Vacancy Trapping Research Articles

Effect of impurities on hydrogen defect stability and migration barrier in yttrium dihydride crystal

The impurity or alloying atoms in YH2 can alter the local electronic structure and so the hydrogen defect stability, as well as the H migration barrier energy. Thus, DFT calculations were employed to determine the effect of foreign elements from alkali and alkaline earth metals to transition metals and one critical impurity element, O, on H vacancy stability and retention characteristics in YH2. Results revealed that alloying elements act as hydrogen vacancy sinks by reducing the vacancy formation energy at neighboring sites. The implantation of non-magnetic foreign elements (s1, s2, and d10 valence electrons) in hydrogen energy landscape was calculated to be minor; while the hydrogen vacancy formation energy was reduced from 1.37 eV to 1.00 eV, the migration energy barrier of hydrogen was increased from 0.87 eV to 1.15 eV for non-magnetic foreign elements. The migration energy barrier monotonically decreased with increasing d-shell occupancy, reaching as low as 0.4 eV for Cr, Mo(d4), and Fe (d4). Alloying with late transition metals (d8 and d9) moderately impacted the hydrogen vacancy formation. Finally, it was found to be O addition into the YH2- lattice did not alter the energy landscape of hydrogen vacancies. Since alloyed YH2 has not been studied extensively, this study provides an atomistic understanding how alloying elements and impurities trap vacancies and affects hydrogen mobility YH2. Meanwhile, the main findings of this study may serve as guidelines for introducing alloying elements in ZrH2 as well.

Uniform Molecular Adsorption Energy‐Driven Homogeneous Crystallization and Dual‐Interface Modification for High Efficiency and Thermal Stability in Inverted Perovskite Solar Cells

AbstractInterfacial defects between perovskite and adjacent charge transport layers present a significant obstacle, hindering the enhancement of power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). To address this challenge, a dual‐interface modification is proposed to aim at improving the performance of mixed‐halide PSCs. Specifically, the hole‐collecting side is modified with 5‐Aminopyridine‐2‐carboxylic Acid (APC), while the electron‐collecting side is modified with 2‐thiopheneethylammonium chloride (TEACl). The multifunctional APC enhances charge transfer by tailoring the interface between the perovskite and poly(triarylamine) (PTAA) through multiple bonding interactions, thereby suppressing interfacial nonradiative recombination. Density functional theory studies reveal that APC on the perovskite surface induces uniform adsorption energy, promoting homogenous crystallization without residual stress. Additionally, APC interlayer eliminates the localized edge states induced by the iodine vacancies near the conduction band edge. Further improvement in the device performance is achieved by passivating the top perovskite surface with TEACl, leading to well‐matched energy bands and reduced vacancy trap states. As a result, champion cell achieves a PCE of 24.87% with an open‐circuit voltage of 1.188 V. Furthermore, The dual‐interface modification improves thermal stability due to enhanced ion‐migration activation energy.

Clustering behaviour of pre-aged Al-Mg-Si alloy during subsequent natural ageing and paint baking

The clustering behaviour of a pre-aged Al-Mg-Si alloy during natural ageing (NA) and paint baking (PB) was systematically investigated by hardness measurements and atom probe tomography (APT). The results show that pre-ageing (PA) can effectively retard the negative effect of NA and enhance the paint-baking response even after long-term NA (up to one year). The pre-aged alloy shows a good resistance against clustering within a short NA period of one week, and the capability is enhanced with increasing pre-ageing time. However, the clustering becomes significant after prolonged NA, especially for one year. The increasing number density of clusters containing 10–30 solute atoms contributes most to the hardness increase after prolonged NA. The mechanism about two synergistic processes of ‘vacancy trap’ and ‘vacancy escape’ is introduced to explain the phenomenon. The alloy with a prior PA above 2 h exhibits a good PB response even after one-year NA. Such performance is attributed to large-size precipitates (GP zones or pre-β″ precipitates) formed during PA, which can serve as nuclei for β″ due to a similar composition. Moreover, a strength model has been established to predict the different strengthening effect and evaluate contributions from clusters, GP zones and pre-β″/β″ precipitates during multi-stage ageing in the Al-Mg-Si alloy.

