The role of defect charge, crystal chemistry, and crystal structure on positron lifetimes of vacancies in oxides.

Density functional theory based positron lifetime (PL) calculations for cation and oxygen monovacancies in a range of oxides-hematite, magnetite, hercynite, and alumina-have been conducted to compare the impact of defect chemistry and crystal structure on the predicted lifetimes. The role of defect charge state has also been examined. A comparison across the same type of crystalline structure but different composition shows that oxygen vacancies only induce a slight increase in the positron-electron overlap and thus barely modify the PL as compared to the bulk.

A new compositional microscopic degree of freedom at grain boundaries in complex compounds: a case study in spinel.

The accurate computational treatment of polycrystalline materials requires the rigorous generation of grain boundary (GB) structures as many quantities of interest depend strongly on the specifics of the macroscopic and microscopic degrees of freedom (DoFs) used in their creation. In complex materials, containing multiple sublattices and where atomic composition can vary spatially through the system, we introduce a new microscopic DoF based on this compositional variation which we find governs observable properties. In spinel - a wide class of complex oxides where this compositional variation manifests as cation inversion - we exploit this DoF to generate and analyze low-energy microstates of two GBs with three spinel chemistries (FeCrO, NiCrO and MgAlO).

How inversion relates to disordering tendencies in complex oxides.

Complex oxides exhibit great functionality due to their varied chemistry and structures. They are quite flexible in terms of the ordering of cations, which can also impact their functional properties to a large extent. Thus, the propensity for a complex oxide to disorder is a key factor in optimizing and discovering new materials.

Atomistic Simulation Informs Interface Engineering of Nanoscale LiCoO.

Lithium-ion batteries continue to be a critical part of the search for enhanced energy storage solutions. Understanding the stability of interfaces (surfaces and grain boundaries) is one of the most crucial aspects of cathode design to improve the capacity and cyclability of batteries. Interfacial engineering through chemical modification offers the opportunity to create metastable states in the cathodes to inhibit common degradation mechanisms.

Modeling Disorder in Pyrochlores and Other Anion-Deficient Fluorite Structural Derivative Oxides.

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.

Band-Edge Engineering To Eliminate Radiation-Induced Defect States in Perovskite Scintillators.

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 Ce-doped LuAlO as a model material system, using density functional theory (DFT)-based DFT + and hybrid Heyd-Scuseria-Ernzerhof (HSE) calculations.

A new mechanism for void-cascade interaction from nondestructive depth-resolved atomic-scale measurements of ion irradiation-induced defects in Fe.

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.

Correction: Spatially-varying inversion near grain boundaries in MgAlO spinel.

[This corrects the article DOI: 10.1039/D0RA00700E.].

Chemical manipulation of hydrogen induced high p-type and n-type conductivity in GaO.

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 β-GaO, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping.

Spatially-varying inversion near grain boundaries in MgAlO spinel.

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 MgAlO spinel, that this is not the case and that the level of inversion at the grain boundary plane is different than in the bulk.

An Overview of Recent Standard and Accelerated Molecular Dynamics Simulations of Helium Behavior in Tungsten.

One of the most critical challenges for the successful adoption of nuclear fusion power corresponds to plasma-facing materials. Due to its favorable properties in this context (low sputtering yield, high thermal conductivity, high melting point, among others), tungsten is a leading candidate material. Nevertheless, tungsten is affected by the plasma and fusion byproducts.

Dissociated vacancies and screw dislocations in MgO and UO: atomistic modeling and linear elasticity analysis.

Understanding the effect of dislocations on the mass transport in ionic ceramics is important for understanding the behavior of these materials in a variety of contexts. In particular, the dissociated nature of vacancies at screw dislocations, or more generally, at a wide range of low-angle twist grain-boundaries, has ramifications for the mechanism of defect migration and thus mass transport at these microstructural features. In this paper, a systematic study of the dissociated vacancies at screw dislocations in MgO is carried out.

Multifidelity Information Fusion with Machine Learning: A Case Study of Dopant Formation Energies in Hafnia.

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.

Hydrogen interactions with low-index surface orientations of tungsten.

We report on density functional theory calculations that have been performed to systematically investigate the hydrogen-surface interaction as a function of surface orientation. The interactions that were analyzed include stable atomic adsorption sites, molecular hydrogen dissociation and absorption energies, migration pathways and barriers on tungsten surfaces, and the saturation coverage limits on the (1 1 1) surface. Stable hydrogen adsorption sites were found for all surfaces.

