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We thank Richard Pattrick and Vicky Coker for help in collecting XMCD on these samples at the Daresbury SRS and subsequently at the Advanced Light Source (ALS), Berkeley. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES) of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231 and we thank Elke Arenholz for her assistance. RJH acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 320750. KMR gratefully acknowledges support from the DOE OBES Chemical Sciences, Geosciences, and Biosciences Division, through the Geosciences Program at Pacific Northwest National Laboratory. We also thank Gerrit van der Laan and Nick Telling for help with XMCD data analysis; David Plant carried out the electron microprobe analyses at Manchester and Paul Schofield provided information on the natural magnesian spinel. We also thank two anonymous referees for constructive comments.

A critical radiation damage assessment of the materials that will be present in a Geological Disposal Facility (GDF) for radioactive waste is a priority for building a safety case. Detailed analysis of the effects of high-energy α-particle damage in phyllosilicates such as mica is a necessity, as these are model structures for both the clay-based backfill material and the highly sorbent components of a crystalline host rock. The α-radiation stability of biotite mica [general formula: K(Mg,Fe) 3 (Al,Si 3 O 10 )(F,OH) 2 ] has been investigated using the 5 MV tandem pelletron at the University of Manchester’s Dalton Cumbrian Facility (DCF) and both the microfocus spectroscopy (I18) and core X-ray absorption spectroscopy (B18) beamlines at Diamond Light Source (U.K.). Microfocus X-ray diffraction mapping has demonstrated extensive structural aberrations in the mica resulting from controlled exposure to the focused 4 He 2+ ion (α-particle) beam. Delivered doses were comparable to a-particle fluences expected in the highly active, near-field of a GDF. At doses up to 6.77 displacements per atom (dpa) in the region of highest particle fluence, biotite mica displays a heterogeneous structural response to irradiation on a micrometer scale, with sequential dilation and contraction of regions of the structure perpendicular to the sheets, as well as a general overall contraction of the phyllosilicate layer spacing. At the peak of ion fluence, the structure collapses under a high point defect density and amorphous areas are pervasive among altered domains of the original lattice. Such structural alterations are likely to affect the material’s capacity to sorb and retain escaped radionuclides over long timescales; increased edge site availability may favor increased sorption while interlayer uptake will likely be reduced due to collapse. Radiation-induced reduction of structural iron at the region of highest structural damage across an α-particle’s track has been demonstrated by Fe K -edge X-ray absorption near edge spectroscopy (XANES) and local structural disorder has been confirmed by analysis of both potassium K -edge XANES and Fe K -edge extended X-ray absorption fine structure analysis. An infrared absorption study of deformations in the OH − stretching region, along with electron probe microanalysis complements the synchrotron data presented here.

The complex, nanometer-scale structural changes resulting from long-term α-particle bombardment of the mineral biotite are revealed for the first time using high-resolution transmission electron microscope (HRTEM) imaging. Radiohaloes are the product of high-energy α-particles emitted from radioactive inclusions penetrating into the surrounding mineral over long (∼1.8 Ga) timescales, resulting in intense discoloration attributed to ionization events and structural damage. HRTEM analysis of these radiohaloes reveals the long-term breakdown of the biotite structure into three distinct domains. Nanometer-scale, neo-phase regions of dilated and contracted mica structure, with periodicities comparable to 1:1 phyllosilicates, are bound by semi-amorphous, high-defect density domains. These are periodically interspersed with areas of near-original biotite structure. Across the halo region, damaged crystallites have become misoriented, revealing changes in the mica layer-to-layer spacing. This nanoscale response of the mica structure has profound implications for understanding the performance of phyllosilicates in barrier systems employed in the safe isolation of nuclear waste, as materials such as these will be relied upon to retard radionuclide migration over the lifetime of a geological disposal facility.