Portevin-Le Chatelier behavior in AlMgScZr alloys: Effects of Al3(Sc,Zr) dispersoid distribution and grain structure

Plastic instability caused by the Portevin-Le Chatelier (PLC) effect remains a challenge for Al–Mg alloy sheets during shape-forming processes, as it causes unsightly stretcher-strain markings on the surface of final products. In this study, the PLC behavior in annealed AlMg and AlMgScZr alloy sheets with varying Al3(Sc,Zr) dispersoid distributions and grain structures were investigated by combining tensile tests, microstructural characterization, and DIC analysis. The results show that the presence of Al3(Sc,Zr) dispersoids can inhibit the PLC effect, which is manifested by a decrease in serration amplitude, PLC band velocity, and an increase in critical strain. The inhibition effect is shown to be highly related to the distribution of Al3(Sc,Zr) dispersoids and resultant grain structures. The alloy with the densest dispersoid distribution and finest subgrain structure showed the slightest PLC effect. The reduction in serration amplitude and PLC band velocity is ascribed to the augmented resistance to local strain softening, caused by inhibition of dislocation motion by Al3(Sc,Zr) dispersoids and subgrain boundaries. In addition, a high density of Al3(Sc,Zr) dispersoids can strongly trap vacancies, which is responsible for the increase in critical strain.

Suppression of thermal diffusion of vacancies across p + -n junction structures in diamond. Application to SnV centers by ion implantation

This work reports on defect engineering related to optical centers in diamond by ion implantation. In particular, we demonstrate that thermal diffusion of vacancies to a few micrometers in depth can be effectively suppressed provided these are electrically charged and located within the depletion region of an abrupt p+ -n junction. The observed effect is complementary to the observations in the previous study (Favaro et al 2017 Nat. Commun. 8 15409) showing that charging of implantation-induced vacancies at such junction structures in diamond inhibits the formation of vacancy complexes in proximity to the targeted optical centers. In the present work we first generate vacancies near the surface of a low nitrogen doped CVD diamond substrate by He and C ion implantation before these are diffused by annealing at 1200∘C into the bulk. In the next step the depth distribution of NV centers generated by trapping of these vacancies is analyzed on a micron scale. For precise tuning of the implantation conditions we derived data on the boron and nitrogen doping by step etching of planar p+ resistors and p+ -n- p+ diode structures combined with electrical characterization and modeling. In the next step, tin-vacancy (SnV) centers were produced by 40 keV Sn implantation across the same junction structures at optimized conditions. In this way we observe an enhancement of the SnV yield and noticeable suppression of NV centers by diffusion and trapping of vacancies along the tracks of tin ions. Such ‘subsidiary’ NVs could significantly affect the emission of SnV and potentially other centers in the same spectral range.

Kinetic Monte Carlo simulations of solute clustering during quenching and aging of Al–Mg–Zn alloys

The physical mechanisms behind cluster formation during quenching and aging of age-hardening metallic alloys are poorly understood based on classical nucleation and growth theories, especially in multicomponent alloys with supersaturated vacancies and fast-diffusing solute atoms. Here, solute clustering in an Al–Mg–Zn alloy during quenching immediately after high-temperature (800 K) solution treatment is modeled with a newly developed kinetic Monte Carlo (kMC) framework. This includes accurate surrogate models trained using first-principles-calculated data to predict vacancy migration energetics on the fly as functions of local lattice occupations. First, kMC simulations at constant aging temperatures revealed that temporal evolution of the number, sizes, and compositions of solute clusters change with aging temperature. Such changes are consistent with our analyses based on classical nucleation theory (CNT) and Monte Carlo simulations. Second, the kMC algorithm was revised for simulating quenching processes by iteratively updating the kMC temperature based on simulated cooling rate profiles. These quenching simulations show that rapid solute clustering mainly occurs from ∼600 to ∼400 K during cooling. Below ∼400 K, the clustering is suppressed to a steady state because vacancies are trapped by stable clusters with sizes of ∼2 nm and high magnitudes of formation energies. The configurations of these steady-state clusters closely resemble the solute clusters we observed experimentally in 7XXX series Al–Mg–Zn-based alloy samples immediately after quenching. A two-stage vacancy trapping mechanism revealed in our simulations can provide physical guidance to tune precipitation kinetics during natural and artificial aging.

