Behavior Of Point Defects Research Articles

Heterogeneity in Point Defect Distribution and Mobility in Solid Ion Conductors.

Alkali metal anodes paired with solid ion conductors offer promising avenues for enhancing battery energy density and safety. To facilitate rapid ion transport crucial for fast charging and discharging of batteries, it is essential to understand the behavior of point defects in these conductors. In this study, we investigate the heterogeneity of defect distribution in two prototypical solid ion conductors, Li3OCl and Li2PO2N (LiPON), by quantifying the defect formation energy (DFE) as a function of distance from the surface and interface through first-principles simulations. To simulate defects at the electrode-electrolyte interface, we perform calculations of Li+ vacancy in Li3OCl near its interface with lithium metal. Our results reveal a significant difference between the bulk and surface/interface DFE which could lead to defect aggregation/depletion near the surface/interface. Interestingly, while Li3OCl has a lower surface DFE than the bulk in most cases, LiPON follows the opposite trend with a higher surface DFE compared to the bulk. Due to this difference between bulk and surface DFE, the defect density can be up to 14 orders of magnitude higher at surfaces compared to the bulk. Further, we reveal that the DFE transition from surface/interface to bulk is precisely characterized by an exponentially decaying function. By incorporating this exponential trend, we develop a revised model for the average behavior of defects in solid ion conductors that offers a more accurate description of the influence of grain sizes. Surface effects dominate for grain sizes ≲1 μm, highlighting the importance of surface defect engineering and the DFE function for accurately capturing ion transport in devices. We further explore the kinetics of defect redistribution by calculating the migration barriers for defect movement between bulk and surfaces. We find a highly asymmetric energy landscape for the lithium vacancies, exhibiting lower migration barriers for movement toward the surface compared to the bulk, while interstitial defects exhibit comparable kinetics between surface and bulk regions. These insights highlight the importance of considering both thermodynamic and kinetic factors in designing solid ion conductors for improved ion transport at surfaces and interfaces.

First-principles study of behaviors of point defects in Mo-14 Re alloy

Molybdenum alloys with rhenium additions of 14 % (Mo-14 Re) are regarded as structural materials for advanced nuclear reactors. The performance of Mo-14 Re alloy under irradiation is crucial for its application. Irradiation performance is related to the behaviors of point defects produced by irradiation. The behaviors of point defects in Mo-14 Re alloy were investigated using first-principles methods in this work. Compared with the results in pure Mo alloy, the formation energy and the migration energy of vacancy in Mo-14 Re were both reduced, and the migration energy of interstitial dumbbell increased while the formation energy decreased in Mo-14 Re. In Mo-14 Re, vacancy preferred to form and diffuse faster in Re-enriched environment. Knowing the number of Re atoms in the 1NN (first nearest neighbor) shell of the vacancy and saddle point, the vacancy formation energy and migration energy can be estimated. Since the interstitial atom has a certain orientation, the distribution of the atoms around it is anisotropic. To estimate the interstitial formation energy, it was necessary to count the number of Re atoms at the tensile and compressive sites at the 1NN shell around the interstitial. The interstitial motion was three-dimensional (3D) in Mo-14 Re. At 1200 K, the diffusion coefficient of Re atom was a little higher than that of Mo atom, which could lead to the segregation of Re atoms at defect sinks and the formation of Re-enriched phase. It is possible to investigate the longer-term evolution of microstructure using cluster dynamics (CD) or kinetic monte carlo (KMC) techniques based on these results.

First-Principles Calculations on Diffusion and Association Behavior of Fluoride Ions in Ba-Doped LaF3 Solid Electrolyte

