Equilibrium Atomic Configuration Research Articles

The Submonolayer Structure of Ni(111)–(√3 × √3)R30–Pb: Atomic Relaxation and Vibrational Properties

The results of a theoretical study of the structural and dynamical parameters of surface phases (adsorption phase and surface alloy phase) formed upon the adsorption of 0.33 Pb monolayer on the Ni(111) plane surface are discussed. Calculations were performed using interatomic potentials obtained within the framework of the embedded atom method. The stability of the surface phases was analyzed on the basis of data on the equilibrium atomic configuration, phonon spectra, local density of phonon states, and polarization of localized vibration modes. It is shown that the Pb–Ni surface alloy has the highest dynamic stability among the two possible surface phases.

Modeling of Gold Adsorption by the Surface of Defect Graphene

The results of the theoretical investigation of the local structural changes and adsorption characteristics of the graphene (GP) surface in the presence of a vacancy + Auads adatom complex are presented. Based on the density functional theory (DFT), the adsorption properties of Auads at the surface of GP supercells containing 50 carbon atoms with vacancies ({text{GP}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle ,{text{ G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle ) are calculated. The most stable configuration of {text{G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle supercells with a vacancy + Auads adatom complex is determined. The effect that an Auads adatom has on the band structure and local magnetic moment in {text{G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle is calculated. The data are analyzed based on the equilibrium atomic configuration {text{G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle , local density of electronic states, and spin polarization. The calculations are made using the exchange-correlation functional in a local electron-spin density approximation (LSDA).

Динамическое поведение субмонослойных пленок щелочных металлов на поверхности (110) Cu

This work is devoted to a theoretical study of the structural and vibrational characteristics of a two-dimensional film of an alkali metal (Li, Na, K) deposited on an unreconstructed and reconstructed Cu (110) surface. Calculations were made of the equilibrium atomic configuration, polarization vectors of localized vibrational modes, and the local density of vibrational states for different degrees of adsorption of an alkali metal. All calculations were performed using interatomic potentials obtained using the embedded atom method. The obtained numerical values of the structural parameters, frequency and polarization of localized modes are in agreement with the available experimental data.

Stochastic Computer Model of the Kinetics of Multicomponent Intercrystalline Adsorption in Solid Solutions

The basic principles of constructing a stochastic computer model of intergranular adsorption kinetics controlled by interatomic pair interactions and the volume diffusion mobility of the components of a solid solution are developed, allowing for the bulk structure of crystals and the equilibrium atomic configuration of intergranular boundaries. The results are presented from computer simulations of the grain boundary segregation of S and P in α-Fe-based alloys.

Critical assessment of Pt surface energy – An atomistic study

Despite the fact that surface energy is a fundamental quantity in understanding surface structure of nanoparticle, the results of experimental measurements and theoretical calculations for the surface energy of pure Pt show a wide range of scattering. It is necessary to further ensure the surface energy of Pt to find the equilibrium shape and atomic configuration in Pt bimetallic nanoparticles accurately. In this article, we critically assess and optimize the Pt surface energy using a semi-empirical atomistic approach based on the second nearest-neighbor modified embedded-atom method interatomic potential. That is, the interatomic potential of pure Pt was adjusted in a way that the surface segregation tendency in a wide range of Pt binary alloys is reproduced in accordance with experimental information. The final optimized Pt surface energy (mJ/m2) is 2036 for (100) surface, 2106 for (110) surface, and 1502 for (111) surface. The potential can be utilized to find the equilibrium shape and atomic configuration of Pt bimetallic nanoparticles more accurately.

Submonolayer adsorption of Na onto the Cu(110) surface: Structure and vibrational properties

The submonolayer adsorption of Na onto the Cu(110) surface is studied. At small Na coverages (Θ = 0.16–0.25 ML), the substrate surface subjected to missing-row reconstruction (1 × 2) is shown to be most stable dynamically. When the coverage increases to Θ = 0.5 ML, the unreconstructed substrate surface with a c(2 × 2) sodium adlayer becomes dynamically stable. For an analysis, we used data on the equilibrium atomic configuration, the adsorption energy, the phonon spectra, the local density of phonon states, and the polarization of localized vibrational modes. All calculations were performed using the interatomic potentials obtained in terms of the embedded-atom method. The calculated frequencies of localized vibrational modes agree well with the existing experimental data.

