Dumbbell Interstitials Research Articles

First-Principles Assessment of the Structure and Stability of 15 Intrinsic Point Defects in Zinc-Blende Indium Arsenide

Point defects are inevitable, at least due to thermodynamics, and essential for engineering semiconductors. Herein, we investigate the formation and electronic structures of fifteen different kinds of intrinsic point defects of zinc blende indium arsenide (zb-InAs ) using first-principles calculations. For As-rich environment, substitutional point defects are the primary intrinsic point defects in zb-InAs until the n-type doping region with Fermi level above 0.32 eV is reached, where the dominant intrinsic point defects are changed to In vacancies. For In-rich environment, In tetrahedral interstitial has the lowest formation energy till n-type doped region with Fermi level 0.24 eV where substitutional point defects In A s take over. The dumbbell interstitials prefer < 110 > configurations. For tetrahedral interstitials, In atoms prefer 4-As tetrahedral site for both As-rich and In-rich environments until the Fermi level goes above 0.26 eV in n-type doped region, where In atoms acquire the same formation energy at both tetrahedral sites and the same charge state. This implies a fast diffusion along the t − T − t path among the tetrahedral sites for In atoms. The In vacancies V I n decrease quickly and monotonically with increasing Fermi level and has a q = − 3 e charge state at the same time. The most popular vacancy-type defect is V I n in an As-rich environment, but switches to V A s in an In-rich environment at light p-doped region when Fermi level below 0.2 eV. This study sheds light on the relative stabilities of these intrinsic point defects, their concentrations and possible diffusions, which is expected useful in defect-engineering zb-InAs based semiconductors, as well as the material design for radiation-tolerant electronics.

Effect of d electrons on defect properties in equiatomic NiCoCr and NiCoFeCr concentrated solid solution alloys

The role of $d$ electrons in determining distributions of formation and migration energies for point defects in equiatomic NiCoCr and NiCoFeCr concentrated solid solution alloys (CSAs) are studied regarding electron density deformation flexibility based on first-principles calculations. The disordered state is taken into account by constructing special quasirandom structures. The migration barriers are determined by directly optimizing the saddle point. It is found that the formation energies of interstitials in CSAs are lower than those in pure Ni, whereas the formation energies of vacancies are higher. In both NiCoCr and NiCoFeCr, Co-related dumbbell interstitials exhibit lower formation energies. Notably, the distributions of migration energies for Cr interstitials and vacancies exhibit a remarkable overlap region. A detailed analysis of electronic properties reveals that the electronic charge deformation flexibility regarding ${e}_{g}$ to ${t}_{2}$g transition has a dominant effect on defect energetics for different elements in CSAs. Thus the electron deformation ability is suggested as a key factor in understanding the peculiar defect behavior in CSAs.

КОМПЬЮТЕРНОЕ МОДЕЛИРОВАНИЕ СТРУКТУРНЫХ ДЕФЕКТОВ В МОНОКРИСТАЛЛИЧЕСКОМ И АМОРФНОМ АЛЮМИНИИ

The paper covers the study of microscopic mechanisms of melting of metals and the structural relaxation of metallic glasses. Despite the extensive efforts and numerous important results obtained in this field, this task does not have a generally accepted final solution. One of the main issues is the microscopic nature of structural defects in metallic glasses – the nanosized regions, which are responsible for the evolution of their physical properties under the external influence. The interstitialcy theory (IT) proposed by Granato provides the advanced interpretation of nature of such defects. The interstitialcy theory is based on the unique hypothesis of the interstitialcy mechanism for melting of metals and associates heat effects in the glass with the shear elasticity of a maternal crystal. The experimental study and computer simulation of the diaelastic effect near the melting temperature T m of crystalline aluminum provided a strong evidence of the avalanche generation of interstitial dumbbells near T m . In this work, the authors carried out the computer simulation to check the presence of interstitial dumbbells (or similar atomic structures) in the solid glassy state produced in the result of melt- quench. The computer simulation shows that the amorphous aluminum produced by rapid melt quenching contains a significant number of “defects” similar in their properties to the interstitial dumbbells in the crystalline state. Although these “defects” do not have any well-defined uniform topological structure, unlike the defects of crystal, and they can be exactly identified by their basic properties – high sensitivity to shear stresses and typical low/high-frequency peculiarities of the spectrum of the vibrational density of states of the “defective” atoms. Using the methods of molecular dynamics and statics, it is shown that the solid non-crystalline aluminum contains the specific atomic configurations similar to the interstitial dumbbells in the crystalline state, which can be considered as the amorphous structure “defects”.

