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Irradiation induced non-equilibrium point defect populations influence mass transport in oxides, which in turn affects their stability and performance in hostile environments. In this study a strong dose rate dependence is observed.

Nuclear materials are often demanded to function for extended time in extreme environments, including high radiation fluxes and transmutation, high temperature and temperature gradients, stresses, and corrosive coolants. They also have a wide range of microstructural and chemical makeup, with multifaceted and often out-of-equilibrium interactions. Machine learning (ML) is increasingly being used to tackle these complex time-dependent interactions and aid researchers in developing models and making predictions, sometimes with better accuracy than traditional modeling that focuses on one or two parameters at a time. Conventional practices of acquiring new experimental data in nuclear materials research are often slow and expensive, limiting the opportunity for data-centric ML, but new methods are changing that paradigm. Here we review high-throughput computational and experimental data approaches, especially robotic experimentation and active learning that based on Gaussian process and Bayesian optimization. We show ML examples in structural materials ( e.g., reactor pressure vessel (RPV) alloys and radiation detecting scintillating materials) and highlight new techniques of high-throughput sample preparation and characterizations, and automated radiation/environmental exposures and real-time online diagnostics. This review suggests that ML models of material constitutive relations in plasticity, damage, and even electronic and optical responses to radiation are likely to become powerful tools as they develop. Finally, we speculate on how the recent trends in artificial intelligence (AI) and machine learning will soon make the utilization of ML techniques as commonplace as the spreadsheet curve-fitting practices of today.

Controlling and predicting the morphology of lanthanide sesquioxides in thin film form is vital to their use in current applications. In the present study, single and codeposited Sm2O3, Er2O3, and Lu2O3 thin films were grown on yttria-stabilized zirconia (8%) substrates by radio frequency magnetron sputtering at room temperature and 500 °C. The effect of two different substrate temperatures and altering the oxide cation on the structural and morphological properties of the films was analyzed. The thin films were characterized by profilometry, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The single-component Lu2O3 and Sm2O3 films obtained were of the cubic phase, and the Er2O3 was a mix of cubic and monoclinic phases. It was observed for both the Er2O3 and Lu2O3 films that increasing the substrate temperature to 500 °C resulted in larger grained polycrystalline films. In contrast, large grained polycrystalline films were obtained at both room temperature and 500 °C for Sm2O3 and uneven granularity increased as temperature increased. Codeposition of Lu2O3 and Sm2O3, and Lu2O3 and Er2O3 resulted in a cubic bixbyite phase (the C phase of the lanthanide sesquioxide) solid solution. It was observed that the structure and morphology of the films can be controlled by manipulating deposition parameters. Both substrate temperature and altering the oxide cation contributed to changes in crystallinity and grain structure, which can modify the chemical and physical properties of the films for their applications.

The next generation of nuclear reactors will expose materials to conditions that, in some cases, are even more extreme than those in current fission reactors, inevitably leading to new materials science challenges. Radiation-induced damage and corrosion are two key phenomena that must be understood both independently and synergistically, but their interactions are often convoluted. In the light water reactor community, a tremendous amount of work has been done to illuminate irradiation-corrosion effects, and similar efforts are under way for heavy liquid metal and molten salt environments. While certain effects, such as radiolysis and irradiation-assisted stress corrosion cracking, are reasonably well established, the basic science of how irradiation-induced defects in the base material and the corrosion layer influence the corrosion process still presents many unanswered questions. In this review, we summarize the work that has been done to understand these coupled extremes, highlight the complex nature of this problem, and identify key knowledge gaps.

