| Summary: | Ferrihydrite (FHYD), a nanocrystalline material has long been described as a poorly crystalline disordered mineral mainly due to its small crystal size which is typically 2−6 𝑛𝑚. The three-dimensional structure of the mineral has long been described by a multi-phase structural model that consists of Fe3+ only in octahedral (Oh) coordination. In this model ferrihydrite is described as a mixture of two major phases (akaganeite/goethite-like f-phase and feroxyhite-like d-phase) and a minor ultradispersed nanohematite phase. This model has been recently challenged and a new, single-phase model was proposed, having a basic structural motif closely related to the Baker-Figgs δ-Keggin cluster and is isostructural with the mineral akdalaite, Al10O14(OH)2. In its ideal form, the proposed new structure of FHYD consist of 80 % Oh and 20 % tetrahedral (Td) Fe3+ polyhedra which can be adequately described by a single-domain model with the hexagonal spacegroup 𝑃63𝑚𝑐 and unit cell dimensions 𝑎=5.95 Å and 𝑐=9.06 Å. In this study, nanoparticles of 2-line FHYD (FHYD2), 2-line FHYD deposited onto SiO2 (FHYD2/SiO2) and 6-line FHYD (FHYD6) synthesised using rapid hydrolysis of Fe(NO3)3.9H2O solutions were characterized using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), Mössbauer spectroscopy (MS) as well as magnetization and magnetic susceptibility measurements. The coordination environment of iron atoms in the structure of FHYD was investigated using TEM and MS. The thermal transformation of FHYD nanoparticles was monitored through changes in the magnetization as a function of temperature and the reduction behaviour in hydrogen environment was studied using temperature programmed reduction (TPR), in-situ XRD and MS. Electron diffraction, TEM/ scanning TEM (STEM) imaging, and electron energy loss (EELS) measurements were carried out on three different microscopes viz. JEOL JEM-2100 LaB6 TEM, aberration corrected Schottky-FEG JEOL JEM-ARM200F HRTEM and cold-FEG Zeiss SESAM TEM. EELS studies were concentrated mainly on the iron 𝐿-edge of FHYD and iron oxides reference spectra with well known crystal structures. The iron oxide Fe 𝐿-edge is usually characterized by two intense sharp peaks termed “white lines”. The fine structures introduced by the crystal field effect on the 𝐿- edge contain information that is highly specific to the Fe3+ site symmetry.
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