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Crystalline ceramics are candidate materials for the immobilization of radionuclides, particularly transuranics (such as U, Pu, and Am), arising from the nuclear fuel cycle. Due to the α-decay of transuranics and the associated recoil of the parent nucleus, crystalline materials may eventually be rendered amorphous through changes to the crystal lattice caused by these recoil events. Previous work has shown irradiation of titanate-based ceramics to change the local cation environment significantly, particularly in the case of Ti which was shown to change from 6- to 5-fold coordination. Here, this work expands the Ti-based study to investigate the behavior in Fe-based materials, using LaFeO3as an example material. Irradiation was simulated by heavy ion implantation of the bulk LaFeO3ceramic, with the resulting amorphous layer characterized with grazing angle X-ray absorption spectroscopy (GA-XAS). Insights into the Fe speciation changes exhibited by the amorphized surface layer were provided through quantitative analysis, including pre-edge analysis, and modeling of the extended X-ray absorption fine structure (EXAFS), of the GA-XAS data.

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy3+incorporation within the self-charge balancing zirconolite Ca1–xZr1–xDy2xTi2O7solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x≤ 1.00 (Δx= 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x= 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x≤ 0.50. Compositions for which x≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L3-edge XANES region confirmed that Dy was present uniformly as Dy3+, remaining analogous to Am3+. Fitting of the EXAFS region was consistent with Dy3+cations distributed across both Ca2+and Zr4+sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy3+was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi2O7–REE3+Ti2O7binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE3+cations.

The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition U1.09(6)Ti1.29(3)Al0.71(3)O6is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTi2–xAlxO6system demonstrated the formation of brannerite-structured compounds with varying Al3+and U5+contents, from U0.93(6)Ti1.64(3)Al0.36(3)O6to U0.89(6)Ti1.00(3)Al1.00(3)O6. Substitution of Al3+for Ti4+, with U5+charge compensation, resulted in near-linear changes in the band cunit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U5+as the dominant oxidation state in near-single-phase brannerite compositions was evidenced by complementary laboratory U L3-edge and high-energy-resolution fluorescence-detected U M4-edge X-ray absorption near-edge spectroscopy. No brannerite phase was found for compositions with Al3+/Ti4+> 1, which would require a U6+contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilization of dominant uranium(V) brannerites by the substitution of trivalent cations, such as Al3+, on the Ti4+site.

The photocatalytic and dielectric behaviors of Aurivillius oxyfluorides such as Bi2TiO4F2depend sensitively on their crystal structure and symmetry but these are not fully understood. Our experimental work combined with symmetry analysis demonstrates the factors that influence anion order and how this might be tuned to break inversion symmetry. We explore an experimental approach to explore anion order, which combines Rietveld analysis with strain analysis.

ABSTRACTZirconolite (CaZrTi2O7) has been identified as a candidate ceramic wasteform for the immobilisation and disposal of Pu inventories, for which there is no foreseen future use. Here, we provide an overview of relevant zirconolite solid solution chemistry with respect to Ce, U and Pu incorporation, alongside a summary of the available literature on zirconolite aqueous durability. The zirconolite phase may accommodate a wide variety of tri- and tetravalent actinide and rare-earth dopants through isovalent and heterovalent solid solution, e.g.CaZr1–xPuxTi2O7or Ca1–xPuxZrTi2–2xFe2xO7. The progressive incorporation of actinides within the zirconolite-2M parent structure is accommodated through the formation of zirconolite polytypoids, such as zirconolite-4M or 3T, depending on the choice of substitution regime and processing route. A variety of standardised durability tests have demonstrated that the zirconolite phase exhibits exceptional chemical durability, with release rates of constituent elements typically <10−5gm−2·d−1. Further work is required to understand the extent to which polytype formation and surrogate choice influence the dissolution behaviour of zirconolite wasteforms.

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH3gas. One such method is the reaction between metal oxides and NaNH2at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe3O4by reaction with NaNH2at 170–190 °C, via a pseudomorphic reaction. The more reduced Fe3O4precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe2O3. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and 57Fe Mössbauer spectroscopy, comprised 49–59 mol % ε-Fe2+xN, accompanied by 29–39 mol % FeO1–xNxand 8–14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH2, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO1–xNxoxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitridephases in high yield, by reaction of a metal oxide substrate with NaNH2, with either careful control of H2O concentration in the system or postsynthetic hydrolysis and crystallization.

Zirconolite (CaZrTi2O7) has been identified as a candidate ceramic wasteform for the immobilisation and disposal of Pu inventories, for which there is no foreseen future use. Here, we provide an overview of relevant zirconolite solid solution chemistry with respect to Ce, U and Pu incorporation, alongside a summary of the available literature on zirconolite aqueous durability. The zirconolite phase may accommodate a wide variety of tri- and tetravalent actinide and rare-earth dopants through isovalent and heterovalent solid solution, e.g. CaZr1–xPuxTi2O7or Ca1–xPuxZrTi2–2xFe2xO7. The progressive incorporation of actinides within the zirconolite-2M parent structure is accommodated through the formation of zirconolite polytypoids, such as zirconolite-4M or 3T, depending on the choice of substitution regime and processing route. A variety of standardised durability tests have demonstrated that the zirconolite phase exhibits exceptional chemical durability, with release rates of constituent elements typically <10-5gm-2·d-1. Further work is required to understand the extent to which polytype formation and surrogate choice influence the dissolution behaviour of zirconolite wasteforms.

