TY - JOUR
T1 - 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
AU - Fock, Jeppe
AU - Bogart, Lara K.
AU - González-Alonso, David
AU - Espeso, Jose I.
AU - Hansen, Mikkel Fougt
AU - Varón, Miriam
AU - Frandsen, Cathrine
AU - Pankhurst, Quentin A.
PY - 2017
Y1 - 2017
N2 - 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, δ̅RT-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 = Femagnetite/Fetotal = (δ̅RT − δo)/m, where δo = 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.
AB - 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, δ̅RT-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 = Femagnetite/Fetotal = (δ̅RT − δo)/m, where δo = 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.
KW - Magnetite
KW - Maghemite
KW - Composition of mixtures
KW - Stoichiometry of solid solutions
U2 - 10.1088/1361-6463/aa73fa
DO - 10.1088/1361-6463/aa73fa
M3 - Journal article
VL - 50
JO - Journal of Physics D: Applied Physics
JF - Journal of Physics D: Applied Physics
SN - 0022-3727
IS - 26
M1 - 265005
ER -