Tuning the efficiency and speed of charge carrier recombination in inorganic scintillators can potentially improve their performance in diverse applications. Recent work suggests that this maybe be achieved via a two-phase scintillator AB that naturally phase separates into A-rich and B-rich domains. In addition, a favorable electronic structure and band-edge alignment such that the charge carriers are confined or are thermodynamically driven to preferentially accumulate in one of the two domains, might lead to an improved radiative recombination rate. Here, we use density functional theory computations and ab initio molecular dynamics (AIMD), including non-adiabatic molecular dynamics (NAMD) simulations, to examine an alternative phase structure and its potential impact on recombination. Using a model perovskite SrTiO$$_3$$ 3 system with one-, two- and three-dimensional Ruddlesden–Popper (RP) phases, we demonstrate that RP faults induce band structure changes in the material that can act as barriers to carrier transport. Our AIMD/NAMD simulations indicate competing effects of a lower mean free path (potentially enhancing the desired radiative recombination and overall scintillating efficiency) and faster non-radiative recombination (undesired) due to enhanced electron–phonon coupling in the faulted system. Full exploitation of such a rational design approach would require tuning of the effective scintillation efficiency by varying the perovskite chemistry using appropriate arrangements of RP faults in the bulk material. Finally, other effects, such as the tendency of point defects to segregate at the interface, that might affect the overall performance, are briefly discussed. We expect the basic results found here to apply to other nanostructured scintillators. Graphical Abstract

Thin film deposition from the vapor phase is a complex process involving adatom adsorption, movement, and incorporation into the growing film. Here, we present quantitative experimental data that reveals anion intermixing over long length scales during the deposition of epitaxial Fe

Mass transport along grain boundaries in alloys depends not only on the atomic structure of the boundary, but also its chemical make-up. In this work, we use molecular dynamics to examine the effect of Cr alloying on interstitial and vacancy-mediated transport at a variety of grain boundaries in Ni. We find that, in general, Cr tends to reduce the rate of mass transport, an effect which is greatest for interstitials at pure tilt boundaries. However, there are special scenarios in which it can greatly enhance atomic mobility. Cr tends to migrate faster than Ni, though again this depends on the structure of the grain boundary. Further, grain boundary mobility, which is sometimes pronounced for pure Ni grain boundaries, is eliminated on the time scales of our simulations when Cr is present. We conclude that the enhanced transport and grain boundary mobility often seen in this system in experimental studies is the result of non-equilibrium effects and is not intrinsic to the alloyed grain boundary. These results provide new insight into the role of grain boundary alloying on transport that can help in the interpretation of experimental results and the development of predictive models of materials evolution.

The influence of radiation-induced (1 MeV energy H+ to ∼0.1 displacements per atom (dpa) at 450 °C), nonequilibrium point defect populations on mass transport is studied with an integrated campaign...

Perovskite oxides continue to attract huge interest due to their fascinating and wide-ranging properties for diverse applications. The tunability of these properties may be further enhanced by incr...

A theoretical and computational analysis of two approaches to simulate luminescent profiles of rare-earth perovskite scintillators is given. This work establishes the importance of many-particle corrections in the prediction of the principal excitation wavelength, revealing that they lead to differences of nearly one hundred nanometers from the standard Δ-SCF approach. We show the electronic structure of this class of materials uniquely necessitates a many-particle treatment because, in contrast to traditional semiconductors, rare-earth scintillator materials are weakly screened and relatively few bands dominate the radiative decay channels. This makes accounting for long-range electron–hole correlations a central issue in accurate predictions, and we discuss the trade-off between accuracy and performance of various popular approaches. Understanding the strengths and weaknesses of available theoretical tools will help define search parameters for new scintillator development.