Synthesis of Platinum–Rare-earth Sub-Nanoclusters via an aluminum vacancy trapping strategy

Synthesizing homogeneous supported bimetallic sub-nanoclusters (with a diameter below 1 nm) that exhibit excellent stability and resistance to sintering at elevated temperatures remains a significant challenge. Herein we report on the synthesis of 14 combinations of platinum with rare earth metal (PtxLn/Al2O3) via an innovative aluminum (Al) vacancy trapping route, resulting in a particle size of approximately 0.4 nm. The salts containing Pt and Ln (Ln denotes rare earth metals) were selected to adsorb on the Al vacancies. When tested in propane dehydrogenation at 580 °C, the supported bimetallic sub-nanoclusters (PtxLn/Al2O3) demonstrated a significant improvement in catalytic performance, with an increase of 2∼7 times compared to Pt-rare earth metal nanoparticle catalysts of similar nature. These sub-nanoclusters exhibited exceptional stability and resistance to carbon deposition.

NV centres by vacancies trapping in irradiated diamond: experiments and modelling

Advances in applications of nitrogen-vacancy (NV) spin centres in diamond for sensing and quantum metrology depend critically on the NV fabrication methods. One such technique combines epitaxial diamond growth and electron or ion irradiation (He, C, etc), where NVs are activated by vacancy trapping at the nitrogen donor atoms upon thermal diffusion. In this work we study the efficiency of such method by analyzing NV depth profiles created by 340 keV and also 4 keV He irradiation in high purity CVD and HPHT diamond crystals and subjected to sequent annealing at 950 °C and 1200 °C temperatures. This analysis is coupled with the measurement of NV density in the bulk of CVD diamonds with nitrogen doping at low-ppb and low-ppm levels, exposed to MeV electrons in a wide range of the doses. For data analysis we developed an atomistic model based on probabilistic atomic jumps in a crystal lattice, which considers competitive trapping between di- (V2) or multi-vacancy defects compared to that of NVs. The efficiency of NV formation was defined as a ratio of the corresponding capture cross sections: σ NV vs. σV2 . Applying this model to the experimental data, the σNV/σV2 ratio was estimated about 0.1–0.5, where the activation energy of vacancy diffusion of about 1.7 eV was evaluated by 3D localization of individual NVs in depth profiles in a confocal microscope and sampling their spin coherence properties ( T2 ). In addition, we noted two subsidiary effects also discussed here: (i) reduction of NV density within the stopping range of the implanted He atoms after 1200∘ annealing and, (ii) partial suppression of NVs at near-surface areas visible only at low-dose electron exposures. The results of this study could be helpful to optimize the NV fabrication process reducing the density of ‘collateral’ lattice damage.

Electronic structure and thermodynamic approaches to the prospect of super abundant vacancies in δ-Pu.

Super abundant vacancies (SAVs) have been suggested to form in the fcc phase of plutonium, δ-Pu, under a low-pressure hydrogen environment. Under these conditions, the vacancy concentration is proposed to reach 10-3 at% due to H trapping in vacancies lowering the effective vacancy formation energy. Previous density functional theory (DFT) results suggest that seven H atoms can be trapped in a single vacancy when a collinear special quasirandom magnetic structure is used to stabilize the δ phase, suggesting SAVs are a possible source of H stored in plutonium. In this report, we present DFT results for δ-Pu in the noncollinear 3Q magnetic state to study the formation of SAVs in mechanically stable δ-Pu. Together with these new simulations, we use publicly available computational and experimental data to provide further constraints on the physical conditions needed to thermodynamically stabilize SAVs in δ-Pu. Using several thermodynamic models, we estimate the vacancy concentrations in δ-Pu and discuss the limits of hydrogen driven formation of vacancies in δ-Pu. We find that, when hydrogen in the lattice is equilibrated with gaseous H2, the formation of SAVs in δ-Pu is unlikely and any excess vacancy concentration beyond thermal vacancies would need to occur by a different mechanism.

