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

We evaluate the application of ⁵⁷Fe Mössbauer spectroscopy to the determination of the composition of magnetite (Fe₃O₄)/maghemite (γ-Fe₂O₃) mixtures and the stoichiometry of magnetite-maghemite solid solutions. In particular, we consider a recently proposed model-independent method which does not rely on a priori assumptions regarding the nature of the sample, other than that it is free of other Fe-containing phases. In it a single parameter, δ̅_RT-the ‘centre of gravity’, or area weighted mean isomer shift at room temperature, T = 295 ± 5 K-is extracted by curve-fitting a sample’s Mössbauer spectrum, and is correlated to the sample’s composition or stoichiometry. We present data on high-purity magnetite and maghemite powders, and mixtures thereof, as well as comparison literature data from nanoparticulate mixtures and solid solutions, to show that a linear correlation exists between δ̅RT and the numerical proportion of Fe atoms in the magnetite environment: α = Fe_magnetite/Fe_total = (δ̅_{RT −} δ_o)/m, where δ_o = 0.3206 ± 0.0022 mm s⁻¹ and m = 0.2135 ± 0.0076 mm s⁻¹. We also present equations to relate α to the weight percentage w of magnetite in mixed phases, and the magnetite stoichiometry x = Fe²⁺/Fe³+ in solid solutions. The analytical method is generally applicable, but is most accurate when the absorption profiles are sharp; in some samples this may require spectra to be recorded at reduced temperatures. We consider such cases and provide equations to relate δ̅(T) to the corresponding α value.