The impact of chemical order on defect transport in mixed pyrochlores.

Using temperature accelerated dynamics, an accelerated molecular dynamics method, we examine the relationship between composition and cation ordering and defect transport in the mixed pyrochlore Gd2(Ti1-xZrx)2O7, using the oxygen vacancy as a representative defect structure. We find that the nature of transport is very sensitive to the cation structure, transitioning, as a function of composition, from three-dimensional migration to two-dimensional to pseudo-one-dimensional to becoming essentially immobile before reverting back to three-dimensional as the Zr content is increased. The rates of migration are also affected by the cation structure in the various compositions.

Physics-informed machine learning for inorganic scintillator discovery.

Applications of inorganic scintillators-activated with lanthanide dopants, such as Ce and Eu-are found in diverse fields. As a strict requirement to exhibit scintillation, the 4f ground state (with the electronic configuration of [Xe]4f 5d) and 5d lowest excited state (with the electronic configuration of [Xe]4f 5d) levels induced by the activator must lie within the host bandgap. Here we introduce a new machine learning (ML) based search strategy for high-throughput chemical space explorations to discover and design novel inorganic scintillators.

Role of Sink Density in Nonequilibrium Chemical Redistribution in Alloys.

Nonequilibrium chemical redistribution in open systems submitted to external forces, such as particle irradiation, leads to changes in the structural properties of the material, potentially driving the system to failure. Such redistribution is controlled by the complex interplay between the production of point defects, atomic transport rates, and the sink character of the microstructure. In this work, we analyze this interplay by means of a kinetic Monte Carlo (KMC) framework with an underlying atomistic model for the Fe-Cr model alloy to study the effect of ideal defect sinks on Cr concentration profiles, with a particular focus on the role of interface density.

Evidence for percolation diffusion of cations and reordering in disordered pyrochlore from accelerated molecular dynamics.

Diffusion in complex oxides is critical to ionic transport, radiation damage evolution, sintering, and aging. In complex oxides such as pyrochlores, anionic diffusion is dramatically affected by cation disorder. However, little is known about how disorder influences cation transport.

On the mobility of carriers at semi-coherent oxide heterointerfaces.

In the quest to develop new materials with enhanced ionic conductivity for battery and fuel cell applications, nano-structured oxides have attracted attention. Experimental reports indicate that oxide heterointerfaces can lead to enhanced ionic conductivity, but these same reports cannot elucidate the origin of this enhancement, often vaguely referring to pipe diffusion at misfit dislocations as a potential explanation. However, this highlights the need to understand the role of misfit dislocation structure at semi-coherent oxide heterointerfaces in modifying carrier mobilities.

The mobility of small vacancy/helium complexes in tungsten and its impact on retention in fusion-relevant conditions.

Tungsten is a promising plasma facing material for fusion reactors. Despite many favorable properties, helium ions incoming from the plasma are known to dramatically affect the microstructure of tungsten, leading to bubble growth, blistering, and/or to the formation of fuzz. In order to develop mitigation strategies, it is essential to understand the atomistic processes that lead to bubble formation and subsequent microstructural changes.

Disorder-induced transition from grain boundary to bulk dominated ionic diffusion in pyrochlores.

We use molecular dynamics simulations to investigate the role of grain boundaries (GBs) on ionic diffusion in pyrochlores, as a function of the GB type, chemistry of the compound, and level of cation disorder. We observe that the presence of GBs promotes oxygen transport in ordered and low-disordered systems, as the GBs are found to have a higher concentration of mobile carriers with higher mobilities than in the bulk. Thus, in ordered samples, the ionic diffusion is 2D, localized along the grain boundary.

Cr incorporated phase transformation in YO under ion irradiation.

Under irradiation, chemical species can redistribute in ways not expected from equilibrium behavior. In oxide-dispersed ferritic alloys, the phenomenon of irradiation-induced Cr redistribution at the metal/oxide interfaces has drawn recent attention. Here, the thermal and irradiation stability of the FeCr/YO interface has been systematically studied.

The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores.

Pyrochlores, a class of complex oxides with formula A2B2O7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. In this work, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores as a function of pyrochlore chemistry. We find that both Pu(3+) and Pu(4+) segregate to the surface for the four low-index pyrochlore surfaces considered, and that the segregation energy varies with the chemistry of the compound.