Metastability of Mn3+ in ZnO driven by strong d (Mn) intrashell Coulomb repulsion: Experiment and theory

Depopulation of the Mn$^{2+}$ state in ZnO:Mn upon illumination, monitored by quenching of the Mn$^{2+}$ EPR signal intensity, was observed at temperatures below 80~K. Mn$^{2+}$ photoquenching is shown to result from the Mn$^{2+}$ $\to$ Mn$^{3+}$ ionization transition, promoting one electron to the conduction band. Temperature dependence of this process indicates the existence of an energy barrier for electron recapture of the order of 1~meV. GGA$+U$ calculations show that after ionization of Mn$^{2+}$ a moderate breathing lattice relaxation in the 3+ charge state occurs, which increases energies of $d$(Mn) levels. At its equilibrium atomic configuration, Mn$^{3+}$ is metastable since the direct capture of photo-electron is not possible. The metastability is mainly driven by the strong intra-shell Coulomb repulsion between $d$(Mn) electrons. Both the estimated barrier for electron capture and the photoionization energy are in good agreement with the experimental values.

A first-principles study on uniaxial strain effects of nonplanar oxygen-functionalized armchair graphene nanoribbons

Based on extensive first-principles calculations, we report the equilibrium atomic configuration, mechanical and electronical properties of nonplanar oxygen-passivated armchair graphene nanoribbons (O-AGNRs) under uniaxial strain. We can get more stable and stiffer structures using uniaxial strain. The mechanical properties of the O-AGNRs such as Young’s modulus exhibits nonlinear behavior with strain, and Poisson’s ratio shows a decreasing trend with increasing tensile strain. In addition, under uniaxial strain the band gaps of the nonplanar O-AGNRs not only experience a transition between direct and indirect but also change their widths and can be classified into three families according to the ribbon width of 3n, 3n+1, and 3n+2.

Semigrand Canonical and Kinetic Monte Carlo simulations of binary B2-ordered nano-films with triple defects

Equilibrium atomic configurations and the kinetics of “order–order” and surface segregation processes in B2-ordering stoichiometric A-50 at.%B binary thin films are investigated by means of Semigrand Canonical Monte Carlo (SGCMC) and Kinetic Monte Carlo (KMC) simulations. The (100)-oriented films are modeled with an Ising-type Hamiltonian with previously evaluated pair-interaction energy parameters yielding the effect of “triple-defect disordering”. The SGCMC simulations provide equilibrium vacancy concentration and atomic configuration in the films with B-atom termination of both free surfaces achieved at high temperatures by the generation of an antiphase boundary. Despite strong vacancy surface segregation, the thermodynamic activation energy for their formation inside the films is the same as in the bulk material. KMC simulations implemented with the SGCMC-determined equilibrium vacancy concentration reveal very slow relaxation of the films towards equilibrium configuration. The B-termination of the (100) free surfaces is produced by A-atom diffusion inwards into the films mediated by vacancies segregating on surfaces.

Atomic Migration as a Mechanism of Superstructure Formation in Intermetallic Compounds

“Order-order” relaxations driven by atomic migration in superstructures proceed in nonsteady- state of a system, which relaxes to the equilibrium atomic configuration. Hence, the corresponding studies are complementary to standard steady-state diffusion investigations. Two time scales operating in “order-order” relaxations in L12-ordered (Ni3Al) and L10-ordered (FePd, FePt) binary intermetallics were experimentally observed. On the other hand, in B2-ordered NiAl – known of a giant vacancy concentration, “order-order” relaxations appeared surprisingly slow. Definite relationships between the activation energies for diffusion ( ) D A E and “order-order” relaxations ( ) O O A E − were revealed: ( ) D A E < ( ) O O A E − in L12-type superstructure; ( ) D A E ³ ( ) O O A E − in L10- and in B2-type superstructures. Corresponding simulation studies elucidated the specific atomistic mechanism of the processes. It has been shown that different time scales active in “order-order” relaxations in L12 and L10-ordered systems follow from specific atomic-jump correlations, which result from non-steady-state conditions and particular superlattice geometries: the availability of easy diffusion channels. A model of “order-order” kinetics in NiAl as controlled by a triple-defect mechanism is proposed.

Oxygen chemisorption effects on the spatial atomic distribution of CuNi, CuPd and NiPt nanostructures

The spatial atomic distribution in cubo-octahedral CuNi, CuPd and NiPt clusters with a total number of atoms, N = 147, in the presence of chemisorbed oxygen, is studied. The equilibrium atomic configuration is obtained by calculating the free energy within the regular solution model and by assuming that the surface of the cluster is covered by oxygen atoms. Depending on the interaction between oxygen and the cluster components, the atomic distribution in the cluster can be completely modified as compared to the case of clusters with a clean surface. We present result for the temperature dependence of the concentration at the different shells around the central atom.

