_2g for 538, 193, and 450–490 rcm⁻¹. The results are compared with those of the earlier Raman studies and possible explanations for the discrepancies are suggested. Some of the inconsistencies can be resolved by considering the effect of oxidation of magnetite during the Raman experiments.

Introduction

Due to its geological and technological importance, and interesting physical properties, the iron oxide magnetite Fe₃O₄ has been subjected to a large number of studies employing different kinds of experimental techniques and theoretical approaches [1], [2], [3], [4], [5], [6]. Yet, despite the intense effort and significant advances, understanding some of its fundamental properties still remains an elusive goal. It is an intimate interplay of structural, electronic, and magnetic properties that makes the studies on this compound so difficult. Effects of order–disorder of cations between octahedral and tetrahedral sites, nonstoichiometry, presence of impurities and defects, oxidation, sample history and origin (e.g., natural vs. synthetic crystals), have to be disentangled in order to reveal the genuine intrinsic properties. Vibrational spectroscopy, both infrared and Raman, proved to be a powerful tool for a direct probing of lattice dynamics of this compound, particularly in the temperature dependent studies of optical phonons across the Verwey transition.

Raman measurements on magnetite have been carried out in several studies. Some of these were devoted to magnetite itself [1], [4], [7], [8], [9], [10], [11] while in others magnetite was considered as a secondary product of reactions [12], [13], [14], [15]. A symmetry analysis and an assignment of magnetite phonon modes based on the spinel structure were carried by Verble [7], Degiorgi et al. [9], Hart et al. [8], Graves et al. [16] and Gasparov et al. [1]. The results of these studies vary significantly either in the number of the observed Raman modes, or their positions and assignments. In this paper we present results obtained from a careful analysis of our extensive polarized Raman measurements, a critical evaluation of previously published spectroscopic studies on magnetite, and from the systematics of Raman spectra of ferrites.

Section snippets

Sample characterization

The sample of natural magnetite comes from the metamorphosed iron formation in Grängesberg, Sweden. The chemical composition is presented in Table 1 [17]. The unit-cell constant determined by X-ray powder diffraction has the value a=8.3945(11) Å, which is in a good agreement with published data for natural Fe₃O₄ a=8.3941(7) Å [18]. The polarized Raman spectra were taken from the polished (100) face of the single crystal of magnetite.

Experimental methods

The light-scattering experiments were conducted using a single

Results and discussion

Above the Verwey transition temperature (T_v=121 K), the iron oxide magnetite Fe³⁺(Fe²⁺Fe³⁺)O₄ crystallizes in the cubic space group Fd3m (O_h⁷), Z=8. It represents the classical example of an inverse spinel ferrite AB₂O₄. Oxygen atoms, having general positions 32e, form approximately a close-packed face centered cubic arrangement. The iron atoms occupy two crystallographically distinct sites being tetrahedrally A(8a) and octahedrally B(16d) coordinated by oxygen anions. The inverse nature of this

Summary and conclusions

Acknowledgements

We thank Prof. Hans Annersten for providing the sample of natural magnetite. The Swedish Research Council (Vetenskapsrådet) is gratefully acknowledged for the financial support.

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Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum

Abstract

Introduction

Section snippets

Sample characterization

Experimental methods

Results and discussion

Summary and conclusions

Acknowledgements

Corros. Sci.

Corros. Sci.

Corros. Sci.

Mater. Res. Bull.

Lithos

Spectrochim. Acta

Mat. Res. Bull.

J. Solid State Chem.

Phys. Rev. B.

Am. Mineral.

Am. Mineral.

Phys. Rev. Lett.

Phys. Rev. B.

Raman spectroscopic study of magnetite (FeFe₂O₄): a new assignment for the vibrational spectrum