Spinels represent an important class of technologically relevant materials, used in diverse applications ranging from dielectrics, sensors and energy materials. While solid solutions combining two “single spinels” have been explored in a number of past studies, no ordered “double” spinels have been reported. Based on our first principles computations, here we predict the existence of such a double spinel compound MgAlGaO4, formed by an equimolar mixing of MgAl2O4 normal and MgGa2O4 inverse spinels. After studying the details of its atomic and electronic structure, we use a cluster expansion based effective Hamiltonian approach with Monte Carlo simulations to study the thermodynamic behavior and cation distribution as a function of temperature. Our simulations provide strong evidence for short-ranged cation order in the double spinel structure, even at significantly elevated temperatures. Finally, an attempt was made to synthesize the predicted double spinel compound. Energy Dispersive X-ray Spectrometry and X-ray diffraction Rietveld refinements were performed to characterize the single-phase chemical composition and local configurational environments, which showed a favorable agreement with the theoretical predictions. These findings suggest that a much larger number of compounds can potentially be realized within this chemical space, opening new avenues for the design of spinel-structured materials with tailored functionality. Materials with a spinel structure are used in various applications, including in the nuclear industry and as dielectrics. Here, first principle calculations and Monte Carlo simulations predict that an ordered double spinel structure is stable, supported by preliminary experimental data.

Fe2O3 hematite is a technologically important material with applications in energy storage as well as being the key phase formed in the rust of iron-based materials. Despite this central importance...

Under radiative environments such as extended hard X- or γ-rays, degradation of scintillation performance is often due to irradiation-induced defects. To overcome the effect of deleterious defects, novel design mitigation strategies are needed to identify and design more resilient materials. The potential for band-edge engineering to eliminate the effect of radiation-induced defect states in rare-earth-doped perovskite scintillators is explored, taking Ce3+-doped LuAlO3 as a model material system, using density functional theory (DFT)-based DFT + U and hybrid Heyd-Scuseria-Ernzerhof (HSE) calculations. From spin-polarized hybrid HSE calculations, the Ce3+ activator ground-state 4f position is determined to be 2.81 eV above the valence band maximum in LuAlO3. Except for the oxygen vacancies which have a deep level inside the band gap, all other radiation-induced defects in LuAlO3 have shallow defect states or are outside the band gap, that is, relatively far away from either the 5d1 or the 4f Ce3+ levels. Finally, we examine the role of Ga doping at the Al site and found that LuGaO3 has a band gap that is more than 2 eV smaller than that of LuAlO3. Specifically, the lowered conduction band edge envelopes the defect gap states, eliminating their potential impact on scintillation performance and providing direct theoretical evidence for how band-edge engineering could be applied to rare-earth-doped perovskite scintillators.

Gadolinium titanate (Gd2Ti2O7, or GTO) and other lanthanide pyrochlores are interesting candidates for actinide waste disposal and fast ion conduction because the relevant material properties are intimately dependent on local cation structure. Therefore, a deep understanding of the kinetics associated with cation ordering and disordering is required if such material properties are to be tuned for specific device designs. To this end, single crystals of GTO were irradiated with 190 keV helium ions to a total fluence of 1 × 1017 ions/cm2, amorphizing the sample surface to a depth of ~1 μm and resulting in significant He bubble accumulation. FIB lamellae lifted out from the irradiated sections were examined during heat treatment in the (scanning) transmission electron microscope. Two distinct stages of the re-crystallization of the amorphized material were observed. The material near the end of the ions’ range transformed first and with the same orientation as the pristine material. This was due to the close proximity of the pristine material and the presence of small defect fluorite seeds, but the propagation of this growth front was frustrated by the large pores (formerly bubbles) in the He accumulation layer. This was followed by heterogeneous nucleation of new crystallites at random orientations at the top of the He accumulation layer, which is attributed to the high surface area associated with the many small He bubbles in that region. It is inferred that the kinetics of grain growth in this material are significantly faster than the kinetics of grain nucleation.

