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Abstract Unlike most native metals, the unit cells of metal oxides tend to expand when crystallite sizes approach the nanoscale. Here we review different models that account for this behavior, and we present structural analyses for goethite (α-FeOOH) crystallites from ~10 to ~30 nm. The goethite was investigated during continuous particle growth via the hydrothermal transformation of 2-line ferrihydrite at pH 13.6 at 80, 90, and 100 °C using time-resolved, angle-dispersive synchrotron X-ray diffraction. Ferrihydrite gels were injected into polyimide capillaries with low background scattering, increasing the sensitivity for detecting diffraction from goethite nanocrystals that nucleated upon heating. Rietveld analysis enabled high-resolution extraction of crystallographic and kinetic data. Crystallite sizes for goethite increased with time at similar rates for all temperatures. With increasing crystallite size, goethite unit-cell volumes decreased, primarily as a result of contraction along the c-axis, the direction of closest-packing (space group Pnma). We introduce the coefficient of nanoscale contraction (CNC) as an analog to the coefficient of thermal expansion (CTE) to compare the dependence of lattice strain on crystallite size for goethite and other metal oxides, and we argue that nanoscale-induced crystallographic expansion is quantitatively similar to that produced when goethite is heated. In addition, our first-order kinetic model based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation yielded an activation energy for the transformation of ferrihydrite to goethite of 72.74 ± 0.2 kJ/mol, below reported values for hematite nucleation and growth.
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Simulation of Natural Iron Oxide Alteration in Soil: Conversion of Synthetic Ferrihydrite to Hematite Without Artificial Dopants, Observed With Magnetic Methods
Abstract We present new results on the conversion of pure, undoped synthetic ferrihydrite, wet‐annealed at pH 6.56 and 90°C without stabilizing ligands, to nanophase goethite, hematite, and an intermediate magnetic phase, nanophase maghemite. Our analyses included magnetic field and temperature‐dependent properties and characterization by powder X‐ray diffraction, Mössbauer spectra, and high‐resolution transmission electron microscopy. We sampled alteration products after 0.5 hr, and then in a geometric progression to 32 hr, yielding a detailed examination of the earliest alteration phases. There are many similarities to the latest studies of pure ferrihydrite alteration but with a significant difference: We observe early appearance of oriented nanophase goethite along with a soft magnetic contribution, while rhombohedral hematite crystals form later, as reported in previous studies. Our observations attest to the non‐uniqueness of the magnetic enhancement process and to its strong dependence on environmental conditions, with important implications for use of the hematite/goethite ratio as a paleoprecipitation proxy.
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- Award ID(s):
- 1642268
- PAR ID:
- 10359816
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 21
- Issue:
- 7
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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