Characterization of Radiation Effects in Ceramics with Neutron Total Scattering

International audienceThe development of durable materials for nuclear-energy related applications has been central to efforts in materials science. There still exist, however, large gaps in the understanding of material degradation under intense radiation. Neutron total scattering measurements with pair distribution function analysis can be utilized to uniquely characterize radiation effects in a wide range of ceramics. This enables detailed analysis of both cation and anion defect behavior, and short-range order, which is important for the investigation of disordered and amorphous materials. Recent results for fluorite-related oxides demonstrate that radiation effects are more complex than previously thought with distinct processes occurring over different length scales. For example, disordered A2B2O7 pyrochlore is composed of local structural building blocks that maintain atomic order and exist in configurations that are different than the expected average structure. Here we will highlight the importance of short-range analysis for a comprehensive description of radiation behavior and amorphization resistance

Characterization of Radiation Effects in Ceramics with Spallation Neutron Probes

International audienceThe development of radiation resistant materials for various nuclear applications and their performance under harsh environments has been central to many research efforts over the past decades. There still exist, however, large gaps in the understanding of fundamental modes of material degradation under extremes such as ion irradiation. We have shown that neutron total scattering measurements with pair distribution function (PDF) analysis can be utilized to uniquely characterize the structural properties of swift heavy ion irradiated materials, including complex (e.g., A2B2O7 pyrochlore) and simple oxides (e.g., AO2). Irradiation experiments were performed at the UNILAC accelerator of the GSI Helmholtz Center with heavy ions of specific energy of 8.6 MeV/u. Such projectiles produce the sufficiently large irradiated sample mass (~100 mg) needed for analysis at the Nanoscale Ordered Materials Diffractometer (NOMAD) beamline at the Spallation Neutron Source (Oak Ridge National Laboratory). Neutron probes enable detailed analysis of both cation and anion defect behavior, and short-range order, which is particularly important for investigating radiation effects with no long-range coherency. Recent results demonstrate that structural changes induced under irradiation are more complex than previously thought with distinct processes occurring over different length scales and a high degree of local atomic order. For example, disordered (crystalline) and amorphous pyrochlore oxides are composed of very similar atomic-scale building blocks despite the very different long-range structure, and ion beam-induced tetragonal zirconia is only a configurational averaged ensample of an orthorhombic nanodomains. These examples highlight the importance of short- and medium-range analysis for a comprehensive description of radiation effects in materials

Multi-scale investigation of heterogeneous swift heavy ion tracks in stannate pyrochlore

Er2Sn2O7 pyrochlore was irradiated with swift heavy Au ions (2.2 GeV), and the induced structural modifications were systematically examined using complementary characterization techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), and neutron total scattering with pair distribution function (PDF) analysis. Each technique probes different aspects and length scales of the transformed material regions. TEM revealed a core–shell ion track structure—an amorphous core surrounded by a disordered, anion-deficient fluorite shell—which was confirmed by XRD. Neutron total scattering, with sensitivity to the oxygen sublattice, provided relative fractions of amorphous and disordered fluorite phases and confirmed the presence of a defective pyrochlore phase, which largely maintains its structural ordering but is clearly distinct from the pristine pyrochlore matrix. This defect-rich pyrochlore phase forms a halo extending radially beyond the well-characterized core–shell track morphology observed in electron micrographs. Despite their differing long-range periodicity, the short-range structures of the amorphous, disordered, and defective pyrochlore phases are all modeled well with a weberite-type configuration. Evolution of the phase fractions with increasing ion fluence was examined to ascertain the phase-to-phase pathways that occur during primary and secondary ion impact. This approach extends knowledge about the multi-scale response of stannate pyrochlores to swift heavy ion irradiation in the electronic energy loss regime and improves existing track-overlap models

Atomic-scale structural analysis of metastable zirconia

International audiencePhases that exist beyond their thermodynamic fields of stability are important to emerging technologies. Metastable phases are prevalent in far-from-equilibrium processing approaches and often exhibit desirable chemical and physical properties. However, there is limited understanding of the atomic-scale structural mechanisms that allow metastable phases to be recovered to ambient conditions. Metastability plays an important role in the synthesis of different zirconia (ZrO2) polymorphs. We utilized a state-of-the-art analytical approach with neutrons from the world’s most intense pulsed neutron source to investigate the atomic-scale recovery process and the structural properties of the metastable tetragonal phase. Grain-size reduction to the nanometer range and bombardment with high-energy (GeV) ions were used to prepare this metastable form of zirconia. We show in this presentation that the formation of a hierarchical network of nanoscale domains of lower symmetry, separated by domain walls, is the underlying structural mechanism that provides the pathway to the metastable state