Heteroanionic oxysulfide perovskite compounds represent an emerging class of new materials allowing for a wide range of tunability in the electronic structure that could lead to a diverse spectrum of novel and improved functionalities. Unlike cation ordered double perovskites—where the origins and design rules of various experimentally observed cation orderings are well known and understood—anion ordering in heteroanionic perovskites remains a largely uncharted territory. In this contribution, we present and discuss insights that have emerged from our first-principles-based electronic structure analysis of a prototypical anion-ordered SrHf(O0.5S0.5)3 oxysulfide chemistry, studied in all possible anion configurations allowed within a finite size supercell. We demonstrate that the preferred anion ordering is always an all-cis arrangement of anions around an HfO3S3 octahedron. As a general finding beyond the specific chemistry, the origins of this ordering tendency are traced back to a combined stabilization effect stemming from electronic, elastic, and electrostatic contributions. These qualitative notions are also quantified using state-of-the-art machine learning models. We further study the relative stability of the identified ordering as a function of A (Ca, Sr, Ba) and B (Ti, Zr, Hf) site chemistries and probe chemistry-dependent trends in the electronic structure and functionality of the material. Most remarkably, we find that the identified ground-state anion ordering breaks the inversion symmetry to create a family of oxysulfide ferroelectrics with a macroscopic polarization >30 μC/cm2, exhibiting a significant promise for electronic materials applications.

Summary Interfaces in semicoherent ceramic heterostructures are characterized by a network of misfit dislocations that plays a key role in dictating structural and functional properties in this important class of materials. Prediction of a misfit dislocation network structure, given the chemistries forming the heterointerface, remains a challenging problem that requires highly specialized experimental synthesis and characterization or computation-intensive atomistic simulations. Here, we present a heuristic approach that allows us to qualitatively predict the structure of misfit dislocation networks in semicoherent ionic heterostructures. The predictions within the proposed and validated approach are based entirely on elastic effects and the nature of electrostatic interactions across the interface and, therefore, do not require any input from explicit simulations. In addition to providing an intuitive and rational justification of misfit dislocation network structure in semicoherent ionic heterostructures, our approach is expected to complement experimental and simulation-based efforts targeted at determining and quantifying misfit dislocation patterns.

Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in beta-Ga2O3, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20 meV and resistivity of 10-4 Ohm.cm. The conductivity was switched to p-type with acceptor ionization energy of 42 meV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga2O3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy confirmed this finding and the interpretation of the results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time it demonstrates for the first time p-type and remarkable n-type conductivity in Ga2O3 which should usher in the development of Ga2O3 devices and advance optoelectronics and high-power devices, Comment: 15 pages, 7 figures

Positron annihilation spectroscopy is a powerful tool to quantify the amount of vacancies and vacancy clusters in materials. The technique has been utilized to study the induced defects in materials after ion beam and neutron irradiations. This paper makes the case for how the technique can and should be utilized to quantify the defects created by irradiation in situ during irradiation to foster a more thorough understanding of the surviving defects after initial collision cascades. This paper outlines a future experimental approach and its meaning for the nuclear materials community, being able to benchmark commonly used rate theory models of damage evolution.

Complex materials, containing multiple chemical species, often exhibit chemical disorder or inversion. Typically, this disorder is viewed as spatially homogeneous throughout the material. Here, we show, using a simple grain boundary in MgAl2O4 spinel, that this is not the case and that the level of inversion at the grain boundary plane is different than in the bulk. This has ramifications for the energetics of the boundary and how defects interact with it, as exemplified by the relative formation energy of vacancies. Using these results as motivation, we construct a simple model of inversion versus grain size that captures the salient behavior observed in experiments and allows us to extract inversion-relevant properties from those same experiments, suggesting that grain boundaries in the experimentally prepared material are essentially fully inverse. Together, these results highlight the role that microstructure plays on the inversion in the material.

