⁵⁷Fe Mössbauer Effect Study of Al-Substituted Lepidocrocites

Abstract

Abstract--Seven A|-containing lepidocrocite samples, ~/-Fe,_xAlxOOH, prepared from FeC12/AI(NO3)3 solutions with initial A1/(A1 + Fe) mole ratios C~ of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 tool/ mol, were examined by means of M6ssbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80~ consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing C i are inconclusive as to whether the A1 cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the therrnoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free ~-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of A1 cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with C~ of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present A1 has been incorporated into the structure.

Fe(II) reduction of pyrolusite (β-MnO2) and secondary mineral evolution

“…9 and Additional file 1 : Table S1). Regular distribution of Mn within Fe precipitates could be evidence for cation substitution of Mn into lepidocrocite, which may also explain the low ordering temperature observed in 57 Fe Mössbauer spectra as was previously observed with Al substitution in lepidocrocite [ 45 ]. Mn and Fe(III) dissolution occurred in an approximately 1:1 ratio rather than a 1:2 ratio predicted by Eq.…”

2H and 27Al Solid-State NMR Study of the Local Environments in Al-Doped 2-Line Ferrihydrite, Goethite, and Lepidocrocite

“…Aluminum substitution into the structural frameworks of iron oxides/oxyhydroxides such as ferrihydrite, goethite (α-FeOOH), and lepidocrocite (γ-FeOOH) has been studied extensively since Al-substituted compounds are prevalent in soil, particularly in weathering environments . A significant number of studies have focused on establishing the limit of solid solutions and local environments of Al in these materials, − using techniques such as elemental analysis, thermal analysis, Mössbauer spectroscopy, − and X-ray powder diffraction (XRD). ,, However, the extent of Al substitution is difficult to determine unambiguously since the materials are not always single phase and are typically nanosized, with poorly crystalline aluminum oxide/hydroxide precipitates often being present in addition to the Al-substituted iron containing phases. Furthermore, Al substitution generally decreases the crystallinity of the iron containing phases.…”

Rethinking the magnetic properties of lepidocrocite: A density functional theory and cluster expansion study

“…[6,12] One of the most common explanations is that the measured low temperature magnetic disorder is due to superparamagnetism arising from the nanoscale domain sizes in lepidocrocite platelets used in experiment. [5][6][7] Evidence for higher temperature magnetic order in bulk lepidocrocite has been determined to be consistent with a model of uncompensated surface magnetic moments present in small particles. [8,13] Yet this study also conclude that superparamagnetism is due to the presence of small diameter maghemite phases, maintaining that the Néel temperature of lepidocrocite is between 50 and 77K.…”

“…[1] While it has wide general relevance, its industrial application derives from its high temperature thermal decomposition to maghemite, [2] whose thin films are ubiquitous in magnetic storage, transistors, and other modern electronics applications. [3] The magnetic properties of lepidocrocite have been studied extensively, [4][5][6][7][8] but there remain discrepancies between the experimentally measured behavior and what might be expected from this class of mineral. The magnetochemistry of iron oxides is governed primarily by the superexchange interactions.…”

“…A number of theories have been presented to account for the experimentally measured low temperature transition of lepidocrocite. These include amorphous crystallinity, [4] the presence of water impurities, [5] or small concentrations of other magnetic ion inclusions. [6,12] One of the most common explanations is that the measured low temperature magnetic disorder is due to superparamagnetism arising from the nanoscale domain sizes in lepidocrocite platelets used in experiment.…”