Emergent Materials under Extremes and Decisive In Situ Characterizations

International audienceRecently, there has been an increased focus on understanding the structural details of disordered materials which are found across all energy technologies [Shamblin et al., Nat. Mater. 2016]. Despite the importance of these materials, there is still little knowledge about the atomic-scale rules governing disordering processes. Here, we present results from neutron total scattering experiments from spinel (AB2O4) and pyrochlore (A2B2O7) model systems which reveal that short-range ordering and associated structural relaxations in disordered materials can be understood as an extension of Pauling’s rules [Pauling, J. Am. Chem. Soc. 1929]. These rules apply whether disorder is induced intrinsically (e.g., chemical substitution), extrinsically (high temperature), or even by highly non-equilibrium conditions (high-energy ion irradiation and mechanical milling). These results provide a framework that can be utilized to predict the atomic configuration in disordered materials, including those that result from exposure to extreme conditions. [O’Quinn et al., Science Advances 2020

Effects of Grain Size on the Radiation Response of CeO2, ThO2, and UO2

International audienceRadiation stability is often a key limiting factor in performance of fluorite-structured materials and determining their suitability for use in energy-related applications. In an effort to mitigate the effects of radiation, nanostructured materials are of interest as they incorporate high defect sink strengths [Rose et al., Nanostructured Materials (1995), Nita et al., Journal of Nuclear Materials (2004)]. Recently, it has been shown that the response of CeO2, ThO2, and UO3 to highly ionizing radiation is strongly dependent on the material’s redox response [Tracy et al., Nature Communications (2015)]. When exposed to swift heavy ions, cations in the material are subject to changes in valence which drives swelling and microstrain as irradiation-induced defects accumulate. In this work, we present new insights into how crystallite size affects irradiation-induced redox response and defect accumulation in fluorite-structured simple oxides. Using 946 MeV Au ions at the UNILAC accelerator of the GSI Helmholtz Center, we irradiated microcrystalline and nanocrystalline materials of different compositions containing cations known to reduce (CeO2), remain univalent (ThO2), and oxidize (UO2) under ionizing conditions. Irradiated samples were characterized by synchrotron X-ray diffraction/absorption, neutron total scattering with pair distribution function (PDF) analysis, transmission electron microscopy, and Raman spectroscopy. Each composition exhibits a distinct response between microcrystalline and nanocrystalline forms, such as magnitude of volumetric swelling and secondary phase formation, driven mainly by redox processes. PDF analysis reveals small peroxide-like defects in CeO2 and mono- and di-interstitial clusters in UO2. Our findings imply that nanocrystallinity has negative effects on a material’s response to highly ionizing radiation. These results shed more light onto the interplay of particle size and cation redox behavior and their effect on defect production in an important class of materials, an insight that is essential in developing advanced materials for energy-related applications

Similar local order in disordered fluorite and aperiodic pyrochlore structures

A major challenge to understanding the response of materials to extreme environments (e.g., nuclear fuels/waste forms and fusion materials) is to unravel the processes by which a material can incorporate atomic-scale disorder, and at the same time, remain crystalline. While it has long been known that all condensed matter, even liquids and glasses, possess short-range order, the relation between fully-ordered, disordered, and aperiodic structures over multiple length scales is not well understood. For example, when defects are introduced (via pressure or irradiation) into materials adopting the pyrochlore structure, these complex oxides either disorder over specific crystallographic sites, remaining crystalline, or become aperiodic. Here we present neutron total scattering results characterizing the irradiation response of two pyrochlores, one that is known to disorder (Er2Sn2O7) and the other to amorphize (Dy2Sn2O7) under ion irradiation. The results demonstrate that in both cases, the local pyrochlore structure is transformed into similar short range configurations that are best fit by the orthorhombic weberite structure, even though the two compositions have distinctly different structures, aperiodic vs. disordered-crystalline, at longer length scales. Thus, a material's resistance to amorphization may not depend primarily on local defect formation energies, but rather on the structure's compatibility with meso-scale modulations of the local order in a way that maintains long-range periodicity