Energetics Of Point Defects Research Articles

Equilibrium configurations and energetics of point defects in two-dimensional colloidal crystals.

We demonstrate a novel method of introducing point defects (mono- and divacancies) in a confined monolayer colloidal crystal by manipulating individual particles with optical tweezers. Digital video microscopy is used to study defect dynamics in real space and time. We verify the numerical predictions that the stable configurations of the defects have reduced symmetry compared to the triangular lattice and discover that in addition they are characterized by distinct topological arrangements of the particles in the defect core. Surprisingly, point defects are thermally excited into separated dislocations, from which we extract the dislocation pair potential.

Point defect energetics in the ZrNi and Zr 2Ni intermetallics

A systematic study of the properties of point defects has been conducted in the ZrNi and Zr 2Ni intermetallic compounds using molecular dynamics. These properties include the stable defect configurations, formation and migration energies, and vacancy migration mechanisms. Zr vacancies (interstitials) are unstable in both compounds; they spontaneously decay to pairs of Ni vacancy (interstitial) and antisite defect. The stable Ni vacancies have formation energies of 0.83 and 0.61 eV in ZrNi and Zr 2Ni, respectively. In ZrNi, vacancy migration occurs preferentially in the [0 2 5] and [1 0 0] directions, with migration energies of 0.67 and 0.73 eV, respectively, and is essentially a two-dimensional process, in the (0 0 1) plane. In Zr 2Ni, vacancy migration is one-dimensional, occurring in the [0 0 1] direction, with a migration energy of 0.67 eV. The stable interstitial configurations for both compounds consist of a Ni atom lying on the (0 0 1) plane between two out-of-plane nearest-neighbor Zr atoms.

Highly optimized empirical potential model ofsilicon

We fit an empirical potential for silicon using the modifiedembedded atom (MEAM) functional form, which containsa nonlinear function of a sum of pairwise and three-body terms.The three-body term is similar to the Stillinger-Weber form.We parametrized our model using five cubic splines,each with 10 fitting parameters,and fitted the parameters to a large databaseusing the force-matching method.Our model provides a reasonable description of energetics for all atomiccoordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). Itaccurately reproduces phonons and elastic constants, as wellas point defect energetics. It also provides a good description ofreconstruction energetics for both the 30° and 90°partial dislocations.Unlike previous models, our model accurately predictsformation energies and geometries of interstitial complexes - smallclusters, interstitial-chain and planar {311} defects.

Defect production, annealing kinetics and damage evolution in α-Fe: An atomic-scale computer simulation

Abstract Radiation-induced microstructural and compositional changes in solids are governed by the interaction between the fraction of defects that escape their nascent cascade and the material. We use a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in α-Fe at 600 K. MD simulations provide information on the nature of the primary damage state as a function of recoil energy, and on the kinetics and energetics of point defects and small defect clusters. The KMC simulations use as input the MD results and provide a description of defect diffusion and interaction over long time and length scales. For the MD simulations, we employ the analytical embedded-atom potential developed by Johnson and Oh for α-Fe, including a modification of the short-range repulsive interaction. We use MD to calculate the diffusivities of point defects and small defect clusters and the binding energy of small ...

Focal cerebral ischemic injury in knockout mice deficient in the ε1 subunit of NMDA receptor

To understand the role of α-Fe/TaC interface on the clustering behavior of helium (He) atoms inside castable nanostructured alloys under irradiation, we built two models (Ta13C14@Fe215 and {1 0 0} 〈1 1 0〉 Fe//{1 0 0} 〈1 0 0〉 TaC interface) and perform systemical ab initio calculations to investigate the energetics of point defects (vacancy, anti-site defect and He), substitutional defect clusters CrFem (1 ≤ m ≤ 4), and Hen (1 ≤ n ≤ 4) clusters, the migration behavior of He atom as well as the effect of Cr atoms and vacancies on the stability of Hen clusters. Vacancy and He at the α-Fe/TaC interface of the Ta13C14@Fe215 are more stable than that in Ta13C14 cluster and Fe matrix. Ta atom at this interface escapes from the lattice site more easily than C atom. In addition, the Cr atoms substituting Fe sites are more stable than Fe vacancies, which reduces the formation energies of Hen clusters in α-Fe/TaC on the Ta13C14@Fe215. Both formation energy and migration energy of He atom in {1 0 0} 〈1 1 0〉 Fe//{1 0 0} 〈1 0 0〉 TaC interface are lower than those in bulk TaC and α-Fe. Hence, {1 0 0} 〈1 1 0〉 Fe//{1 0 0} 〈1 0 0〉 TaC interface as a sink can trap more helium atoms and vacancies than α-Fe matrix.