Aliquots of well-characterized Ce-brannerite were annealed at different temperatures under N2and synthetic air atmospheres. The autoreduction of cerium at temperature was observed using thermogravimetry to monitor the mass lost as O2was evolved. It has been shown that the brannerite structure is stable with a small fraction of Ce3+, charge-balanced by O vacancies. The range of stability was determined to be Ce4+0.975Ti2O5.95, the fully oxidized end-member, to Ce3.87+0.975Ti2O5.886, as reduced by annealing under N2at 1075 °C. Higher temperatures under N2led to further reduction of Ce and collapse of the brannerite structure. Ce-brannerite remained stable on heating to 1300 °C in synthetic air, with multiple steps of oxidation and reduction corresponding to changes in the average Ce oxidation state. We propose that the autoreduction of Ce at temperature is an important factor in the overall thermodynamic stability of Ce-brannerite at temperature and has a large impact on the energetics of formation of Ce-brannerite.

The Dion-Jacobson (DJ) family of perovskite-related materials have recently attracted interest due to their polar structures and properties, resulting from hybrid-improper mechanisms for ferroelectricity in n= 2 systems and from proper mechanisms in n= 3 CsBi2Ti2NbO10. We report here a combined experimental and computational study on analogous n= 3 CsLn2Ti2NbO10(Ln= La, Nd) materials. Density functional theory calculations reveal the shallow energy landscape in these systems and give an understanding of the competing structural models suggested by neutron and electron diffraction studies. The structural disorder resulting from the shallow energy landscape breaks inversion symmetry at a local level, consistent with the observed second-harmonic generation. This study reveals the potential to tune between proper and hybrid-improper mechanisms by composition in the DJ family. The disorder and shallow energy landscape have implications for designing functional materials with properties reliant on competing low-energy phases such as relaxors and antiferroelectrics.

The crystallization of iron-containing sodium silicate phases holds particular importance, both in the management of high-level nuclear wastes and in geosciences. Here, we study three as-quenched glasses and their heat-treated chemical analogs, NaFeSiO4, NaFeSi2O6, and NaFeSi3O8(with nominal stoichiometries from feldspathoid, pyroxene, and feldspar mineral groups, i.e., Si/Fe = 1, 2, and 3, respectively) using various techniques. Phase analyses revealed that as-quenched NaFeSiO4could not accommodate all Fe in the glass phase (some Fe crystallizes as Fe3O4), whereas as-quenched NaFeSi2O6and NaFeSi3O8form amorphous glasses. NaFeSi2O6glass is the only composition that crystallizes into its respective isochemical crystalline polymorph, i.e., aegirine, upon isothermal heat-treatment. As revealed by Mössbauer spectroscopy, iron is predominantly present as fourfold-coordinated Fe3+in all glasses, though it is present as sixfold-coordinated Fe3+in the aegirine crystals (NaFeSi2O6), as expected from crystallography. Thus, Na-Fe silicate can form a crystalline phase in which it is octahedrally coordinated, even though it is mostly tetrahedrally coordinated in the parent glasses. Thermal behavior, magnetic properties, iron redox state (including Fe K-edge X‑ray absorption), and vibrational properties (Raman spectra) of the above compositions are discussed.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2analogue for UO2fuel. Dissolution was performed on the following: CeO2annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-xannealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90 °C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-xgave rise to rates that were an order of magnitude greater than for CeO2with fewer oxygen vacancies. While enhanced solubility of Ce3+influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthetic CeO2and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure characteristics of fuel-grade UO2but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) was investigated during dissolution. The effects of surface polishing on dissolution rate were also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called “instant release fraction” of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude.

X-ray powder diffraction data are presented for nine new phases with the tetragonal tungsten bronze structure. By comparison with the structure of the M=Nb,RE=Laanalogue, these contain Ba in the large pentagonal sites with coordination number 15, a range of large rare-earth cations and Bi in the twelve coordinate sites, and the smaller cations, Ti and either Nb or Ta, in the octahedral framework sites. The alattice parameter data of the tetragonal unit cell show an approximately linear variation with trivalent ion radius, apart from Dy, whose value appears to be anomalous; data for cshow little variation with trivalent ion radius.

Ceramic samples of the tetragonal tungsten bronze Ba₂LaTi₂Nb₃O₁₅, containing B-site disorder of Ti,Nb, have a maximum permittivity of 250–300 at T_c ≈ 200 K and frequency-dependent permittivity below T_c, characteristic of relaxor behaviour.

Application of pressure (1 GPa, 150 °C) enables the direct ion exchange of Cs+for Na+in both the alumino‐ and gallosilicate forms of natrolite, which does not occur at ambient pressure.