Magnetite or maghemite? There is a simple answer. (Vol. 48, No. 5-6)

Correlation between δRT, the ‘centre of gravity’ of the room temperature 57Fe Mössbauer spectrum of a magnetite/maghemite sample vs. the fraction of 57Fe atoms present in magnetite, α.

The composition or stoichiometry of magnetite (Fe3O4) and maghemite (γ-Fe2O3) mixtures or solid solutions is important for the physical, geological and material sciences. It is also significant in biomedical science, where magnetic nanoparticles are used both in vitro and clinically, and where both ferrous and ferric iron ions play active roles in the production of reactive oxygen species. However, the accurate determination of the composition/stoichiometry can be tricky, as it requires either well-crystallised samples suitable for x-ray diffraction, or it relies on chemical dissolution methods that, depending on the nature of the sample, are often either unfeasible or inappropriate.

However, there is a simple answer, in the form of the recently proposed 57Fe Mössbauer spectroscopic ‘centre of gravity method’. The COG method is non-destructive and determines the composition/stoichiometry from the mean isomer shift, ¬δRT. It is well suited to nanomaterials, is simple and straightforward, and as long as appropriate measures and protocols are observed – all of which are explained in the paper – even inexperienced users will find little difficulty in its implementation.

J. Fock, L. K. Bogart, D. González-Alonso, J. I. Espeso, M. F. Hansen, M. Varón, C. Frandsen and Q. A. Pankhurst, On the ‘centre of gravity’ method for measuring the composition of magnetite/ maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via 57Fe Mössbauer spectroscopy, J. Phys. D: Appl. Phys. 50, 265005 (2017)