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. (13th June 2017)

• Record Type: Journal Article
• Authors: Fock, Jeppe; Bogart, Lara K; González-Alonso, David; Espeso, Jose I; Hansen, Mikkel F; Varón, Miriam; Frandsen, Cathrine; Pankhurst, Quentin A
• Is Part Of: Journal of physics. Volume 50:Number 26(2017)
• Publication Date: 2017-06-13
• Subjects: magnetite -- maghemite -- composition of mixtures -- stoichiometry of solid solutions -- Mössbauer spectroscopy; Physics -- Periodicals; 530
• DOI: 10.1088/1361-6463/aa73fa
• Languages: English
• ISSNs: 0022-3727

Abstract

Abstract: We evaluate the application of 57 Fe Mössbauer spectroscopy to the determination of the composition of magnetite (Fe3 O4 )/maghemite ( γ -Fe2 O3 ) 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, δ ¯ 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 δ ¯ RT and the numerical proportion of Fe atoms in the magnetite environment: α = Femagnetite /Fetotal = ( δ ¯ 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 = Fe 2+ /Fe 3+ 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 δ ¯ ( T ) to the corresponding α value.