Influence of Protons on the Lithium Transport Mechanism in Antiperovskite Solid Electrolytes from Molecular Dynamics Simulations

Battery energy storage has become a critical component of civil and military operations because of the rapidly growing demands in consumer electronics, electric vehicles and the power-consuming systems carried by soldiers on the battlefield. Currently utilized batteries are flammable and suffer from an insufficient high specific energy, which poses major safety issues creating a tactical burden for the soldiers. The use of solid state batteries may offer enhanced safety and increased energy density. Previous modeling studies of the superionic lithium-rich anti-perovskites conductors focused on Li3OX (X = Cl, Br) structures and reported a variety of the charge transport mechanisms ranging from the low-barrier three-atom hop mechanism involving vacancies, Li hops to the nearest neighbor, Li interstitial dumbbells, and diffusion of the Li+ interstitials in LiCl-deficient Li3OCl that outnumber vacancies by 2–3 orders of magnitude as predicted by the shell model. It is, however, quite challenging to completely remove protons from these anti-perovskite conductors during synthesis, highlighting the importance of understanding the influence of protons on conductivity and electrochemical properties for these superionic lithium-rich anti-perovskite conductors.(1, 2) Born-Oppenheimer molecular dynamics (BOMD) simulations of Li2+xOH1-xCl (0≤x≤1) solid electrolytes were performed in order to gain insights into the mechanism of lithium transport and the influence of H concentration, complementing previous modeling studies of H-free antiperovskites. The simulation cell was comprised of 320 atoms. Density Functional Theory (DFT) calculations were performed with the QUICKSTEP module of the CP2K code, which implements the dual Gaussian and Plane Waves (GPW) method. The gamma point supercell approach was used in combination with 3-dimensioanl Periodic Boundary Conditions (PBC). Calculations were performed using the spin-polarized Perdew-Burke-Ernzerhof (PBE) exchange correlation functional with Grimme’s D3 dispersion correction. BOMD simulations predicted a significant enhancement of the lithium diffusion with the increased fraction of protons in agreement with experimental data. The lithium transport mechanism was found to be closely coupled with the rotation of a relatively short OH-group within the protonated electrolytes. Moreover, lithium jump events were found to be highly-correlated. Figure 1a shows a snapshot of the simulation box. In order to gain further insight into the transport mechanics, the motion of the fastest lithium cations in Li2.5OH0.5Cl solid electrolyte are highlighted with different colors. Overlap between trajectories of the fastest lithium cations shown by red arrows is indicative of the highly-correlated fashion of the lithium jumps.(2) We will also discuss changes of band gaps and band alignments that were calculated using DFT hybrid functionals as markers for electrochemical stability. We anticipate that our findings will reduce the existing confusions and show new avenues for tuning solid electrolyte compositions for further improved Li-ion conductivities. Different aspects of this work were supported by NASA Minority University Research and Education Project (MUREP; NASA grant NNX15AP44A and interagency agreement NND16AA29I with ARL for modeling) and ARPA-E (grant DE-AR0000779).

Identification of interstitial-like defects in a computer model of glassy aluminum

Computer simulation shows that glassy aluminum produced by rapid melt quenching contains a significant number of ‘defects’ similar to dumbbell (split) interstitials in the crystalline state. Although these ‘defects’ do not have any clear topological pattern as opposed to the crystal, they can be uniquely identified with the same properties which are characteristic of these defects in the crystalline structure, i.e. strong sensitivity to applied shear stress, specific local shear strain fields and distinctive low-/high-frequency peculiarities in the vibration spectra of ‘defective’ atoms. This conclusion provides new support for the interstitialcy theory, which was found to give consistent and verifiable explanations for a number of relaxation phenomena in metallic glasses and their relationship with the maternal crystalline state.