Off-Stoichiometry, Vacancy Trapping, and Pseudo-irreversible First-Cycle Capacity in LiNiO2

Off-Stoichiometry, Vacancy Trapping, and Pseudo-irreversible First-Cycle Capacity in LiNiO₂

Natural aging and vacancy trapping in Al-6xxx

Undesirable natural aging (NA) in Al-6xxx delays subsequent artificial aging (AA) but the size, composition, and evolution of clustering are challenging to measure. Here, atomistic details of early-stage clustering in Al–1%Mg–0.6%Si during NA are studied computationally using a chemically-accurate neural-network potential. Feasible growth paths for the preferred beta '' precipitates identify: dominant clusters differing from beta '' motifs; spontaneous vacancy-interstitial formation creating 14–18 solute atom beta ''-like motifs; and lower-energy clusters requiring chemical re-arrangement to form beta '' nuclei. Quasi-on-lattice kinetic Monte Carlo simulations reveal that 8–14 solute atom clusters form within 1000 s but that growth slows considerably due to vacancy trapping inside clusters, with trapping energies of 0.3-0.5 eV. These findings rationalize why cluster growth and alloy hardness saturate during NA, confirm the concept of “vacancy prisons”, and suggest why clusters must be dissolved during AA before formation of beta ''. This atomistic understanding of NA may enable design of strategies to mitigate negative effects of NA.Graphical abstractEnergy and RMS vacancy displacement vs simulation time for three different kMC simulations of Al matrix containing Mg (green) and Si atoms(orange), and 1 vacancy (pink). Geometries extracted from the third trajectory at four different times show solute clustering and vacancy trapping.

Analysis of ion conduction behavior of Nb- and Zr-doped Li3InCl6-based materials via material simulation

The Li-ion conductivities of Li3InCl6 (LIC), which is a promising chloride solid electrolyte, and its compositional derivatives, Nb5+- and Zr4+-doped LIC, i.e., Li3−2xIn1−xNbxCl6 and Li3−yIn1−yZryCl6, respectively, were experimentally and computationally investigated. An increase in the ionic conductivity caused by Nb5+ or Zr4+ doping, which was due to the increase in Li vacancies, was observed in both the experimental and computational results. Nb5+ doping yielded a larger increase in conductivity at 60 °C. First-principles molecular dynamics studies indicated two factors affecting the Li-ion conductivity under doping with higher-valent ions: (1) the vacancy trapping effect and (2) the reduction in the phase-transition temperature from a Li/vacancy ordered structure to a disordered structure. In particular, in factor (2), the effect of Nb5+ doping is larger than that of Zr4+ doping, which supports the improvement in ionic conductivity at 333 K in the experiment.

Modelling the spatial evolution of excess vacancies and its influence on age hardening behaviors in multicomponent aluminium alloys

Excess vacancies play crucial roles in the precipitation of age-hardening precipitates in aluminium alloys, but their spatial evolution across grains during heat treatments is less known. In this work, a numerical model is developed to predict the spatial evolution of non-equilibrium excess vacancies during cooling from solution treatment and during ageing heat treatments of multicomponent aluminium alloys. A finite volume scheme is applied to derive the spatial distribution of vacancy site fraction across grains by solving the diffusion equations of vacancies under the influence of solute elements. Binding energies between solute atoms and vacancies predicted by first-principles calculations has been used to handle the trapping of excess vacancies by solute atoms and atom clusters. The annihilation rates of excess vacancies at grain boundaries and at dislocation jogs have been derived based on a rigorous description of the annihilation mechanisms of vacancies. The evolution of the density of dislocation jogs due to vacancy annihilation has been taken into account. The model is successfully applied to interpret the age hardening behaviors of experimental alloys subjected to different thermomechanical processing conditions. This model will help to reach a deeper understanding of the roles of excess vacancies in precipitation kinetics and therefore is important to further optimize thermomechanical processing parameters and alloy composition to improve the macroscopic mechanical properties of age hardening aluminium alloys.

