Instability of the rhodium magnetic moment as origin of the metamagnetic phase transition in alpha-FeRh

Based on ab initio total energy calculations we show that two magnetic states of rhodium atoms together with competing ferromagnetic and antiferromagnetic exchange interactions are responsible for a temperature induced metamagnetic phase transition, which experimentally is observed for stoichiometric alpha-FeRh. A first-principle spin-based model allows to reproduce this first-order metamagnetic transition by means of Monte Carlo simulations. Further inclusion of spacial variation of exchange parameters leads to a realistic description of the experimental magneto-volume effects in alpha-FeRh.Comment: 10 pages, 13 figures, accepted for publication in Phys. Rev.

Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments

Iron oxides affect many elemental cycles in aquatic sediments via numerous redox reactions and their large sorption capacities for phosphate and trace elements. The reactive ferric oxides and oxyhydroxides are usually quantified by operationally defined selective chemical extractions that are not mineral specific. We have used cryogenic 57Fe Mössbauer spectroscopy to show that the reactive iron oxyhydroxide phase in a large variety of lacustrine and marine environments is nanophase goethite (α-FeOOH), rather than the assumed surface-complex–stabilized, two-line ferrihydrite and accompanying mixture of clay and oxyhydroxide Fe-bearing phases. This result implies that the kinetic and stability parameters of the type of nanogoethite that we observe to be present in sediments should be first determined and then used in models of early diagenesis. The identity and characteristics of the reactive phase will also set constraints on the mechanisms of its authigenesis

Control of site populations, at synthesis, by inter-sheet differential thermal expansion in a 2:1 layer silicate

We have measured octahedral Fe3+ and tetrahedral Fe3+ site populations, in annite samples equilibrated at different temperatures under the C-CH4 buffer at 2 kbar, with sufficient accuracy (0.2-1 %total-Fe) to test a lateral constraint model that includes differential thermal expansion of the tetrahedral and octahedral bonds, in addition to previously proposed tetrahedral rotation (a) and octahedral flattening (1Jr). We used Mossbauer spectroscopy with data treatment and spectral analysis methods including: (1) analytic methods for removing spectral distortions associated with the effec ts of absorber thickness; and (2) a Voigt-based fitting method that allows arbitrary-shape quadrupole splitting distributions (QSDs) for each site. The usual constant-temperature crystal chemical models based on regular polyhedral constructions are shown to give correct b lattice parameter predictions at room temperature, if tetrahedral elongation (along c) is taken into account by using effective tetrahedral bond lengths based on basal 0-0 distances. When, in addition, we include thennal expansion coefficients for the tetrahedral and octahedral bonds (a, and a 0 , respectively), we lind that our measured site populations are consistent with reasonable parameter values: "' = 56.8 \ub1 0.6 degrees, at synthesis, and ao - a, = 32.3-33. I X I 0"6 \ub0C '' where a, is taken to lie in the physical range 0 - 20 x 10\ub76 \ub0C1 and a (the tetrahedral rotation angle) is assumed to be zero at synthesis. This demonstrates the possibility of undenstanding site populations in annite in terms of such models and suggests that differential thermal expansion plays a key role. We also obtain an accurate lower bound for Fe3~1Fe, .. in annite of9.59(9) %, which represents the limit imposed by lateral misfit structural constraints under highly reducing conditions. Finally, we observe a clear migration from octahedral Fe\ub7'- to tetrahedral Feh as equilibration temperature increases. This feature is not elucidated by our model and is presumably due to more subtle effects. such as the temperature dependence of hydrogen fugacity under a given buffer.Peer reviewed: YesNRC publication: Ye

A Mössbauer spectroscopic study of the iron redox transition in eastern Mediterranean sediments

Fe cycling at two sites in the Mediterranean Sea (southwest of Rhodes and in the North Aegean) has been studied, combining the pore water determination of nutrients, manganese, and iron, citrate-bicarbonate-dithionite (CDB) and total sediment extractions, X-ray diffraction, and 57Fe Mössbauer spectroscopy (MBS). At the Rhodes site, double peaks in the CDB-extractable Mn and Fe profiles indicate non-steady-state diagenesis. The crystalline iron oxide hematite, identified at both sites by room temperature (RT) MBS, appears to contribute little to the overall Fe reduction. MBS at liquid helium temperature (LHT) revealed that the reactive sedimentary Fe oxide phase was nanophase goethite, not ferrihydrite as is usually assumed. The pore water data at both sites indicates that upon reductive dissolution of nanophase goethite, the upward diffusing dissolved Fe2+ is oxidized by Mn oxides, rather than by nitrate or oxygen. The observed oxidation of Fe2+ by Mn oxides may be more common than previously thought but not obvious in sediments where the nitrate penetration depth coincides with the Mn oxide peak. At the Rhodes site, the solid-phase Fe(II) increase occurred at a shallower depth than the accumulation of dissolved Fe2+ in the pore water. The deeper relict Mn oxide peak acts as an oxidation barrier for the upward diffusing dissolved Fe2+, thereby keeping the pore water Fe2+ at depth. At the North Aegean site, the solid-phase Fe(II) increase occurs at approximately the same depth as the increase in dissolved Fe2+ in the pore water. Overall, the use of RT and cryogenic MBS provided insight into the solid-phase Fe(II) gradient and allowed identification of the sedimentary Fe oxides: hematite, maghemite, and nanophase goethite

A Mössbauer spectroscopic study of the iron redox transition in eastern Mediterranean sediments

Fe cycling at two sites in the Mediterranean Sea (southwest of Rhodes and in the North Aegean) has been studied, combining the pore water determination of nutrients, manganese, and iron, citrate-bicarbonate-dithionite (CDB) and total sediment extractions, X-ray diffraction, and 57Fe Mössbauer spectroscopy (MBS). At the Rhodes site, double peaks in the CDB-extractable Mn and Fe profiles indicate non-steady-state diagenesis. The crystalline iron oxide hematite, identified at both sites by room temperature (RT) MBS, appears to contribute little to the overall Fe reduction. MBS at liquid helium temperature (LHT) revealed that the reactive sedimentary Fe oxide phase was nanophase goethite, not ferrihydrite as is usually assumed. The pore water data at both sites indicates that upon reductive dissolution of nanophase goethite, the upward diffusing dissolved Fe2+ is oxidized by Mn oxides, rather than by nitrate or oxygen. The observed oxidation of Fe2+ by Mn oxides may be more common than previously thought but not obvious in sediments where the nitrate penetration depth coincides with the Mn oxide peak. At the Rhodes site, the solid-phase Fe(II) increase occurred at a shallower depth than the accumulation of dissolved Fe2+ in the pore water. The deeper relict Mn oxide peak acts as an oxidation barrier for the upward diffusing dissolved Fe2+, thereby keeping the pore water Fe2+ at depth. At the North Aegean site, the solid-phase Fe(II) increase occurs at approximately the same depth as the increase in dissolved Fe2+ in the pore water. Overall, the use of RT and cryogenic MBS provided insight into the solid-phase Fe(II) gradient and allowed identification of the sedimentary Fe oxides: hematite, maghemite, and nanophase goethite. Copyright © 2005 Elsevier Ltd.SCOPUS: ar.jinfo:eu-repo/semantics/publishe