Fluoride ion batteries, which generate electrical energy through the fluorination/de-fluorination reaction of electrode active materials, are expected to be the next-generation storage batteries with high energy density. LaF3 with a tysonite-type structure is known as one of the representative solid-state fluoride ion conductors. Substitutional solid solution of divalent cations such as Ba or Sr ions at La sites forms fluoride ion vacancies as charge compensation, resulting in fluoride ion conductivity. The main objective of this study is to analyze the formation behavior of point defects in LaF3 through quantitative evaluation of their formation energies using first-principles calculations, and to elucidate the conduction mechanism of fluoride ions in LaF3.The plane-wave basis PAW method implemented in VASP code was used for the electronic structure calculations. The electron correlation treatment uses the PBEsol-type potential based on the generalized gradient approximation. Calculations for given point defects were performed. The cutoff energy of the plane wave basis sets was set to 420 eV, and the structure optimization calculations were performed until the Hellmann-Feynman force acting on each atom was less than 0.02 eV/Å.There are three fluoride ion sites (12g, 4d, and 2a) in the unit cell of LaF3. Using the supercell model, we calculated the defect formation energies of fluoride ion vacancies at each site. The formation energies at the 12g, 4d, and 2a sites are 0.17 eV, 0.52 eV, and 0.52 eV, respectively. According to the defect formation energies, fluoride ion vacancies are most likely to form at the 12g sites.To elucidate the elementary processes of fluoride ion transfer between 12g sites by the vacancy mechanism, we performed first-principles calculations and transition state search using the Nudged Elastic Band method. From symmetry, four different pathways exist in the lattice of undoped LaF3. The pathway with the highest energy barrier is shown to have very low transfer energies, 0.17 eV, and the energy barriers of the other pathways are less than 0.1 eV. These results suggest that LaF3 has a potential for very fast fluoride ionic conductivity by the vacancy mechanism. In the presentation, the mechanism of fluoride ion conduction in Ba-doped LaF3 will also be reported. This work was supported by the RISING2 (JPNP16001) and the RISING3 (JPNP21006) projects from the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Effect of grain size on gas bubble evolution in nuclear fuel: Phase-field investigations

Numerous irradiation-induced gas bubbles are created in the nuclear fuel during irradiation, leading to the change of microstructure and the degradation of mechanical and thermal properties. The grain size of fuel is one of the important factors affecting bubble evolution. In current study, we first predict the thermodynamic behaviors of point defects as well as the interplay between vacancy and gas atom in both UO2 and U3Si2 according to ab initio approach. Then, we establish the irradiation-induced bubble phase-field model to investigate the formation and evolution of intra- and inter-granular gas bubbles. The effects of fission rate and temperature on the evolutions of bubble morphologies in UO2 and U3Si2 have been revealed. Especially, a comparison of porosities under different grain sizes is examined and analyzed. To understand the thermal conductivity as functions of grain size and porosity, the heat transfer capability of U3Si2 is evaluated.

Intermittent Defect Fluctuations in Oxide Heterostructures.

The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X-ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoOx with different degrees of stability of the oxygen vacancies are observed. Ab-initio-informed phase-field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X-rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemicalapplications.

Peculiarities of the Behavior of Point Defects under the Optoplastic Effect in Copper

The authors previously discovered a new optoplastic effect and observed it under the action of a nanosecond UV laser pulse irradiation of subcritical intensity. In this paper it is shown that under this effect no micropores arise in the subsurface layer of metal. This proves the statement that swelling of metal under laser impact of moderate (subcritical) intensity occurs due to interstitial atoms migrating to the surface and not due to melting with formation of bubbles. At a abrupt cooling (for ~20 μs) interstitial atoms migrate to the surface by the Schottky mechanism due to abnormal mass transfer and the less mobile vacancies have no time to coagulate with formation of micropores in the time of the process.

Peculiarities of the Behavior of Point Defects under the Optoplastic Effect in Copper

The authors previously discovered a new optoplastic effect and observed it under the action of a nanosecond UV laser pulse irradiation of subcritical intensity. In this paper it is shown that under this effect no microporesarise in the subsurface layer of metal. This proves the statement that swelling of metal under laser impactof moderate (subcritical) intensity occurs due to interstitial atoms migrating to the surface and not due to melting with formation of bubbles. At a abrupt cooling (for ~20 μs) interstitial atoms migrate to the surfaceby the Schottky mechanism due to abnormal mass transfer and the less mobile vacancies have no time tocoagulate with formation of micropores in the time of the process.

The formation and migration of point defects and the thermodynamic behavior of TiD2 by first-principles calculations

TiD2 has important applications in nuclear materials, for example neutron generator. Therefore, it is of great importance to study the evolution of the defects in TiD2 during the service. In this work, first-principles calculations are performed to study the formation and migration behaviors of point defects in TiD2, as well as the phase stability of TiD2. The formation energies of Ti vacancy (VTi) and D vacancy (VD) in TiD2 are 2.34 and 0.73 eV, respectively. The migration barriers of VD are 0.80 and 0.66 eV for the migration path within ab plane and bc plane, respectively, which is much lower than the those of the interstitial D atoms. So, the predominant diffusion mechanism of a D atom in TiD2 is the vacancy hopping mechanism. Furthermore, we find that VTi will trap interstitial D atoms and D vacancies. At last, the thermal stability of TiD2 is investigated and we find that interstitial D atoms and D vacancies are formed during the molecular dynamics simulations at 1200 K. This work presents a microscopic picture for the formation and transformation of the point defects in TiD2.