Spinel compounds, with the general chemical formula AB2O4, are a wide class of materials, where A and B can be a variety of cations, providing this structure with a great deal of functional flexibility and giving rise to its considerable scientific interest. Recently, a spinel with the general formula ABB′O4 has been predicted, increasing the possible usability of the spinels due to the higher cation diversity in the so-called double spinel structure. Here, we use density functional theory calculations to predict if double spinels can be formed between experimentally synthesized normal and inverse single spinels. Our computations reveal that 49 double spinels have negative mixing enthalpies and are thus thermodynamically stable, with most of the stable compounds being formed from one of two distinct cation orderings. We show that the 17 different cations that form the different double spinels have a preferred site, tetrahedral or octahedral, except for Mn, Fe and Co which can occupy both sites interchangeably. We also study the relation between mixing enthalpies and cation-specific properties, as well as ways to classify the double spinels into distinct types and spinel groups depending on the cation ordering and cation oxidation states, respectively. By applying the Sure Independence Screening and Sparsifying Operator (SISSO) approach on the coordination-dependent ionic radii of the elemental constituents, we show that an interplay of local strain and electrostatic dominated terms can be used to separate the double spinels into distinct structural types depending on the cation order and their oxidation states.

Their very flexible chemistry gives oxide materials a richness in functionality and wide technological application. A specific group of oxides that have a structure related to fluorite but with less oxygen, termed anion-deficient fluorite structural derivatives and with pyrochlores being the most notable example, has been shown to exhibit a diversity of useful properties. For example, the possibility to undergo a transition from an ordered to disordered state allows these oxides to have high radiation tolerance. Atomistic-scale calculations in the form of molecular dynamics (MD) and density functional theory (DFT) have been extensively used to understand what drives this order/disorder transition. Here we give a brief overview of how atomistic-scale calculations are utilized in modeling disorder in pyrochlores and other anion-deficient fluorite structural derivatives. We discuss the modeling process from simple point defects to completely disordered structures, the dynamics during the disordering process, and the use of mathematical models to generate ordered solid-solution configurations. We also attempt to identify the challenges in modeling short range order and discuss future directions to more comprehensive models of the disordered structures.

Quantum-radiative decay is a fundamental process in many optoelectronic systems such as laser diodes and solar cells. The bright exciton lifetime is a critical factor in determining the performance of these systems. Motivated by the ever-increasing need for laser systems in space and nuclear applications, we develop a many-particle approach to predict the radiative lifetime under harsh radiation environments. Using GaAs as a model system, we find that radiation-induced band tailing reduces the bright exciton lifetime. This result shows that the efficiency of radiative recombination, in addition to nonradiative recombination, can be affected by ionization radiation. Our approach enables a detailed understanding of the interplay between correlation, localization, and radiation that affect the performance of gain media.

Diffusion of Xe atoms in UO2 fuel is important for nuclear fuel performance, and is enabled by interaction with U and O vacancies. Previous work using atomistic calculations based on density functional theory (DFT) and empirical potentials (EP) focused on the role of small vacancy clusters ( Xe U 2 O y , y = 0, 1, 2) for Xe transport, but the model predictions failed to reproduce the experimental observations by consistently underestimating Xe diffusivity. In this work, where we focused on out-of-pile conditions (i.e. in the absence of irradiation), we have explored two of the uncertainties associated with the model: the DFT methodology and the types of clusters considered. We found that using the energy barriers obtained with GGA + U DFT allows to reconcile simulation results and experimental observations in the intrinsic regime (i.e. out-of-pile conditions). The XeUO cluster is the preferred configuration at high temperature, while the mobile Xe U 2 O cluster takes over below 1720 K. The latter cluster is also the main contributor to Xe diffusion and good agreement is obtained with current models empirically fitted to experiments for diffusion at high temperature. A simple expression that captures most of Xe diffusivity is proposed, relying on XeUO and Xe U 2 O clusters only. We also determined the out-of-pile concentration and diffusivity of extended clusters Xe U x O y , 3 ≤ x ≤ 9 , and determined that these do not contribute significantly to the overall diffusivity under intrinsic conditions. However, we speculate that the Xe U 4 O 3 and Xe U 8 O 9 clusters, which have relatively low migration barrier, may play a role under irradiation conditions, because the radiation-induced enhanced vacancy concentration would rapidly increase the fraction of large Xe-vacancy clusters.