The Multiple Roles of Small-Angle Tilt Grain Boundaries in Annihilating Radiation Damage in SiC

Lattice defects generated by radiation damage can diffuse to grain boundaries (GBs) and be annihilated at GBs. However, the precise role of GBs in annihilating the segregated defects remains unclear. Here, we employed multi-scale models to determine how interstitials are annihilated at small-angle tilt GBs (STGBs) in SiC. First of all, we found the pipe diffusion of interstitials in STGBs is slower than bulk diffusion. This is because the increased interatomic distance at dislocation cores raises the migration barrier of interstitial dumbbells. Furthermore, we found both the annihilation of interstitials at jogs and jog nucleation from clusters are diffusion-controlled and can occur under off-stoichiometric interstitial fluxes. Finally, a dislocation line model is developed to predict the role of STGBs in annihilating radiation damage. This model includes defect flux to GBs, pipe diffusion in STGBs, and the interaction of defects with jogs. The model predicts the role of STGBs in annihilating defects depends on the rate of defects segregation to and diffusion along STGBs. STGBs mainly serve as diffusion channel for defects to reach other sinks when defect diffusivity is high at boundaries. When defect diffusivity is low, most of the defects segregated to STGBs are annihilated by dislocation climb.

Stability of β″ nano-phases in Al-Mg-Si(-Cu) alloy under high dose ion irradiation

The microstructure of a 6061-T6 Al alloy subjected to ion irradiation at 95 and 165 displacements per atom (dpa) has been evaluated by transmission electron microscopy and atom-probe tomography. The initial microstructure of the alloy is dominated by needle-shaped β″ precipitates. After 95 dpa irradiation, the β″ precipitates display a slight lattice distortion and are partially dissolved, together with the formation of a new phase. After 165 dpa irradiation, the β″ precipitates are completely dissolved, the new phase has grown and a high density of clusters rich in Mg, Si, Cu and Cr is observed. The determination of ballistic versus radiation enhanced diffusion coefficients shows that enhanced diffusion is predominant for β” dissolution. The formation and growth of the new particles may be caused by radiation induced segregation. Solute drag by vacancies or mixed dumbbell interstitials migration could explain the diffusion of some elements as Si or Cr towards the new particles.

Modeling of Phosphorus Transport by Interstitial Dumbbell in α−Iron Using First-Principles-Based Kinetic Monte Carlo

Modeling of Phosphorus Transport by Interstitial Dumbbell in α−Iron Using First-Principles-Based Kinetic Monte Carlo

First-principles study of vacancy, interstitial, noble gas atom interstitial and vacancy clusters in bcc-W

Based on first-principles calculations, the vacancy and self-interstitial formation energy in bcc-W are 3.19eV and 9.97eV. Binding energy between the dumbbell interstitials can be up to 2.29eV. Binding energy for the first and second nearest neighbor vacancy pair are −0.12eV and −0.41eV. The migration barrier of vacancy, He, Ne and Ar interstitial are 1.70eV, 0.07eV, 0.15eV and 0.25eV. The migration barrier of self-interstitial along 〈111〉 is 0.05eV. The so-called rotation barrier of self-interstitial is 0.35eV. The formation energy of He, Ne, Ar substitutional and He, Ne, Ar tetrahedral interstitial are 4.85eV, 6.42eV, 9.54eV and 6.23eV, 10.40eV, 15.10eV, respectively. Binding energy for di-gas atom (He, Ne and Ar) interstitial are 0.95eV, 2.28eV and 1.70eV. The binding energy of noble gas atom interstitial and vacancy cluster are obtained and can be used as an input to build a molecular dynamics (MD) W-Ne potential. Then molecular dynamics (MD) simulations can be used to investigate the mechanism and temperature dependence of the surface modification of plasma-facing tungsten in the application of future fusion reactors in the following investigations.

Experimental evidence for thermal generation of interstitials in a metallic crystal near the melting temperature

The only intrinsic point defects of simple crystalline metals known from solid state physics are vacancies and interstitials. It is widely believed that while vacancies play a major role in crystal properties and their concentration reaches relatively big values near the melting temperature Tm, interstitials essentially do not occur in thermodynamic equilibrium and their influence on properties is minor. Here, taking aluminum single crystals as an example, we present compelling experimental evidence for rapid thermoactivated growth of interstitial concentration upon approaching Tm.Using high precision measurements of the shear modulus we found a diaelastic effect of up to near Tm. It is argued that this effect is mostly due to the generation of dumbbell (split) interstitials. The interstitial concentration ci rapidly increases upon approaching Tm and becomes only 2–3 times smaller than that of vacancies just below Tm. The reason for this ci-increase is conditioned by a decrease of the Gibbs free energy with temperature, which in turn originates from the high formation entropy of dumbbell interstitials and a decrease of their formation enthalpy at high ci. Special molecular dynamic simulation confirmed all basic aspects of the proposed interpretation.The results obtained (i) demonstrate the significance of interstitial concentration near Tm that could lead to the revaluation of vacancy concentration at high temperatures, (ii) suggest that dumbbell interstitials play a major role in the melting mechanism of monatomic metallic crystals and (iii) support a new avenue for in-depth understanding of glassy metals.