Ultra-wide void denuded zone near composite grain boundary in micro-nano crystalline 304L steels

Grain boundaries in materials alter the behaviors of point defects and can subsequently affect their properties and performance. Here we report the formation of the ultra-wide void denuded zone (VDZ) with a width of up to about 650 nm, near the composite grain boundary (CGB) in Micro-Nano composite 304 L stainless steels (MN304-Las). Three kinds of MN304-Las with different nanocrystalline grain sizes but the same microcrystalline grain sizes were irradiated by 6 MeV Au3+ ions to a peak dose of 216 dpa at 600 ℃. The strong ability to absorb and accommodate vacancies of CGB plays an important role in the formation of ultra-wide VDZ.

Atomic study of the trapped and migration patterns of point defects around screw dislocation in tungsten

The interaction of 1/2 〈1 1 1〉 screw dislocation and point defects, including vacancy and self-interstitial atom (SIA), was investigated by molecular dynamics (MD) simulations. Molecular statics (MS) are used to analyze the factors affecting the binding energy of point defects around the dislocation. The results show that the point defects around the dislocation core have the highest binding energy. In addition, we use elasticity theory to find that the point defects binding is interrelated to strain distribution of dislocation and the residual stress of point defects. Furthermore, nudged elastic band (NEB) calculations have been applied to find that vacancy and SIA tend to migrate along the dislocation line around the core region. The results can clearly show the interaction behavior of point defects and screw dislocation, and provide a reasonable explanation for the diffusion mechanism of mono-vacancy and a single SIA around the dislocation line.

Spatial inhomogeneity of point defect properties in refractory multi-principal element alloy with short-range order: A first-principles study

Short-range order can be developed in multi-principal element alloys and influences the point defect behavior due to the large variation of the local chemical environment. The effect of short-range order on vacancy and interstitial formation energy and migration behavior was studied in body-centered cubic multi-principal element alloy NbZrTi by first-principles calculations. Two short-range order structures created by density functional theory and Monte Carlo method at 500 and 800 K were compared with the structure of random solid solution. Both vacancy and interstitial formation energies increase with the degree of short-range order. Point defect formation energies tend to be higher in regions enriched in Nb and lower in regions enriched in Zr and Ti. Both vacancies and interstitials prefer to migrate toward Zr,Ti-rich regions and away from Nb-rich regions, suggesting that Zr,Ti-rich regions can potentially act as recombination centers for point defect annihilation. Compared to an ideal random solid solution, the short-range order increases the spatial inhomogeneity of point defect energy landscape. Tuning the degree of short-range order by different processing techniques can be a viable strategy to optimize the point defect behavior to achieve enhanced radiation resistance in multi-principal element alloys.

Anomalous temperature dependence of self-interstitial diffusivity in metallic lithium and sodium

Despite the abundance and importance of alkali metals for various applications, the behavior of point and dimensional defects in them has remained terra incognita. At the same time, the growing interest to high-capacity Li-metal batteries and the study of Li electrodeposition mechanisms dictates the need to investigate the defect dynamics. Here, we report the results of MD simulations of atomic motions in bulk Li and Na, including vacancy and interstitial (particularly <111>-crowdion) defect dynamics. Our main finding is an anomalous temperature dependence of the apparent diffusion coefficient for self-interstitial atoms (SIA), namely, the diffusivity decrease with the temperature rise. To uncover the underlying mechanism, we analyzed the interplay between 1D motions in terms of mean squared displacement (MSD) and 3D motions due to SIA rotations or switching. We found that the coefficient for 1D crowdion migration along the 〈111〉 axes is virtually temperature independent in case of both Li and Na. This implies linear temperature dependence of the drag coefficient γ, in contrast to other BCC metals. High rotation frequency reaching several THz at 300 K seems to drive the 1D diffusion into a semi-ballistic mode, therefore also affecting the temperature dependence of the apparent 3D diffusion coefficient. We believe that our results contribute to the atomistic understanding of low-barrier processes in a crystal lattice.