Bridging the gap between theory and experiment in vacancy concentration, C/N/O diffusivity, and divacancy interaction in tungsten: Role of vacancy-C/N/O interaction

Light element solutes such as carbon (C), nitrogen (N), and oxygen (O) are impurities commonly seen in tungsten alloys, often exerting great impact on materials properties due to their strong interaction with defects. In this work, we conducted a comprehensive investigation into the atomistic properties of small vacancy-impurity defect complexes in tungsten using rigorous first-principles calculations. Through the integration of a statistical approach, we meticulously examined the concentration distributions of these complexes across various temperatures and impurity concentrations. Our findings reveal that the introduction of C/N/O impurities significantly augments the concentration of vacancy-type defects to a level well above the thermal equilibrium vacancy concentration in pure tungsten. Moreover, we conducted a thorough assessment of the diffusivity of C/N/O atoms, conclusively demonstrating that the inclusion of the vacancy trapping effect is paramount in comprehending their unexpectedly low diffusivity observed in experiments. Finally, in conjunction with an object kinetics Monte Carlo method, we revisited the experimental determination of the divacancy binding energy in tungsten. We showed that the presence of C/N/O impurities may have conceivably impacted the experimentally ascertained divacancy binding energy. This work provides accurate data on evaluating vacancy-impurity interaction in tungsten, unveils the pivotal role played by C/N/O in affecting vacancy concentration and divacancy binding energy, and sheds new insights toward understanding the diffusion behavior of impurities in tungsten.

Resistive switching behavior and mechanism of HfO x films with large on/off ratio by structure design

Different bilayer structures of HfO x /Ti(TiO x ) are designed for hafnium-based memory to investigate the switching characteristics. The chemical states in the films and near the interface are characterized by x-ray photoelectron spectroscopy, and the oxygen vacancies are analyzed. Highly improved on/off ratio (∼104) and much uniform switching parameters are observed for bilayer structures compared to single layer HfO x sample, which can be attributed to the modulation of oxygen vacancies at the interface and better control of the growth of filaments. Furthermore, the reliability of the prepared samples is investigated. The carrier conduction behaviors of HfO x -based samples can be attributed to the trapping and de-trapping process of oxygen vacancies and a filamentary model is proposed. In addition, the rupture of filaments during the reset process for the bilayer structures occur at the weak points near the interface by the recovery of oxygen vacancies accompanied by the variation of barrier height. The re-formation of fixed filaments due to the residual filaments as lightning rods results in the better switching performance of the bilayer structure.

Revitalizing platinum: Alkali-promoted formation of active metallic nanoparticles from inert Pt entities for enhanced benzene combustion

Pt-based catalysts play a crucial role in the catalytic combustion of benzene. Previous studies have highlighted the significance of metal Pt atoms as the active phase in benzene combustion, while oxidized Pt sites exhibit lower activity. In practical catalysts synthesized through wet chemical methods, a range of metal species is present, including highly dispersed oxidized metal atoms, small oxidized metal clusters, and metal nanoparticles. This paper explores an effective strategy to enhance metal utilization and develop high-performance platinum-based catalysts for benzene catalytic combustion by introducing alkali metals. This approach transforms low-activity oxidized metal atoms and small oxidized metal clusters into active small metal nanoparticles. The introduction of K-O groups displaces Pt from oxygen vacancy trapping sites on antimony-doped tin oxide (ATO) support, enhancing Pt mobility and converting them into active metallic Pt nanoparticles, ultimately improving benzene combustion. These findings highlight the role of alkali metals in promoting catalytic activity and provide insights for designing high-performance noble-metal-based catalysts.

Synergistic effect of helium and hydrogen for vacancy-like defects in pure Fe and Fe9Cr alloy

A synergistic effect between the helium and hydrogen elements exists in fusion reactors materials. To explore its microscopic mechanism, this paper meticulously investigates the H and He action for vacancy-like defects in pure Fe and Fe9Cr alloys by using first-principles calculations. The He and H vacancy-like defects were performed within atomistic simulations in body-centered cubic Fe and Fe9Cr alloys. Via analyzing the structures of the clusters, we find that He atoms in the VacHenHm clusters are more energetically favorable to occupy the center of the vacancy and the H atoms will attach to the free surface. Based on the comprehensive analysis of the vacancy trapping energy, vacancy formation energy, formation volumes, and the electron density maps, He elements play a more dominant role within the H/He synergistic effect, while H elements aggravate the bubble growth. There is no evident chemical bonding between He and H atoms. Concerning the Cr atoms, the Cr-He possesses the strongest repulsive energy within the considered diatomic molecules. The Cr element could weaken the aggregation and reduce the diffusion barriers of the vacancies, thus inhibiting the growth of vacancy-like defects to some extent. These results would help to provide insights into the synergistic effect of He and H for vacancy-like defects in Fe9Cr alloy.