Changes in the equilibrium shape of bimetallic nanostructures induced by chemisorption

The equilibrium shape of bimetallic nanostructures in the presence or absence of chemisorbed species is studied. It is found that chemisorption may induce a structural change from icosahedral to cubo-octahedral structure. The equilibrium atomic configuration for each one of the structures is obtained by calculating the free energy within the regular solution model and by assuming that the surface of the cluster is covered by chemisorbed atoms. Depending on the interaction between the chemisorbed species and the cluster components, the atomic distribution and the structure of the cluster can be completely modified as compared to the case of clusters with a clean surface. This model is applied to the chemisorption of H and O on CuPd clusters.

Chemisorption effects on the spatial atomic distribution of bimetallic nanostructures

The spatial atomic distribution in icosahedral and cubo-octahedral bimetallic clusters with a total number of atoms, N=55 and 147, in the presence of chemisorbed species, is studied. The equilibrium atomic configuration is obtained by calculating the free energy within the regular solution model and by assuming that the surface of the cluster is covered by chemisorbed atoms. Depending on the interaction between chemisorbed species and the cluster components, the atomic distribution in the cluster can be completely modified as compared to the case of clusters with a clean surface. Applications to the chemisorption of H and O on CuNi, CuPd, and NiPt clusters are discussed. We present results for the temperature dependence of the concentration at the different shells around the central atom.

Atomistic Studies of Interfacial Structure and Properties

The computer simulation of the structure and fracture of interfaces on an atomic scale requires a computationally efficient prescription for the total energy that is reliable both for small deviations from the bulk as well as for the free surfaces produced during fracture. The recently developed Embedded Atom Method is such a method. It will be briefly described and compared to traditional pair interaction approaches. In particular, it will be shown that the many-body effects inherent in the Embedded Atom Method are essential to correctly describe the experimentally observed surface reconstructions of Au surfaces. The necessary first step in simulating the fracture of an interface, such as a grain boundary, is the determination of the initial or equilibrium atomic configuration of the interface. Equilibrium Monte Carlo simulations using the Embedded Atom Method can determine this structure. This approach will be outlined and various results for grain boundary structure in fcc metals will be presented. The atomic structure of symmetric tilt boundaries is found to be significantly different from that deduced from energy minimization techniques. In addition, the Monte Carlo technique allows for the determination of thermal effects such as the vibrational amplitudes at the interface and the thermal expansion of the interface.

Atomic oxygen in silicon: The formation of the Si-O-Si bond.

We study the equilibrium atomic configuration of atomic oxygen in silicon. We calculate the total energy of a cluster of silicon atoms before and after the incorporation of atomic oxygen. The energy is calculated for various atomic configurations to obtain the stable one. The equilibrium atomic distribution involves the separation of the silicon atoms forming a covalent bond to accommodate the oxygen atom. Analysis of the charge distribution reveals the break of the covalent silicon--silicon bond which is replaced by a Si---O---Si puckered bond. A careful analysis of the energy spectrum as well as of the charge-density matrix in the equilibrium configuration indicates the similarity between the new Si---O---Si bond formed and the basic chemical unit of ${\mathrm{SiO}}_{2}$. Analysis of the rotational and vibrational modes around the defect is in good agreement with infrared-absorption measurements.

Static lattice distortions in substitutional alloys: a computer simulation

A computer simulation of the static lattice distortions in substitutional potassium-rubidium alloys is presented. Interatomic forces and volume energies are derived from a non-local first-principles pseudopotential; a novel variant of the cluster relaxation technique is used to calculate the equilibrium atomic configuration of finite clusters. It is shown that substantial lattice distortions occur in K-Rb alloys and that they have a non-negligible influence on the heat of formation. The authors find that the distribution of the interatomic spacings and the relaxation energy are well described by the linear harmonic response theory of Froyen and Herring (1981).

The Ram an Effect and its Chemical Applications

MODERN students of molecular structure have before them the double objective of the determination of the positions and mobilities of the atoms in a molecule, or, in other words, the equilibrium atomic configuration and the intramolecular force field; and their ideal is to achieve a complete and quantitative description of these complementary features of structure. Two types of method of outstanding value are available for the quantitative study of atomic configuration, namely, a group of interferometric methods, either with electrons or X-rays, and a group of spectro-scopic methods, which may be summarized under the name long-wave spectroscopy. One method only is available for the complementary study of intramolecular force fields, namely, the method of long-wave spectroscopy. The Ram an Effect and its Chemical Applications By James H. Hibben. With a Theoretical Discussion by James H. Hibben and Prof. Edward Teller. (American Chemical Society, Monograph Series, No. 80.) Pp. 544. (New York: Reinhold Publishing Corporation; London: Chapman and Hall, Ltd., 1939.) 66s. net.