Numerical simulations considering effects of thermal stress and impurities on the behavior of intrinsic point defects in Si crystal growing by Cz method

Numerical simulations considering effects of thermal stress and impurities on the behavior of intrinsic point defects in Si crystal growing by Cz method

The role of point defects related with carbon impurity on the kink of log J–V in GaN-on-Si epitaxial layers

Carbon impurity as point defects makes key impact on the leakage in GaN-on-Si structures. GaN-based epitaxial layers with different point defects by changing carbon-doped concentration were used to investigate the point defects behavior. It was found that leakage mechanisms correspond with space-charge-limited current models at low voltages, and after 1st kink, electron injection from silicon to GaN and PF conduction play a key role in the leakage of both point defects case with low carbon and high carbon doped. In addition, high carbon in GaN-on-Si epitaxial layers obtained lower leakage and larger breakdown voltage. The slope of log J–V has two kinks and effective energy barrier E a has two peaks, 0.4247 eV at about 300 V and 0.3485 eV at about 900 V, respectively, which is related to accepted states and donor states related with carbon impurity. While the slope of log J–V has one kink and effective energy barrier E a has one peak, 0.4794 eV at about 400 V of low carbon in GaN-on-Si epitaxial layers, indicating only field-induced accepted ionized makes impact on leakage. The comparative results of more donor trap density in high carbon indicate point defects related with carbon impurity play a key role in the kinks of log J–V slope.

Trends in the Behavior of Point Defects in MB and MAB Phases

Binary and ternary transition metal borides exhibit many outstanding properties, such as high melting temperature, high temperature strength, and electrical and thermal conductivities. However, defect properties have been rarely studied so far. Here, using first-principles calculations, we investigate structural effects on defect recovery processes in MB and MAB phases focusing on how Al layers and the type of transition metal affect annihilation of defects. First, we find that metal interstitials cannot be accommodated in MAB phases that have two Al layers, but instead they form antisites, which are largely immobile and therefore detrimental for defect recovery processes. We also find that the absence of Al layers in MB phases that have B chains leads to faster migration of metal interstitials and lower recombination barriers of B Frenkel pairs and hence contributes to efficient defect recovery processes of those MB phases as compared to the corresponding MAB phases. In the other types of MB and MAB phases that have B rings, we find that the role of Al layers varies depending on the type of metal elements. Lastly, we find that a larger M–B bond strength, which depends on the type of transition metal in MAB phases, correlates with a higher recombination barrier of B Frenkel pairs. Based on the discovered trends, we provide insights into the selection of materials to help design diverse applications where defect recovery processes are important.

Using neural network potentials to study defect formation and phonon properties of nitrogen vacancies with multiple charge states in GaN

Investigation of charged defects is necessary to understand the properties of semiconductors. Whereas density functional theory calculations can accurately describe the relevant physical quantities, these calculations increase the computational loads substantially, which often limits the application of this method to large-scale systems. In this paper, we propose a different scheme of neural network potential (NNP) to analyze the point defect behavior in multiple charge states. The proposed scheme necessitates only minimal modifications to the conventional scheme. We demonstrated the prediction performance of the proposed NNP using wurzite-GaN with a nitrogen vacancy with charge states of 0, $1+$, $2+$, and $3+$. The proposed scheme accurately trained the total energies and atomic forces for all the charge states. Furthermore, it fairly reproduced the phonon band structures and thermodynamics properties of the defective structures. Based on the results of this paper, we expect that the proposed scheme can enable us to study more complicated defective systems and lead to breakthroughs in novel semiconductor applications.

A multimodal approach to revisiting oxidation defects in Cr2O3

The oxidation of chromium in air at 700 °C was investigated with a focus on point defect behavior and transport during oxide layer growth. A comprehensive set of characterization techniques targeted characteristics of chromium oxide microstructure and chemical composition analysis. TEM showed that the oxide was thicker with longer oxidation times and that, for the thicker oxides, voids formed at the metal/oxide interface. PAS revealed that the longer the oxidation time, there was an overall reduction in vacancy-type defects, though chromium monovacancies were not found in either case. EIS found that the longer oxidized material was more electrochemically stable and that, while all oxides displayed p-type character, the thicker oxide had an overall lower charge carrier density. Together, the results suggest anion oxygen interstitials and chromium vacancy cluster complexes drive transport in an oxidizing environment at this temperature, providing invaluable insight into the mechanisms that regulate corrosion.

