An 57Fe Mössbauer effect study of poorly crystalline γ-FeOOH

Four synthetic lepidocrocite samples with a significant difference in crystallinity have been studied by the ⁵⁷Fe Mössbauer effect technique at temperatures ranging between 12 K and 360 K. In the magnetically ordered region, the spectra display broad hyperfine field distributions, the profile of which could be derived numerically. In a broad transition region of approximately 20 K, an additional doublet must be included in the fitting model. From Mössbauer thermoscanning measurements at zero-velocity and with and without the application of an external magnetic field, it is found that the doublet is due to Fe³⁺ species in a paramagnetic state. Neither the hyperfine field nor the Néel temperature distributions are markedly affected by the crystallinity. In the pure paramagnetic state, the broadened doublets are best described by a distribution of quadrupole splittings in the range 0.3–1.9 mm/s. By numerical manipulations of the derived probability profiles it was possible to distinguish between the iron species in the outer surface layers of the γ-FeOOH particles and those in the innermost layers, the relative amount of each being correlated with the measured surface areas. The centre shift is quite uniform and its temperature dependence, determined in detail for one of the samples, is perfectly described by the Debije approximation for the second-order Doppler shift. From the obtained Debije temperature, a Mössbauer fraction f of 0.79 at room temperature was calculated, which is in good agreement with the results obtained for a reference lepidocrocite/hematite mixture. Finally, line shape simulations aimed to explain the external field spectrum at 180 K provided strong indications that both the sign and the asymmetry parameter of the electric field gradient are non-uniform as well.