Cost versus accuracy trade-offs are frequently encountered in materials science and engineering, where a particular property of interest can be measured/computed at different levels of accuracy or fidelity. Naturally, the most accurate measurement is also the most resource and time intensive, while the inexpensive quicker alternatives tend to be noisy. In such situations, a number of machine learning (ML) based multifidelity information fusion (MFIF) strategies can be employed to fuse information accessible from varying sources of fidelity and make predictions at the highest level of accuracy. In this work, we perform a comparative study on traditionally employed single-fidelity and three MFIF strategies, namely, (1) Δ-learning, (2) low-fidelity as a feature, and (3) multifidelity cokriging (CK) to compare their relative prediction accuracies and efficiencies for accelerated property predictions and high throughput chemical space explorations. We perform our analysis using a dopant formation energy data set for hafnia, which is a well-known high-k material and is being extensively studied for its promising ferroelectric, piezoelectric, and pyroelectric properties. We use a dopant formation energy data set of 42 dopants in hafnia-each studied in six different hafnia phases-computed at two levels of fidelities to find merits and limitations of these ML strategies. The findings of this work indicate that the MFIF based learning schemes outperform the traditional SF machine learning methods, such as Gaussian process regression and CK provides an accurate, inexpensive and flexible alternative to other MFIF strategies. While the results presented here are for the case study of hafnia, they are expected to be general. Therefore, materials discovery problems that involve huge chemical space explorations can be studied efficiently (or even made feasible in some situations) through a combination of a large number of low-fidelity and a few high-fidelity measurements/computations, in conjunction with the CK approach.

The complexity of adatom diffusion on the Al (1 0 0) surface is reflected by the existence of several low-energy non-trivial atomic exchange or vacancy formation mechanisms involving concerted motion of several (3–7) atoms. Interestingly, these mechanisms have energy barriers lower than or comparable to that of the simple (and intuitive) hopping mechanism commonly observed on other surface facets. While prior studies mainly used classical potentials to understand diffusion processes active on Al (1 0 0) surface, here we use accurate (and expensive) density functional theory (DFT) computations to estimate barriers associated with nine low-energy and non-trivial adatom diffusion mechanisms. We find that there exist several exchange mechanisms with energy barriers less than or equal to that of the trivial hop mechanism. Furthermore, several of the atomic exchange mechanisms have barriers within 0.2 eV of that of the simple hop, thereby highlighting mechanisms that can be relevant during surface/crystal growth. Our results paint a highly complex picture of the diffusion landscape on Al (1 0 0) and provide insights into how such mechanisms may contribute toward large length- and time-scale surface phenomena. The results presented in this work may also have implications for other fcc metals. Further, we show that some of the commonly used interatomic potentials fail to accurately capture the details of adatom diffusion on Al (1 0 0). The presented benchmark DFT dataset can thus be utilized to parameterize/retrain such potentials.

Refining grain size and adding alloying elements are two complementary approaches for enhancing the radiation tolerance of existing nuclear materials. Here, we present detailed in-situ irradiation research on defect evolution behavior and irradiation tolerance of ultrafine W-TiC alloys (thin foils) irradiated with 1 MeV Kr+2 at RT and 1073 K, and compare their overall performance to pure coarse grained tungsten. Loop Burgers vector was studied confirming the presence of loops whose population increased at high temperature. Loop density, average loop area, and overall damage are reported as a function of irradiation dose revealing distinct defect evolution behavior from pure materials. The overall damage generally followed the average loop size trend, which decreased with time for both temperatures, but was higher at 1073 K and attributed to biased vacancy sink behavior of the TiC dispersoids evidenced by large vacancy clusters on their interfaces. By comparison, the overall loop and void damage in pure tungsten was larger by a factor of six and two, respectively. The improved irradiation damage resistance in the alloys is thus attributed to the effect of dispersoids in 1) the enhancement in annihilating defects and mutual defect recombination due to both dispersoids and a higher grain boundary density; 2) decreasing the loop mobility, causing shrinkage and annihilation of loop density, which was confirmed via in-situ video. Several mechanisms are illustrated to describe the performance of the complex alloy system. The results motivate further experimental and modeling research that aims to understand the many different phenomena occurring at different time scales.