Dynamic changes in interaction between prefrontal neurons revealed by joint-PSTH analysis

To understand the role of α-Fe/TaC interface on the clustering behavior of helium (He) atoms inside castable nanostructured alloys under irradiation, we built two models (Ta13C14@Fe215 and {1 0 0} 〈1 1 0〉 Fe//{1 0 0} 〈1 0 0〉 TaC interface) and perform systemical ab initio calculations to investigate the energetics of point defects (vacancy, anti-site defect and He), substitutional defect clusters CrFem (1 ≤ m ≤ 4), and Hen (1 ≤ n ≤ 4) clusters, the migration behavior of He atom as well as the effect of Cr atoms and vacancies on the stability of Hen clusters. Vacancy and He at the α-Fe/TaC interface of the Ta13C14@Fe215 are more stable than that in Ta13C14 cluster and Fe matrix. Ta atom at this interface escapes from the lattice site more easily than C atom. In addition, the Cr atoms substituting Fe sites are more stable than Fe vacancies, which reduces the formation energies of Hen clusters in α-Fe/TaC on the Ta13C14@Fe215. Both formation energy and migration energy of He atom in {1 0 0} 〈1 1 0〉 Fe//{1 0 0} 〈1 0 0〉 TaC interface are lower than those in bulk TaC and α-Fe. Hence, {1 0 0} 〈1 1 0〉 Fe//{1 0 0} 〈1 0 0〉 TaC interface as a sink can trap more helium atoms and vacancies than α-Fe matrix.

Development of an effective interatomic potential for transition metals and alloys: Modified Wills-Harrison model

This paper deals with the development of an effective interionic interaction for transition metals and alloys in the spirit of a hybridized pseudopotential tight-binding formalism, developed among others by Wills and Harrison. Stated briefly, the recipe consists in writing the total interionic interaction as a sum of contributions arising from nearly free s-electrons and that of tightly bound d-electrons. The s-d hybridization is also accounted for by allowing for the relative change in the occupancy of s and d electron counts. This potential, although not taking into account the magnetic contribution, contains otherwise all the essentials of a physically meaningful interatomic interaction. In this paper, a modified version of the Wills-Harrison model is used to calculate the structural and cohesive properties of transition metals at 0 K. The calculated values are in reasonable agreement with the experimental ones. This method is also used to derive an effective interatomic interaction for γ-TiAl. The use of this potential in obtaining an estimate of point defect energetics in TiAl is discussed.

Theoretical analysis of chemical bonding and solute systematics in RE2TM17 and RE2TM4B compounds (abstract)

Recent advances in the theory of bonding and the structural properties of solids have shown that simplified many-atom bonding theories utilizing various tight-binding-based approximations to the electronic density-of-states distributions are capable of systematizing experimental data as well as the results of ab initio theoretical calculations for a wide variety of solids including metallic compounds.123 Such theories are also capable of making useful predictions of the energetics of point defects and disorder, solution energetics, and deviations from stoichiometry.4 In this article, I will describe a simple second-moment-approximation-based many-atom bonding theory, including magnetic contributions to cohesive energy, for the families of permanent magnet compounds RE2TM17 and RE2TM14B (where RE is a rare-earth metal and TM a transition metal). The use of the theory to describe the systematics of energies and volumes of formation, solute locations, and energetics, as well as the energetics of point-defect formation and nonstoichiometry, will be discussed.

Atomistic simulation of point defects and diffusion in B2 NiAl

Abstract In part I of this work we studied point defect energetics in the ordered B2 compound NiAl by means of computer simulations using ‘molecular statics’ and the embedded atom method. In the present paper we apply the computation technique and results of part I to study atomic mechanisms of tracer self-diffusion in NiAl. We calculate the activation energy of Ni and Al self-diffusion in perfectly stoichiometric NiAl for three atomic mechanisms: the mechanism of next-nearest-neighbour (NNN) vacancy jumps, the 6-jump vacancy mechanism and the 4-ring mechanism. The results of our simulations indicate that self-diffusion in stoichiometric NiAl is dominated by the mechanism of next-nearest-neighbour vacancy jumps. Diffusion of Al by this mechanism is likely to occur more slowly and with a higher activation energy than diffusion of Ni. The mechanism of 6-jump cycles is less favourable but still highly competitive to the NNN vacancy mechanism. The 4-ring mechanism is the least effective for both Ni and Al dif...