A defect-based model of radiation-induced segregation to free surfaces in binary alloys

A defect-based model of radiation-induced segregation in binary solid solutions is presented. The model consists of a set of reaction–diffusion equations governing the space and time evolution of vacancies, dumbbell interstitials and lattice atoms under irradiation. Irradiation, the mechanism driving evolution, is represented by stochastic and spatially-resolved defect generation events. A key feature of the model presented here is that the role of boundaries as defect sinks is ensured by a set of defect–boundary reaction boundary conditions. Defining defect–boundary interactions in this way makes it possible to capture both segregation and boundary motion simultaneously. The model is tested with Cu–Au solid solution. Enrichment of Cu and depletion of Au has been observed near the boundaries, in agreement with experimental observations. For a particular dose rate, the amount of segregation after a given period of irradiation has been found to be highest at an intermediate temperature. At lower temperatures, maximum segregation is observed by lowering the dose rates, and vice versa. The activation barrier for the defect–surface reactions plays a significant role in segregation near the boundaries. A slight increase in the sample size is also noticed during the simulations due to rapid migration of interstitials to the surface.

Dependence of the glass shear modulus on the crystal shear modulus and the structural relaxation kinetics for the Zr46Cu46Al8 system

It has been shown that the temperature coefficient of the shear modulus of the Zr46Cu46Al8 glass at a constant concentration of frozen-in “defects” such as dumbbell interstitials is identical to that of the same alloy in the crystalline state. Relaxation of the “defect” subsystem leads to positive or negative changes in this coefficient, depending on the sign of change in the “defect” concentration. The “defect” concentration is calculated in the initial and relaxed glass states as functions of temperature for different heating rates and thermal prehistories.

Determination of the activation energy spectrum of structural relaxation in metallic glasses using calorimetric and shear modulus relaxation data

We consider two methods for reconstruction of the activation energy spectra of structural relaxation from calorimetric and shear modulus relaxation data. The methods are based on the assumption that structural relaxation of metallic glasses can be understood as a relaxation change of the concentration of frozen-in defects similar to dumbbell interstitials (interstitialcies) in simple crystals. For three Pd- and Zr-based glasses taken as an example, it is shown that both methods give nearly the same results. We calculated a decrease of the defect concentration and showed that the heat of structural relaxation per defect for all three glasses practically coincides with the theoretical value of the interstitialcy formation enthalpy.

Influence of the prefactor to defect motion in α-Iron during long time scale simulations

We present a study of the influence of the prefactor in the Arrhenius equation for the long time scale motion of defects in α-Fe. It is shown that calculated prefactors vary widely between different defect types and it is thus important to determine these accurately when implementing on-the-fly kinetic Monte Carlo (otf-KMC) simulations. The results were verified by reproducing many events using Molecular Dynamics (MD) and Temperature-Accelerated Dynamics (TAD). The calculated prefactor was shown to increase the relative interstitial-vacancy diffusion rates by an order of magnitude compared to the assumption of a constant prefactor value and the ordering of the rate table for the interstitial defect migration mechanisms was also changed. In addition, low prefactor values were observed for the 4 interstitial dumbbells configuration with low barrier transitions.

Thermal effects induced during heating of the bulk metallic glass Zr46Cu46Al8

It has been shown that thermal effects measured calorimetrically during heating of the bulk metallic glass Zr46Cu46Al8 can be interpreted as a result of the annihilation or generation of structural “defects,” which are similar in their properties to interstitial dumbbells in simple crystalline metals. The shear susceptibility, which characterizes the sensitivity of the shear modulus of the glass to the concentration of “defects” of the type of interstitial dumbbells, has been calculated.

Effect of vacancies and interstitials in the dumbbell configuration on the shear modulus and vibrational density of states of copper

It has been shown by the molecular dynamics method that the introduction of interstitials in the dumbbell configuration into the copper crystal leads to a significantly stronger shear modulus reduction than the vacancy introduction. Specific low-frequency modes appear in the spectrum of vibrational states of “defect” atoms. The vacancy formation enthalpy weakly depends on their concentration, and the interstitial dumbbell formation enthalpy at high concentrations can decrease by a factor of 8. In this case, the radial distribution function takes the form characteristic of noncrystalline materials. The results obtained confirm the interstitial theory of the condensed state.

