Prediction of irradiation spectrum effects in pyrochlores

The formation energy of cation antisites in pyrochlores (A2B2O7) has been correlated with the susceptibility to amorphize under irradiation, and thus, density functional theory calculations of antisite energetics can provide insights into the radiation tolerance of pyrochlores. Here, we show that the formation energy of antisite pairs in titanate pyrochlores, as opposed to other families of pyrochlores (B = Zr, Hf, or Sn), exhibits a strong dependence on the separation distance between the antisites. Classical molecular dynamics simulations of collision cascades in Er2Ti2O7 show that the average separation of antisite pairs is a function of the primary knock-on atom energy that creates the collision cascades. Together, these results suggest that the radiation tolerance of titanate pyrochlores may be sensitive to the irradiation conditions and might be controllable via the appropriate selection of ion beam parameters

Molecular dynamics modelling of radiation damage in normal, partly inverse and inverse spinels

The radiation response of perfect crystals of MgAl2O4, partially inverted MgGa2O4 and fully inverse MgIn2O4 were investigated using molecular dynamics. Dynamical cascades were initiated in these spinels over a range of trajectories with energies of 400 eV and 2 keV for the primary knock-on event. Collision cascades were set up on each of the cation and anion sublattices and were monitored up to 10 ps. Simulations in the normal MgAl2O4 spinel for the 2 keV energy regime resulted in similar defect structures as obtained at the post-threshold 400 eV energies, with little clustering occurring. The predominant defect configurations were split interstitials and cation antisites. For the inverse spinels, a much wider variety of lattice imperfections was observed. More defects were also produced due to the formation of interstitialvacancy cation chains and oxygen crowdions

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinel that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. These results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives

Radiation Effects in Solids - NATO Science Series II - Mathematics, Physics and Chemistry

The purpose of this book is to provide students with a comprehensive overview of fundamental principles and relevant technical issues associated with the behavior of solids exposed to high-energy radiation. These issues are important to the development of materials for existing fission reactors or future fusion and advanced reactors for energy production; to the development of electronic devices such as high-energy detectors; and to the development of novel materials for electronic and photonic applications (particularly on the nanoscale). The book details a broad range of topics falling into three general categories: (i) radiation damage fundamentals; (ii) materials dependent radiation damage phenomena; (iii) special topics (including swift ion irradiation effects, nanostructure design via irradiation, radiation detectors, and many other topics). This book serves to demonstrate the crucial interplay between experimental and theoretical investigations of radiation damage phenomena. The book explores computer simulation methods for the examination of radiation effects, ranging from molecular dynamics (MD) simulations of events occurring on short timescales (ps – ns), to methods such as kinetic Monte Carlo and kinetic rate theory, which consider damage evolution over times ranging from μs to hours beyond the initial damage event. The book also examines some of the experimental techniques used to assess radiation damage accumulation in solids, including transmission electron microscopy, ion channeling, nanoindentation, and positron annihilation, to name only a few techniques.JRC.E.3-Materials researc

Diffusion of small self-interstitial clusters in silicon: Temperature-accelerated tight-binding molecular dynamics simulations

Temperature-accelerated tight-binding molecular dynamics simulations show that self-interstitial clusters formed from two and three defects are mobile species. In particular, the di-interstitial (I2) cluster is found to diffuse nearly as fast as the single self-interstitial (dumbbell) over a wide temperature range. In particular, at room temperature I2 is found to diffuse at a rate similar to the dumbbell, thus making an important contribution to silicon self-diffusion at temperatures relevant for silicon bulk processing. The simulations also reveal the atomistic mechanisms responsible for the defects’ mobility, showing that the I2 cluster must be promoted to a metastable state in which it executes several diffusive events before decaying to a new ground-state configuration equivalent to the initial one

Theoretical Studies of Self-Diffusion and Dopant Clustering in Semiconductors

n limited. 1. Introduction Diffusion in semiconductors is of great importance for semiconductor technology. As the dimensions of circuits shrink, an understanding of the atomistic-scale mechanisms of diffusion processes will become crucial in order to accurately model and design future devices. A prerequisite to the understanding of dopant diffusion is the mechanism of self-diffusion in semiconductors since, for example, Ge, Sb, and As are believed to diffuse in Si by a mechanism involving a vacancy in the Si lattice [1]. Accurate estimates for the formation energy and entropy of defects are therefore essential for determining dopant diffusion rates. It is also important to understand the clustering mechanisms since the formation rate of dopant clusters, e.g. B in Si, and the identity of the clusters actually formed will strongly influence the determination of the final profiles of active B in the substrate [2]. Theoretical calculations can give valuable insight into the diffusion

The conundrum of relaxation volumes in first-principles calculations of charged defects in UO2

The defect relaxation volumes obtained from density-functional theory (DFT) calculations of charged vacancies and interstitials are much larger than their neutral counterparts, seemingly unphysically large. We focus on UO 2 as our primary material of interest, but also consider Si and GaAs to reveal the generality of our results. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defects in periodic DFT. We probe the dependence of the proposed energy corrections to charged defect formation energies on relaxation volumes and find that corrections such as potential alignment remain ambiguous with regards to its contribution to the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprising individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights the issue that, as is well-known for defect formation energies, the defect formation volumes depend on the choice of reservoir. We show that considering the change in volume of the electron reservoir in the formation reaction of the charged defects, analogous to how volumes of atoms are accounted for in defect formation volumes, can renormalize the formation volumes of charged defects such that they are comparable to neutral defects. This approach enables the description of the elastic properties of isolated charged defects within an overall neutral material

Structure and mobility of defects formed from collision cascades in MgO

We study radiation-damage events in MgO on experimental time scales by augmenting molecular dynamics cascade simulations with temperature accelerated dynamics, molecular statics, and density functional theory. At 400 eV, vacancies and mono- and di-interstitials form, but often annihilate within milliseconds. At 2 and 5 keV, larger clusters can form and persist. While vacancies are immobile, interstitials aggregate into clusters (In) with surprising properties; e.g., an I4 is immobile, but an impinging I2 can create a metastable I6 that diffuses on the nanosecond time scale but is stable for years

Thermally Induced Interdiffusion and Precipitation in a Ni/Ni₃Al System

<div><p>Ordered Ni₃Al intermetallic precipitates constitute the main hardening sources of Ni-based superalloys. Here, we report the interdiffusion and precipitation behavior in a Ni/Ni₃Al model system. The deposition of Ni₃Al on a pure Ni layer at 500°C generated L1₂-structured γ′ (Ni₃Al) precipitates, preferentially at the interface. After annealing at 800°C for 1 h, interdiffusion between Ni and Ni₃Al layers occurred, and the γ′ precipitates that grew near the parent Ni/Ni₃Al interface are ∼2.8 times larger in size than those formed in the matrix. Monte Carlo simulations indicate that vacancies preferentially diffuse along the Ni/Ni₃Al interface, increasing the probability of precipitation.</p></div