Nanocrystalline metals are often postulated as irradiation tolerant materials due to higher grain boundary densities. The efficiency of these materials in mitigating irradiation damage is still under investigation. Here, we present an in-situ transmission electron microscopy with ion irradiation study on equiaxed 35 nm grained tungsten (NCW-35 nm) and compare its radiation tolerance, in terms of dislocation loop damage, to several other grades of tungsten with different grain sizes at two temperatures (RT and 1073 K). The NCW-35 nm was shown to possess significant higher radiation tolerance in terms of loop damage. As demonstrated by Kinetic Monte Carlo simulations, at least part of the higher radiation tolerance of the small grains is due to higher interstitial storage (at the grain boundaries) and defect recombination (in the grain interiors) in the small grain material. In addition, experimental observations reveal rapid and efficient dislocation loop absorption by the grain boundaries and this is considered the dominant factor for mass transport to the boundaries during irradiation, enabling the remarkable radiation tolerance of 35 nm grained tungsten. This study demonstrates the possibility of attaining high radiation tolerant materials, in terms of dislocation loop damage, by minimizing grain sizes in the nanocrystalline regime.

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with B Ti amorphize much more readily than B Zr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr++ ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

Pyrochlore oxides (A2B2O7) are interesting for a number of technological applications, including radiation damage tolerance and as ionic conductors. Mixed pyrochlores—containing two A and/or two B site cations—provide even more flexibility for tailoring properties owing to the diverse chemical and configurational degrees of freedom accessible within this chemical space. Here, we examine relative stability of different cation orderings in one model double pyrochlore Gd2(ZrxTi1−x)2O7, as a function of Zr content x. Our results show that, in the presence of some very specific local cation arrangements, certain cation-ordered compositions in this system are highly stabilized as a result of large oxygen relaxation displacements, leading to the formation of an ordered ‘double’ pyrochlore structure. The origins of these anomalous oxygen relaxations are traced back to both the local cation symmetry and a strong chemical preference of Zr atoms towards adopting a 7-fold coordination environment, as opposed to a 6-fold coordination available in a regular pyrochlore structure. Subsequently, we examine the stability of this type of ordering in 131 other pyrochlore compositions. Implications of our findings are discussed in relation to the observed composition-dependent ionic conductivity in these systems and connections with previously reported experimental findings are made.

Crystalline disorder in complex oxides can have a pronounced effect on mass transport, but the role of disorder within the complex landscape of material and microstructural defects is not uniquely understood. In this work, we use radiation damage, not to study the fundamental damage response, but rather as a tool to induce and control the extent of disorder of a single oxide composition in order to explore the relationship between intrinsic disorder and transport. Thin films of the ordered pyrochlore Gd2Ti2O7 (GTO) were irradiated with light ions and transitioned from ordered pyrochlore to disordered crystalline defect fluorite to an amorphous structure, disordering monotonically with increasing fluence, but nonhomogeneously throughout the film. Conductivity increased substantially with increasing disorder, where only slight disordering results in the majority of the change in conductivity. Additionally, while the pristine GTO films exhibited mixed ionic and p-type electronic conductivity, the total conductivity of the material that exhibited maximum disorder prior to the onset of amorphization was ionic in nature.