The thermodynamics of mixing and its dependence on cation distribution in the Fe3O4–Fe2TiO4 (magnetite-ulvospinel) spinel solid solution were studied using high-temperature oxide melt solution calorimetry and a range of structural and spectroscopic probes. The enthalpies of formation of ilmenite and ulvospinel from the oxides and from the elements were obtained using oxidative drop solution calorimetry at 973 K in molten sodium molybdate. The enthalpy of mixing, determined from the fit to the measured enthalpies of drop solution calorimetry, is endothermic and represented by a quadratic formalism, Δ H mix = (22.60 ± 8.46) x (1 − x ) kJ/mol, where x is the mole fraction of ulvospinel. The entropies of mixing are more complex than those for a regular solution and have been calculated based on average measured and theoretical cation distributions. Calculated free energies of mixing show evidence for a solvus at low temperature in good agreement with that observed experimentally.

A consistent methodology for obtaining enthalpy of formation of Fe{sup 2+}-containing binary and multicomponent oxides using high temperature oxide melt solution calorimetry has been developed. The enthalpies of wuestite (FeO) and magnetite (Fe{sub 3}O{sub 4}) oxidation to hematite (Fe{sub 2}O{sub 3}) were measured using oxidative drop solution calorimetry in which the final product is dissolved ferric oxide. Two methods were applied: drop solution calorimetry at 1073 K in lead borate solvent and at 973 K in sodium molybdate, each under both oxygen flowing over and bubbling through the solvent, giving consistent results in agreement with literature values. The enthalpies of formation of all three iron oxides from the elements were obtained using a thermodynamic cycle involving the directly measured oxidative dissolution enthalpy of iron metal in sodium molybdate at 973 K and gave excellent consistency with literature data. The methodology was then applied to the magnetite - maghemite system. The enthalpy of mixing of the Fe{sub 3}O{sub 4}-Fe{sub 8/3}O{sub 4} spinel solid solution is exothermic and, 2 represented by a subregular (Margules) formalism, {Delta}H{sub mix} = x(1-x)(-63.36 {+-} 8.60(1-x) + 17.65 {+-} 6.40x) kJ/mol, where x is the mole fraction of magnetite. The entropies of mixing of the solid solutionmore » were calculated for different assumptions about the distribution of cations, charges, and vacancies in these defect spinels. The different models lead to only small differences in the entropy of mixing. Calculated free energies of mixing show no evidence for a solvus in the magnetite - maghemite system.« less

Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was > 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrentmore » with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.« less

Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe3+-reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe3– x O4, where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe3+-oxyhydroxides containing the appropriate substitutional cations. A combination of L -edge and K -edge X-ray absorption spectroscopy (XAS) and L -edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni2+, with at least 80% in Oh coordination. Cobalt ferrites contained only Co2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn2+ only on the Oh sites but a mixture of Mn2+ and Mn3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.

Cation distributions in natural ferrite spinels, some containing large amounts of Mg, Ti, Mn, and Zn, have been investigated using the element-, site-, and symmetry-selective spectroscopic techniques of L 2,3 X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). By comparing XMCD data with calculated spectra, the site occupancies of the Fe cations have been determined. From the analysis of natural ferrite spinels with formulae very close to that of pure magnetite (Fe3O4), a standard XMCD spectrum for natural magnetite is proposed. Magnetites with small numbers of cation vacancies due to oxidation (solid solutions with maghemite, γ-Fe2O3) show that all the vacancies occur in octahedral sites. Ti L 2,3 XAS of oxidized Ti-bearing magnetites (hereafter referred to titanomagnetites) shows that Ti is tetravalent occurring on the octahedral site with 10 Dq ~2eV; Fe L 2,3 XMCD spectra indicate that the vacancies occur in both tetrahedral and octahedral sites. Mn L 2,3 XAS of the Mn-rich ferrite spinels shows that Mn is predominantly ordered onto the tetrahedral site with an Mn2+:Mn3+ ratio of 0.85:0.15. Mn- and Zn-rich ferrite spinels have an excess of cations over 3.0 per 4-oxygen formula unit. The sign of the XMCD for Mn corresponds to a parallel alignment of the Mn moments with the Fe3+ moments in the tetrahedral sublattice. This work demonstrates clearly that combined XAS and XMCD provides direct information on the distribution of multivalent cations in chemically complex magnetic spinels.