Oxygen diffusion in ThO2-CeO2 and ThO2-UO2 solid solutions from atomistic calculations.

We elucidate oxygen diffusivity in ThO2-CeO2 and ThO2-UO2 solid solutions across their whole concentration ranges in the phase diagram using static pair-potential calculations and molecular dynamics simulations. Between pure CeO2 (and UO2) and pure ThO2, oxygen diffusivity is higher in CeO2 (and UO2) due to lower oxygen migration barriers. With the addition of Th to CeO2 (and UO2) in the phase diagram, the diffusivity decreases due to the increase in the migration barriers introduced by a larger ionic radius of Th. On the other side of the phase diagram, with the addition of Ce to ThO2 oxygen diffusion decreases due to oxygen vacancy binding with Ce, even though the migration barriers decrease due to the smaller size of Ce than the host Th. Using these calculations, we provide a schematic of high oxygen diffusivity regions in the phase diagram. We also compare the impact of tetravalent dopants (e.g. actinides) on oxygen vacancy energetics to that of trivalent dopants (e.g. lanthanides). We find that trivalent dopants bind much more strongly with oxygen vacancy than the tetravalent dopants. We also find that the tetravalent dopants that have larger radii than the host cation have negative oxygen vacancy binding energy, whereas all trivalent dopants have positive binding energy irrespective of their ionic radii. This work thus highlights key differences in the oxygen vacancy energetics between the trivalent and tetravalent cations.

The effect of electronic energy loss on irradiation-induced grain growth in nanocrystalline oxides.

Grain growth of nanocrystalline materials is generally thermally activated, but can also be driven by irradiation at much lower temperature. In nanocrystalline ceria and zirconia, energetic ions deposit their energy to both atomic nuclei and electrons. Our experimental results have shown that irradiation-induced grain growth is dependent on the total energy deposited, where electronic energy loss and elastic collisions between atomic nuclei both contribute to the production of disorder and grain growth. Our atomistic simulations reveal that a high density of disorder near grain boundaries leads to locally rapid grain movement. The additive effect from both electronic excitation and atomic collision cascades on grain growth demonstrated in this work opens up new possibilities for controlling grain sizes to improve functionality of nanocrystalline materials.

Stabilizing nanocrystalline grains in ceramic-oxides.

Nanocrystalline ceramic-oxides are prone to grain growth rendering their highly attractive properties practically unusable. Using atomistic simulations of ceria as a model material system, we elucidate a framework to design dopant-pinned grain boundaries that prevent this grain growth. While in metallic systems it has been shown that a large mismatch between host and dopant atomic sizes prevents grain growth, in ceramic-oxides we find that this concept is not applicable. Instead, we find that dopant-oxygen vacancy interaction, i.e., dopant migration energy in the presence of an oxygen vacancy, and dopant-oxygen vacancy binding energy are the controlling factors in grain growth. Our prediction agrees with and explains previous experimental observations.

Nanoprecipitates to Enhance Radiation Tolerance in High-Entropy Alloys.

The growth of advanced energy technologies for power generation is enabled by the design, development, and integration of structural materials that can withstand extreme environments, such as high temperatures, radiation damage, and corrosion. High-entropy alloys (HEAs) are a class of structural materials in which suitable chemical elements in four or more numbers are mixed to typically produce single-phase concentrated solid solution alloys (CSAs). Many of these alloys exhibit good radiation tolerance like limited void swelling and hardening up to relatively medium radiation doses (tens of displacements per atom (dpa)); however, at higher radiation damage levels (>50 dpa), some HEAs suffer from considerable void swelling limiting their near-term acceptance for advanced nuclear reactor concepts. In this study, we developed a HEA containing a high density of Cu-rich nanoprecipitates distributed in the HEA matrix. The Cu-added HEA, NiCoFeCrCu0.12, shows excellent void swelling resistance and negligible radiation-induced hardening upon irradiation up to high radiation doses (i.e., higher than 100 dpa). The void swelling resistance of the alloy is measured to be significantly better than NiCoFeCr CSA and austenitic stainless steels. Density functional theory simulations predict lower vacancy and interstitial formation energies at the coherent interfaces between Cu-rich nanoprecipitates and the HEA matrix. The alloy maintained a high sink strength achieved via nanoprecipitates and the coherent interface with the matrix at a high radiation dose (∼50 dpa). From our experiments and simulations, the effective recombination of radiation-produced vacancies and interstitials at the coherent interfaces of the nanoprecipitates is suggested to be the critical mechanism responsible for the radiation tolerance of the alloy. The materials design strategy based on incorporating a high density of interfaces can be applied to high-entropy alloy systems to improve their radiation tolerance.

∑3 Twin Boundaries in Gd2Ti2O7 Pyrochlore: Pathways for Oxygen Migration.

Understanding the chemistry at twin boundaries (TB) is a well-recognized challenge, which could enable the capabilities to manipulate the functional properties in complex oxides. The study of this atomic imperfection becomes even more important, as the presence of twin boundaries has been widely observed in materials, regardless of the dimensionalities, due to the complexities in growth methods. In the present study, we provide atomic-scale insights into a ∑3(111̅) ⟨11̅0⟩ twin boundary present in pyrochlore-structured Gd2Ti2O7 using atomic-resolution electron microscopy and atomistic modeling. The formation of the observed TB occurs along (111̅) with a 71° angle between two symmetrically arranged crystals. We observe distortions (∼3 to 5% strain) in the atomic structure at the TB with an increase in Gd-Gd (0.66 ± 0.03 nm) and Ti-Ti (0.65 ± 0.02 nm) bond lengths in the (11̅0) plane, as compared to 0.63 nm in the ordered structure. Using atomistic modeling, we further calculate the oxygen migration barrier for vacancy hopping at 48f-48f sites in the pyrochlore structure, which is the primary diffusion pathway for fast oxygen transport. The mean migration barrier is lowered by ∼25% to 0.9 eV at the TB as compared to 1.23 eV in the bulk, suggesting the ease in oxygen transport through the ∑3 twin boundaries. Overall, these results offer a critical understanding of the atomic arrangement at the twin boundaries in pyrochlores, leading to control of the interplay between defects and properties.

Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7.

The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

(001) SrTiO3 | (001) MgO interface and oxygen-vacancy stability from first-principles calculations.

In-depth understanding of interfacial atomistic structures is required to design heterointerfaces with controlled functionalities. Using density functional theory calculations, we investigate the interfacial structure of (001) SrTiO3 | (001) MgO, and characterize the stable interface structure. Among the four types of possible interface structures, we show that the TiO2-terminated SrTiO3 containing electrostatically attractive Mg-O and Ti-O ion-ion interactions forms the most stable interface. We also show that oxygen vacancies can be preferentially stabilized across the interface via manipulating interfacial strain. We elucidate that oxygen vacancies are most stable in the tensile-strain material, and unstable in compressive strain material. This stability is explained from equation-of-state analysis using a single crystal, where the oxygen vacancy shows a larger volume than the oxygen ion, thus explaining its stability under tensile-strained conditions.