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, ${\overline{\delta }}_{\text{RT}}$—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 ${\overline{\delta }}_{\text{RT}}$ and the numerical proportion of Fe atoms in the magnetite environment: $\alpha$  =  Fe_magnetite/Fe_total $=({\overline{\delta }}_{\text{RT}}-{\delta }_{\text{o}})/m$, where ${\delta }_{\text{o}}$  =  0.3206  ±  0.0022 mm s⁻¹ and $m$  =  0.2135  ±  0.0076 mm s⁻¹. We also present equations to relate $\alpha$ 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 $\overline{\delta }\left(T\right)$ to the corresponding α value. (paper)