Structural and Magnetic Properties of Ferrihydrite

Abstract

Abstract--57Fe MSssbauer spectra of two synthetic samples of ferrihydrite, recorded at 4.2 K in applied fields of up to 9 T, have been analysed by a mean-field model. The samples exhibit two and six X-ray diffraction peaks. It is shown that only one ferric ion site is present in the mineral, and that in this site the ions are octahedrally coordinated. The spectra show the presence of different magnetic states: ferrimagnetism in two-line ferrihydrite, and antiferromagnetism in six-line ferrihydrite. The ferrimagnetism in two-line ferrihydrite is analysed in terms of random fluctuations arising from the small numbers of ferric ions per particle, and it is shown that the different magnetic states may arise purely as a result of these fluctuations.

Transformation of 2-line ferrihydrite to 6-line ferrihydrite under oxic and anoxic conditions

“…The room temperature parameters were similar to those recently reported for a room temperature spectrum of a synthetic 2-line ferrihydrite, and are typical of Fe 3+ in octahedral coordination (Rancourt et al 2001). The assignment of Fe 3+ sites to octahedral coordination is in agreement with the Mössbauer spectroscopy data of ferrihydrite by Pankhurst and Pollard (1992). The objective of the modeling of the 77 K spectrum was to utilize its fit parameters in describing the 35 month aged sample (discussed below).…”

Ferrihydrite: Surface Structure and Its Effects on Phase Transformation

“…The spectra for bulk Fe304 and "y-Fe203 are asymmetric with zero field and the asymmetry becomes more pronounced with an applied field, because the 6 and A values for the tetrahedral Fe(II, III) sites are significantly different from those for the octahedral Fe(III) sites (Amstrong et al 1966;Hargrove and Kiindig 1970). Based on the above observation, Pankhurst and Pollard (1992) concluded that iron ions in ferrihydrite are exclusively octahedrally coordinated.…”

“…Recently, Pankhurst and Pollard (1992) reported M/Sssbauer spectra for 2-line and 6-line ferrihydrites at 4.2 K and with external magnetic fields from zero to 9 T. For both samples, the Mrssbauer spectra exhibit very symmetric absorption lines and, as the external field increases, the spectra split into two sets of six-line magnetic components, although the two sextets for the 6-line ferrihydrite are less resolved and the spectrum was fitted with one sextet by the authors. Because the spec-trum for the 2-line ferrihydrite is very symmetric, a single value of isomer shift (b = 0.48 mm/s) was used by Pankhurst and Pollard (1992) to fit the spectra and the quadruple splittings (A) for the two sextets were found to be equal within their uncertainties (no specific data were given by Pankhurst and Pollard 1992). The 6 and A values obtained for the two sextets are characteristic of the octahedrally coordinated Fe(III) at liquid helium temperature.…”

“…Their conclusion was disputed by Manceau et al (1990) on the basis of the XAFS spectra collected at synchrotron radiation facilities. The presence of tetrahedral sites in ferrihydrite has also been a controversial subject in the interpretation of the Mrssbauer spectra (Cardile 1988;Murad 1988;Pankhurst and Pollard 1992).…”

Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

“…As such, the basic crystal structure (4)(5)(6)(7) and physical properties of ferrihydrite [e.g., density, composition (7,8), and magnetic properties (9)(10)(11)(12)(13)(14)] have remained controversial. A variety of structural models have been proposed for ferrihydrite (see (4) for review) but all have proven difficult to confirm unequivocally by conventional crystallographic techniques.…”