Strain effects on the behavior of intrinsic point defects within the GaN/AlN interface

Defect behaviors in GaN-based compounds studied by using density functional theory have exhibited widely practical applications for optoelectronic devices. Ga is usually used to solubilize in AlN to release the lattice mismatch and the interface strain. In this study, we investigate the formation energies and diffusion barriers of intrinsic point defects within the GaN/AlN interface. The intrinsic defect with the lowest formation energy is positively charged N vacancy, and negatively charged Ga vacancy is the defect with second lower formation energy. Compared with the case in bulk GaN, the GaN/AlN interface promotes the generation of Ga vacancy and inhibits the generation of N vacancy. Meanwhile, the defect diffusion barrier of N vacancy within the GaN/AlN interface is higher than that in the bulk GaN. The hydrostatic compressive/ tensile strain significantly promotes the generation of these two vacancy defects, while biaxial tensile strain and the biaxial compressive strain slightly promote and inhibit the generation of these two vacancy defects, respectively. These interface and strain effects provide the significant information in understanding the electrical properties of GaN-based devices, especially under the extreme conditions. Our work therefore not only elucidates defect behaviors under different strains in GaN/AlN interface, but also paves the way for understanding and designing promising electronic devices with interface engineering.

Determining the diffusion behavior of point defects in zirconium by a multiscale modelling approach

Zr alloys are commonly used materials for in-core components of pressurized water reactors. The evolution of defects in Zr under irradiation conditions will directly contribute to the degradation of reactor components. For a better understanding of the evolution process of defects in Zr, a multiscale simulation was performed to study the diffusion behavior of radiation-induced point defects. Because the existing potentials cannot accurately describe the energy differences of stable states self-interstitial atoms (SIAs) in Zr, an embedded atom method potential that can exactly reproduce the energy difference between metastable states and ground states, and the binding energy of divacancy of Zr, was first developed by fitting a potential to point defect properties calculated by density functional theory and other basic crystal properties. Based on the constructed potential, molecular dynamics (MD) and kinetic Monte Carlo simulations were conducted to investigate the migration of point defects. We determined several frequent migration paths of SIAs by kinetic study, which is rarely reported in previous MD studies. The SIA exhibits obvious anisotropic diffusion characteristics at low temperatures (<600 K). The most frequent migration path for SIAs is the jump between the two nearest basal octahedral (BO) sites in the basal plane, which means that SIAs diffuse faster along the basal plane than along the c axis. It is found that diffusion within the basal plane tends to be two-dimensional (2D) diffusion rather than 1D diffusion, and there is a significant correlation effect for diffusion of SIAs along the basal plane. Additionally, the existence of trivacancy-SIA complex was found in the process of divacancy migration, which can inhibit divacancy migration. Monovacancies and divacancies exhibit anisotropic diffusion characteristics in the considered temperature range Divacancies have a much faster diffusion rate than monovacancies in the present MD simulation and can easily dissociate at high temperatures (>900 K). The rapid migration of the divacancies may contribute to the formation of vacancy dislocation loops. These results are meaningful to understand the evolution process of radiation-induced defects.

Chemical environment dependent Stabilities, electronic properties and diffusions behaviors of intrinsic point defects in novel Two-Dimensional MoSi2N4 monolayer

A novel 2D semiconductor, MoSi2N4 monolayer, was very recently successfully fabricated and shown great potential for future broad applications in nanoelectronic devices and catalysis. For the synthesis and application of the material, intrinsic point defects are required to be considered, because they are inevitably formed during the bottom-up synthesis and following substrate transfer process, and also can be intentionally introduced by defect engineering to modulate properties and explore applications of materials. Herein, eight types of intrinsic point defects including two vacancies (N mono- and di-vacancies; VN(m) and VN2(m)), four antisites (SiMo, SiN(t), MoN(m) and MoSi) and two adatoms (Siada and Nada) are found to have highly thermodynamic stabilities in MoSi2N4 monolayer under various chemical environments. Electronic properties calculations suggest the existence of VN(m) and VN2(m) leads to the emergence of the recombination center, reducing the carrier lifetime and thus showing the potential for high-speed switching device applications. SiN(t) and Siada-defected structures show p-type and n-type conducting characters, respectively, which can be explored for possible PN junction applications. Also, the existence of MoSi may induce unusual resistance–temperature dependence behavior, providing some chances to design high-mobility semiconductors. Combined with the migration behavior of point defects (monovacancies and adatoms), the Schottky defect formation mechanism of VN(m) could be reasonably explained. Besides, high migration barriers of monovacancy defects (VN(m), VSi and VMo) demonstrate their higher kinetic stability in monolayer, compared with monovacancies in graphene and silicene. The migration of N adatom is relatively difficult than that of Si adatom, increasing opportunities to detect it in experiments. This work will provide insight into defect engineering of MoSi2N4 monolayer and the MA2Z4 monolayer family for various applications.