Atomistic simulation of point defects and diffusion in B2 NiAl

In part I of this work we studied point defect energetics in the ordered B2 compound NiAI by means of computer simulations using 'molecular statics' and the embedded atom method. In the present paper we apply the computation technique and results of part I to study atomic mechanisms of tracer self-diffusion in NiAl. We calculate the activation energy of Ni and Al self-diffusion in perfectly stoichiometric NiAl for three atomic mechanisms: the mechanism of next-nearest-neighbour (NNN) vacancy jumps, the 6-jump vacancy mechanism and the 4-ring mechanism. The results of our simulations indicate that self-diffusion in stoichiometric NiAI is dominated by the mechanism of next-nearest-neighbour vacancy jumps. Diffusion of Al by this mechanism is likely to occur more slowly and with a higher activation energy than diffusion of Ni. The mechanism of 6-jump cycles is less favourable but still highly competitive to the NNN vacancy mechanism. The 4-ring mechanism is the least effective for both Ni and Al diffusion. The effect of off-stoichiometry on diffusion in NiAI is briefly discussed.

Energetics of point defects in γ-TiAl

{gamma}TiAl has been receiving a great deal of attention in recent times owing to its industrial importance. This structural intermetallic is a candidate material for high temperature aerospace applications. Therefore, a study of point defect properties is useful in elucidating its physical metallurgy. In this brief communication, the authors discuss the vacancy and antisite defect properties of {gamma}-TiAl.

Simulations of point-defect properties in graphite by a tight-binding-force model.

Point-defect energetics and diffusion mechanisms in graphite are investigated using a semi-empirical tight-binding-force model. Possible diffusion processes associated with point-defect (i.e., vacancies and interstitials) and nondefect (i.e., atomic exchange) mechanisms are analyzed. It is found that self-diffusion in graphite in the direction parallel to the basal plane can be mediated by vacancies. However, since the calculated vacancy- and interstital-formation energies are nearly equal, it is argued that Frenkel pairs could exist as equilibrium defects. In this case, at high enough temperatures, self-diffusion parallel to the basal plane should occur by an interstitial mechanism because the migration energy of the interstitial is much lower.

Role of induced quadrupoles in the simulation of intrinsic point defects in AgCl and NaCl.

The paper presents a detailed formulation of the energetics of point defects in ionic crystals that takes into account the quadrupolar deformation of the ions surrounding the defect within a polarizable-point-ion picture. The dipolar polarizabilities are modeled to represent correctly the static dielectric response of the crystal. Consideration of the axial symmetry of the electric-field-gradient tensor, the residual site symmetries in the defect crystal, and the symmetry of the quadrupolar-polarizability tensor leads to a cylindrical symmetry for the induced quadrupoles for both the vacancy and interstitial type of defects. These quadrupoles give rise to an additional polarization contribution to the defect formation besides affecting the dipolar energy. Computer codes have been developed based on this model and results are presented for vacancies and interstitials in AgCl and NaCl and discussed in detail. The calculations use a two-body central-force potential with well-represented van der Waals interactions. It is found that there is a substantial contribution from the quadrupoles to the defect enthalpy. The investigations reveal the comparative importance of accurate modeling of the polarization features vis \`a vis subtle refinements in the interionic potential.

Point defect structures and energetics in Si using an empirical potential

Statics calculations on silicon using the Tersoff 2 and Tersoff 3 potential energy functions are presented. We have evaluated the following at 0 K: (1) neutral monovacancy and divacancy formation and migration energies; (2) neutral bond‐centered, site sharing, tetrahedral, and hexagonal self‐interstitial formation and migration energies; and (3) the variation of these energies with distance from a bulk site of Frenkel defect formation and from [100] and [111] surfaces. The structural variations around these defects are also determined and discussed.