The dissolution behavior of He atom in AlNbTiZr high entropy alloy by first principles

The dissolution behavior of He atom in AlNbTiZr high entropy alloy (HEA) was studied from first principles and a new mechanism for the enhanced irradiation resistance with the unusual interaction between AlNbTiZr and He is found. It is interesting to note that the dissolution energy is inversely proportional to Al atom and proportional to Nb atom, i.e., the helium tends to occupy the sites where there exists a local chemical ordering characterized by an atomic environment with high content of Al or low content of Nb atoms, and thus Al atoms are conducive to dissolving He atoms. And interesting enough, differing from traditional metal, the site-to-site interaction is different in AlNbTiZr HEA, which leads to a special sub-nanoscale structure, Al cage. What's more, the Al cage can not only promote the ability of vacancy trapping for He, but also greatly suppress the growth of He bubbles by pinning the He atoms, further improving the irradiation resistance. Consequently, it is feasible to produce some Al cage in dispersion state to improve irradiation resistance of AlNbTiZr. Due to the infinite possibilities of the composition of HEA, this finding can guide future design of novel HEA with improved irradiation resistance performances.

First-Principles Study of the Stability of Ti n O m Clusters and He Behavior Inside Ti n O2Va1 Clusters in Vanadium Alloy

In order to characterize the behaviors of interstitial oxygen (O) in the vanadium (V) alloy, the interactions between O and Ti with respect to atomic separation distance have been investigated using first-principles calculations. We observe an attractive interaction between Ti and O within the third nearest neighbor (nn) (3nn) distance. The stability of the Ti-vacancy (Ti-Va) clusters has been studied by calculating the binding energy between Ti and monovacancy in the vanadium alloy, and our results show that the stable configurations are Ti1Va1, Ti2Va1, and Ti4Va1 clusters. The TinVa1 clusters prefer to trap two O atoms and form stable Ti1O2Va1, Ti2O2Va1, and Ti4O2Va1 clusters. Furthermore, the self-trapping energies of the Hex clusters by the TinO2Va1 clusters have been calculated. When four He atoms are trapped, the Hex clusters are stable. Furthermore, the trapping energies for the multiple He atoms captured by the TinO2Va1 clusters are calculated, and the TinO2 clusters are found to impede the vacancy trapping of He atoms to form He bubbles.

Comprehensive Analysis of Ce1−xSmxO2−δ Solid Electrolytes: Structural, Microstructural, and Electrochemical Characterization for Intermediate Temperature Solid Oxide Fuel Cells

Samarium substituted ceria has been studied as a potential electrolyte material for intermediate temperature range solid oxide fuel cells. The structural, microstructural, morphological and electrochemical properties of the Ce1−xSmxO2−δ solid electrolytes were analyzed, with different substitutions from 0.05 to 0.50. The difference in the ionic radii of Sm3+ and Ce4+ resulted in lattice strain and expansion, which was found to decrease the average size of crystallites. The ionic conductivity of ceria increases as Sm3+ concentration is increased upto a limit of 20%. This is due to the combination of vacancy-vacancy repulsion and vacancy trapping by the substitution at a higher level. However, the conductivity of Ce0.80Sm0.20O2−δ was the highest, at 4.04 × 10−2 S cm−1 at 600 °C. The grain activation energy and grain boundary activation energy were also found to be ∼0.87 eV and ∼0.74 eV respectively. The low values of activation energies indicate that Ce0.80Sm0.20O2−δ could be a suitable electrolyte material for intermediate-temperature solid oxide fuel cells.