First-principles study of point defects in an fcc Fe-10Ni-20Cr model alloy

The influence of the local environment on vacancy and self-interstitial formation energies has been investigated in a face-centered-cubic (fcc) Fe-10Ni-20Cr model alloy by analyzing an extensive set of first-principle calculations based on density functional theory. Chemical disorder has been considered by designing special quasirandom structures and four different collinear magnetic structures have been investigated in order to determine a relevant reference state to perform point defect calculations at 0 K. Two different convergence methods have also been used to characterize the importance of the method on the results. Although our fcc Fe-10Ni-20Cr would be better represented in terms of applications by the paramagnetic state, we found that the antiferromagnetic single-layer magnetic structure was the most stable at 0 K and we chose it as a reference state to determine the point defect properties. Point defects have been introduced in this reference state, i.e., vacancies and Fe-Fe, Fe-Ni, Fe-Cr, Cr-Cr, Ni-Ni, and Ni-Cr dumbbell interstitials oriented either parallel or perpendicular to the single layer antiferromagnetic planes. Each point defect studied was introduced at different lattice sites to consider a sufficient variety of local environments and analyze its influence on the formation energy values. We have estimated the point defect formation energies with linear regressions using variables which describe the local environment surrounding the point defects. The number and the position of Ni and Cr first nearest neighbors to the point defects were found to drive the evolution of the formation energies. In particular, Ni is found to decrease and Cr to increase the vacancy formation energy of the model alloy, while the opposite trends are found for the dumbbell interstitials. This study suggested that, to a first approximation, the first nearest atoms to point defects can provide reliable estimates of point defect formation energies.

Phase Stability and Transport Mechanisms in Antiperovskite Li3OCl and Li3OBr Superionic Conductors

We investigate phase stability and ionic transport mechanisms in two recently discovered superionic conductors, Li3OX (X = Cl, Br), from first principles. These compounds, which have an antiperovskite crystal structure, have potential applications as solid electrolytes in Li-ion batteries. We identify a low-barrier three-atom hop mechanism involving Li interstitial dumbbells. This hop mechanism is facile within the (001) crystallographic planes of the perovskite crystal structure and is evidence for the occurrence of concerted motion, similar to ionic transport in other solid electrolytes. Our first-principles analysis of phase stability predicts that antiperovskite Li3OCl (Li3OBr) is metastable relative to Li2O and LiCl (LiBr) at room temperature. We also find that although the band gap of Li3OCl exceeds 5 eV, the metastable antiperovskite becomes susceptible to decomposition into Li2O2, LiCl and LiClO4 above an applied voltage of 2.5 V, suggesting that these compounds are most suited for low-voltage Li ...

“Defect”-induced heat flow and shear modulus relaxation in a metallic glass

Calorimetric and shear modulus measurements on bulk glassy Pd40Ni40P20 are performed. The obtained data is used for the calculation of structural relaxation-induced heat flow within the framework of the Interstitialcy theory. A good correspondence between the calculated and experimental heat release and absorption both below and above Tg is found. It is concluded that a description based on the population of “defect” states that are similar to dumbbell interstitials is capable to correctly describe the observed heat flow.

Isochronal annealing study of X-ray induced defects in single- and double-walled carbon nanotubes

X-ray induced defects in single-walled (SWCNTs) and double-walled carbon nanotubes (DWCNTs) were characterized by Raman scattering spectroscopy. Frenkel defects, interstitial-vacancy pairs, were revealed to form in both SWCNTs and DWCNTs after X-ray irradiation because these defects were entirely healed by thermal annealing. In order to clarify the structure of the X-ray induced defect in SWCNT and DWCNT, isochronal-annealing experiments were performed on the irradiated samples and the activation energy for defect healing was estimated. The intensity of D band (defect induced band) on Raman spectra was used as a measure of the density of X-ray induced defects. The experimental results were in good agreement with the simulated values using second order reaction model, which indicated that the defect healing was determined by the migration energy of interstitials on the carbon layer. We also found that the activation energy for defect healing of SWCNT and DWCNT were around 0.5 eV and 0.32 eV, respectively. The X-ray induced defects in SWCNTs were more stable than those in DWCNTs. Compared these estimated activation energies to previous theoretical reports, we concluded that bridge and/or dumbbell interstitials are formed in both SWCNT and DWCNT by X-ray irradiation.