Polycrystalline texture causes magnetic instability in greigite

Magnetic stability of iron mineral phases is a key for their use as paleomagnetic information carrier and their applications in nanotechnology, and it critically depends on the size of the particles and their texture. Ferrimagnetic greigite (Fe$_{3}$S$_{4}$) in nature and synthesized in the laboratory forms almost exclusively polycrystalline particles. Textural effects of inter-grown, nano-sized crystallites on the macroscopic magnetization remain unresolved because their experimental detection is challenging. Here, we use ferromagnetic resonance (FMR) spectroscopy and static magnetization measurements in concert with micromagnetic simulations to detect and explain textural effects on the magnetic stability in synthetic, polycrystalline greigite flakes. We demonstrate that these effects stem from inter-grown crystallites with mean coherence length (MCL) of about 20 nm in single-domain magnetic state, which generate modifiable coherent magnetization volume (CMV) configurations in the flakes. At room temperature, the instability of the CVM configuration is exhibited by the angular dependence of the FMR spectra in fields of less than 100 mT and its reset by stronger fields. This finding highlights the magnetic manipulation of polycrystalline greigite, which is a novel trait to detect this mineral phase in Earth systems and to assess its fidelity as paleomagnetic information carrier. Additionally, our magneto-spectroscopic approach to analyse instable CMV opens the door for a new more rigorous magnetic assessment and interpretation of polycrystalline nano-materials

Early Oxidation Processes on the Greigite Fe₃S⁴(001) Surface by Water: A Density Functional Theory Study

Greigite (Fe3S4), the sulfide counterpart of the spinel-structured oxide material magnetite (Fe3O4), is a mineral widely identified in anoxic aquatic environments and certain soils, which can be oxidized, thereby producing extremely acid solutions of sulfur-rich wastewaters, so-called acid mine drainage (AMD) or acid rock drainage (ARD). Here we report a computational study of the partial replacement of sulfur (forming H2S) by oxygen (from H2O) in the Fe3S4(001) surface, derived from density functional theory calculations with on-site Coulomb approach and long-range dispersion corrections (DFT+U–D2). We have proposed three pathways for the oxidation of the surface as a function of H2O coverage and pH. Different pathways give different intermediates, some of which are followed by a solid-state diffusion of the O atom. Low levels of H2O coverage, and especially basic conditions, seem to be essential, leading to the most favorable energetic landscape for the oxidation of the Fe3S4(001) surface. We have derived the thermodynamic and kinetic profile for each mechanism and plotted the concentration of H2S and protons in aqueous solution and thermodynamic equilibrium with the stoichiometric and partially oxidized Fe3S4(001) surface as a function of the temperature. Changes in the calculated vibrational frequencies of the adsorbed intermediates are used as a means to characterize their transformation. We have taken into account statistical entropies for H2S and H2O and other experimental parameters, showing that this mineral may well be among those responsible for the generation of AMD

Computer modelling studies of mackinawite, greigite and cubic FeS

In this thesis we develop rigorous theoretical models for the simulation of the iron sulfides mackinawite, greigite and cubic FeS using both ab initio and interatomic potential methods. The mineral mackinawite (tetragonal FeS) takes a layered PbO-type structure, with Fe atoms coordinated tetrahedrally to S ligands. We have used GGA+U calculations to show that the inter-layer interaction is very difficult to accurately describe using this form of DFT, and instead a single-layer formulism is developed which allows the modelling of the electronic and magnetic properties of a single layer of mackinawite. These results are used to derive an interatomic potential to investigate the surfaces of this phase, and we use the calculated surface energies to successfully reproduce the observed crystal morphology of mackinawite. The effect of impurity atoms in the interlayer sites is investigated, and it is found that these contribute considerably to the stabilisation of the mackinawite structure. Greigite (Fe3S4) is the iron sulfide analogue of the famous iron oxide magnetite. We use spin-polarised GGA+U calculations to model the magnetic and electronic structure of greigite, and this phase is found to be most accurately described using an applied Ueff value of 1 eV. Further calculations show that a Verwey-type low temperature transition in greigite is energetically unfavourable. Cubic FeS takes the cubic sphalerite structure at room temperature. A low temperature transition to an antiferromagnetic orthorhombic structure has been observed experimentally. GGA+U calculations demonstrate that applying a value for the Hubbard Ueff parameter of 2 eV provides an excellent description of both the lowand high-temperature structures. It is found that the previously derived potential for mackinawite predicts the cubic FeS structure as well as non-spin-polarised GGA. The work described in this thesis has provided a greater understanding of the electronic, magnetic and structural properties of these iron sulfides