Segregation of interstitials at a grain boundary (GB) is known to generally lower its mobility. This phenomenon, called ‘solute-drag’, has important ramifications on the process of recrystallization and microstructural evolution. In this manuscript, we present predictions from molecular dynamics (MD) simulations which demonstrate that interstitial hydrogen in tungsten can in fact increase the mobility of some GBs which exhibit shear coupling. Assuming a disconnection-based mechanism, activation energies and pre-factors for disconnection nucleation are predicted from simulations of shear-coupled motion. In GBs where enhanced mobility is predicted, interstitial H reduces both the activation energy and the pre-factor for disconnection nucleation, thus effectively increasing the mobility. For GBs with diminished mobility, MD predicts that presence of interstitial H reduces the pre-factor and, in some cases, increases the activation energy. The reduction in the activation energy inferred from MD simulations are confirmed by nudged elastic band calculations. Temperature-dependent structural transitions are observed for some GBs, and the effect of interstitial H is found to change with the changes in structure. The effect of interstitial H is predicted to be complex and highly variable, providing some plausible explanations for experimental observations on the recrystallization of tungsten in presence of H-loaded plasma.

Radiation resistant materials are needed for a large number of applications. One route to enhancing radiation resistance is to introduce a high density of defect sinks such as grain boundaries. However, there are still important questions regarding the role of grain boundaries in enhancing radiation tolerance, particularly in ionic materials. Experiments have found large improvements in the amorphization resistance of oxides at temperatures where defect mobilities are too low to easily reach the boundaries. Standard reaction-diffusion models are inadequate in explaining this behavior. Here, we examine the role of electro-migration in the overall transport of irradiation-induced defects in ionic systems. We find that electro-migration can have a large impact on the steady state point defect concentrations as compared to models that do not include the effect of electro-migration. In particular, defect concentrations can change by nearly seven orders of magnitude for certain conditions. We conclude that radiation tolerance in nanocrystalline ionic ceramics will be driven both by enhanced sink density via small grain sizes and, critically, the effects of electro-migration induced by charge segregation to grain boundaries. Keywords: Radiation damage, Electro-migration, Ionic materials, Computer modeling

Despite the fundamental importance of mass transport, there is still a lack of knowledge regarding the mechanisms in many materials. In particular, how the magnetic spin structure interacts with migrating defects is unclear. Here, using density functional theory to examine cation interstitial transport in antiferromagnetic corundum-structured oxides, we reveal an interplay between the migration of the interstitial and the magnetic structure of the oxide. The magnetic spin configuration impacts the migration energy of the migrating defect but, critically, the defect modifies the spin configuration. Thus, the two aspects change in concert to modify one another. This has profound implications for mass transport in magnetic materials.

Many important processes in materials and chemical systems are intrinsically atomistic in nature but involve timescales that span many orders of magnitude, thus exceeding what can be directly simulated using molecular dynamics. In this paper, dedicated to William A. Goddard III, we give a brief introduction to the accelerated molecular dynamics (AMD) methodology, which is aimed at this problem. AMD methods exploit the infrequent-event nature of the activated processes that typically comprise this long-time evolution. In favorable cases, these methods can predict state-to-state evolution that approximates what would result from an extremely long molecular dynamics simulation, and the most accurate of the methods can do this to arbitrary accuracy. We present some examples of applications of these methods and discuss the greatest ongoing challenge for the methods.

Positron annihilation spectroscopy and transmission electron microscopy yield previously unknown insights on radiation damage., The nondestructive investigation of single vacancies and vacancy clusters in ion-irradiated samples requires a depth-resolved probe with atomic sensitivity to defects. The recent development of short-pulsed positron beams provides such a probe. Here, we combine depth-resolved Doppler broadening and positron annihilation lifetime spectroscopies to identify vacancy clusters in ion-irradiated Fe and measure their density as a function of depth. Despite large concentrations of dislocations and voids in the pristine samples, positron annihilation measurements uncovered the structure of vacancy clusters and the change in their size and density with irradiation dose. When combined with transmission electron microscopy measurements, the study demonstrates an association between the increase in the density of small vacancy clusters with irradiation and a remarkable reduction in the size of large voids. This, previously unknown, mechanism for the interaction of cascade damage with voids in ion-irradiated materials is a consequence of the high porosity of the initial microstructure.