Energetics of point and planar defects in aluminium from first-principles calculations

Formation energies of the vacancy and self-interstitial in Al, as well as energies of intrinsic, extrinsic, and twin-boundary stacking faults are calculated from first-principles. The electronic structure and forces on the atoms are calculated in the framework of the Augmented Plane Wave method using new algorithms proposed by Williams and Soler, enabling an ab initio approach to long-standing questions on defects in metals.

Energetics of point defects in the two-dimensional Wigner crystal.

Calcul numerique de l'energie coulombienne de l'etat fondamental et de l'energie vibrationnelle au point zero, pour un cristal de Wigner bidimensionnel parfait ou contenant des interstitiels et des lacunes. Etude de l'interaction entre defauts de meme type, en fonction de la distance les separant; comparaison aux predictions de la theorie de l'elasticite. Chacun des types de defauts ponctuels abaisse l'energie vibrationnelle du systeme au point zero. Application au role des defauts ponctuels dans la fusion a la temperature nulle

Calculation of the Defect and Interface Properties of Ni3Al

AbstractThe structure and energetics of point defects, surfaces and grain boundaries in Ni3A1 are investigated using the Embedded Atom Method. The approach is shown to reproduce the experimental phase diagram of the Ni-Al system and the elastic properties of Ni3AL. The vacancy and anti-site defect energies are calculated and used to predict the vacancy concentration as a function of bulk composition. The preferred geometries and energies of the low index surfaces are also computed. The equilibrium structure of certain ideal grain boundaries are computed by Monte Carlo computer simulations as a function of bulk composition. It is found that the boundaries act as a sink for anti-sitedefects and the degree of ordering at the boundaries is strongly affected by the bulk composition. The cohesive energy of grain boundaries in Ni3A1 is computed and is found to be comparable to that for pure Ni.

Defects and diffusion mechanisms in Nb3Sn

Abstract The structure and energetics of point defects in A15 Nb 3 Sn were investigated by means of computer simulations based on a pair-potential model of cohesion. The repulsive part of the potential was calculated theoretically using electron-gas methods while the attractive part was derived by matching experimentally determined crystal properties. The properties of vacancies on both the Nb and Sn sublattice, as well as those of simple antisite defects, were investigated. The results show an unusual structure for the vacancy in the Nb sublattice: the vacancy dissociates into a “split” configuration consisting of “partial vacancies” separated by a one-dimensional “stacking fault”. The Sn vacancy was found to be metastable; it decomposes by an activated process into a Nb atom on a Sn site (an antisite defect) adjacent to a “split” Nb vacancy. The defect of lowest energy compatible with the proper three-to-one ratio of sublattice sites is the antisite defect pair. The lowest energy grouping which contains vacancies is found to consist of Nb-sublattice vacancies and Nb-on-Sn-sublattice antisite defects in the ratio of four of the former to one of the latter (quintuple defects). The results also suggest that bulk Sn diffusion is slower than Nb diffusion; this is consistent with the belief that rapid Sn diffusion during Nb 3 Sn layer growth does not occur by bulk but by grain-boundary diffusion. Nb vacancy migration was found to be quasi-one-dimensional with much lower migration energy required for motion along the Nb chains than that for jumps between chains.

Theoretical studies of point defects and diffusion in Nb<inf>3</inf>Sn

The structure and energetics of several simple point defects in A15 Nb 3 Sn were investigated by means of computer simulations based on a pair-potential model of cohesion. The properties of vacancies on both the Nb and Sn sublattices, as well as those of simple antisite defects, were examined, and estimates were made of the energetics of several types of atom-vacancy exchange (jump) processes. The results show an unusual structure for the vacancy on the Nb sublattice: the vacancy is between two adjacent sites along the Nb chain with an atom midway between them. We find the Sn vacancy (on the Sn sublattice) to be metastable; this vacancy will decompose by an activated process into a more stable configuration consisting of a Nb atom on a Sn site adjacent to a split Nb-sublattice vacancy. We find that the lowest energy grouping of defects compatible with maintaining sublattice sites in the proper three-to-one ratio is the antisite defect pair; the lowest energy grouping which contains vacancies is found to consist of Nb-sublattice vacancies and Nb-on-Sn-sublattice antisite defects in the ratio of four of the former to one of the latter (quintuple defects). Our results also suggest that bulk Sn diffusion is slower than Nb diffusion; this is consistent with the belief that rapid Sn diffusion during Nb 3 Sn layer growth does not occur by bulk but by grain-boundary diffusion.