Formation and electrocatalysis studies of nickel and iron sulfide catalysts using in situ XAS

X-ray absorption spectroscopy (XAS) is an atom specific characterisation technique. It is particularly useful for understanding dilute systems of short range order. When used in situ during reaction processes, the technique is a powerful tool to study the evolving local structure of materials. In this thesis, XAS is used to understand the synthesis, stability, and electrocatalytic activity of iron sulfides. A popular route to the synthesis of nanoparticulate sulfides is the solvothermal decomposition of transition metal dithiocarbamates. These single source precursors are typically dissolved in a coordinating solvent and heated rapidly. The coordinating solvent acts as a heat sink and capping agent but its role in each decomposition process is less understood. Here, it is shown through ex situ and in situ XAS that the coordinating solvent has significant effect on the starting material and decomposition process through coordination to the transition metal centre and reaction with the dithiocarbamate backbone. Case studies include the decomposition of zinc dimethyl, and nickel and iron diisobutyl dithiocarbamates in oleylamine. The solvothermal decomposition of iron or iron and nickel diisobutyl dithiocarbamate in the presence of thiuram disulfide results in the formation of Fe3S4 or NiFe2S4 nanoparticles. These inverse spinel structures are formed as hexagonal sheets with 001 and 111 surfaces which are predicted to be electrocatalytically active. The oxidation of the systems in electrolyte during cyclic voltammetry – a process that is thought to reduce catalytic activity – is investigated by in situ XAS

The application of environmental magnetism to archaeological prospection: a semi-quantitative approach

Magnetic survey is the most widely used shallow geophysical techniquefor the location of archaeological activity in the United Kingdom but is often discountedftom use within alluviated landscapes. Results presented in this thesis from fluxgate gradiometer surveys conducted over an alluviated flood plain near the village of Yarnton, Oxfordshire, UK, demonstrate a wide variation of magnetic response between adjacent sites. This suggests a more complicated relationship between the rock magnetic properties of underlying archaeological sediments and the resultant magnetic anomalies recorded during surface magnetometer surveys. This study aims to investigate this relationship further and determine the influence of post-depositional mineral dissolution in water logged soils; a factor that together with increased alluvial overburden, has often been cited as an explanation for disappointing magnetic results over similar sites. A wide ranging study has been conducted including geophysical surveys, environmental magnetic analysis of archaeological sediments recovered during excavation and experimental work to investigate the influence of burning. Initial, rapid determination of isothermal magnetic parameters, such as susceptibility, has been complemented, for selected samples, by more detailed hysteresis measurements and thermomagnetic variation over a temperature range from 20 to 973K. Interpretation of the resulting data has been assisted through the development of semi-quantitative numerical models to describe the complex magnetic mixtures present. The results, including over 20ha (20, OOOOm2)of geophysical survey and the analysis of more than 500 samples, demonstrate the important role of fire for magnetic enhancement and also provides evidence, under suitable conditions, for more esoteric biogenic mechanisms. The main conclusions reached suggest archaeological features magnetised before the onset of floodplain conditions may still be detected through geophysical survey, particularly if more sensitive, caesium vapour magnetometers are applied In addition, semi-quantitative unmixing models allow both the thermal history of burnt sediments to be estimated and provide a means for identifying biogenically enhanced samples