Basis Of Total-energy Calculations Research Articles

DFT investigations on mechanical stability, electronic structure and magnetism in Co2TaZ (Z = Al, Ga, In) heusler alloys

Ferromagnetic Heusler compounds have vast and imminent applications for novel devices, smart materials thanks to density functional theory (DFT) based simulations, which have scored out a new approach to study these materials. We forecast the structural stability of Co2TaZ alloys on the basis of total energy calculations and mechanical stability criteria. The elastic constants, robust spin-polarized ferromagnetism and electron densities in these half-metallic alloys are also discussed. The observed structural aspects calculated to predict the stability and equilibrium lattice parameters agree well with the experimental results. The elastic parameters like elastic constants, bulk, Young’s and shear moduli, poison’s and Pugh ratios, melting temperatures, etc have been put together to establish their mechanical properties. The elaborated electronic band structures along with indirect band gaps and spin polarization favour the application of these materials in spintronics and memory device technology.

Emergence of Metallic Properties at LiFePO4 Surfaces and LiFePO4/Li2S Interfaces: An Ab Initio Study

The atomic and electronic structures of the LiFePO4 (LFP) surface, both bare and reconstructed upon possible oxygenation, are theoretically studied by ab initio methods. On the basis of total energy calculations, the atomic structure of the oxygenated surface is proposed, and the effect of surface reconstruction on the electronic properties of the surface is clarified. While bare LFP(010) surface is insulating, adsorption of oxygen leads to the emergence of semimetallic behavior by inducing the conducting states in the band gap of the system. The physical origin of these conducting states is investigated. We further demonstrate that deposition of Li2S layers on top of oxygenated LFP(010) surface leads to the formation of additional conducting hole states in the first layer of Li2S surface because of the charge transfer from sulfur p-states to the gap states of LFP surface. This demonstrates that oxygenated LFP surface not only provides conducting layers itself, but also induces conducting channels in the top layer of Li2S. These results help to achieve further understanding of potential role of LFP particles in improving the performance of Li-S batteries through emergent interface conductivity.

Structural and magnetic properties of CrSb compounds: NiAs structure

The structural and magnetic properties of CrSb compounds with NiAs structure have been studied by means of the Korringa–Kohn–Rostoker (KKR) band structure method. An analysis of the structural and magnetic stability has been performed on the basis of total energy calculations for various magnetic states. The magnetic properties at finite temperature have been investigated by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed us to determine the critical temperature in good agreement with experiment.

Magnetism of ultrathin wires suspended in free space and adsorbed on vicinal surfaces

On the basis of total-energy calculations within density functional theory the possibility of magnetic ordering in ultrathin, one or two atom wide nanowires is studied. Specifically, we investigate nanowires composed of fifth and sixth row elements, which are nonmagnetic in the solid phase. At first, the unsupported straight wires are discussed and then, similar to experimental conditions, the wires are placed along step ledges of vicinal surfaces of copper and silver Free-standing wires show only a weak tendency towards magnetic ordering at the equilibrium bond length. In analogy with their 3d homologues Mo, Tc, W, Re are found to order antiferromagnetically, Ru, Rh, and Ir ferromagnetically. Surprisingly, ferromagnetism is also predicted for the early transition metals Zr and Ta and for the simple metals In and Tl. This picture is profoundly modified for supported wires, where the expansion of the bond length enforced through the epitaxial relationship with the substrate favors magnetic ordering but hybridization with the substrate electrons tends to quench magnetism. It turns out that wires on a Cu substrate prefer a ferromagnetic order, whereas on a Ag substrate most elements tend to antiferromagnetism. A second row of atoms added to the wires destroys the magnetism in wires on a Cu substrate, and reduces it in wires on a Ag substrate, except for the late transition metals (Rh, Ir) where an enhancement of magnetic moments is observed. Two possible growth modes of nanowires - a row-by-row growth and island growth - are explored. The results allow us to suggest that Ru, Rh, and Os wires on Ag stepped surfaces are the most promising systems in which magnetism could be verified experimentally.

Structure and stability of the modulated phase Sb-II

The crystal structure of Sb-II has been determined using angle-dispersive x-ray diffraction of synchrotron radiation. The crystal structure comprises an interpenetrating assembly of a tetragonal host sublattice with symmetry $I422$ and a tetragonal guest sublattice of symmetry $I422,$ which realize common a axes but different c axes. Weak extra peaks that are also reported in recent investigations performed independently are attributed to modulation waves of the atomic positions in both sublattices. A refinement using full profiles of powder-diffraction data was performed in the four-dimensional superspace group ${\mathrm{L}}_{\ensuremath{-}111}^{I422}:{\mathrm{L}}_{\ensuremath{-}111}^{I422}.$ The structural investigation is accompanied by ab initio full-potential band-structure calculations confirming the experimentally determined modification sequence. Fitting equations of state to the results of these computations generates pressure-volume relations which are in excellent agreement with the experimental data. On the basis of total-energy calculations, an earlier proposed crystal structure of the tetragonal phase has to be reconsidered in the light of the composite arrangement in Sb-II.

Do WE UNDERSTAND THE STRUCTURE OF THE GALLIUM-RICH SURFACE of GaAs(001)? EXPERIMENTAL AND THEORETICAL APPROACHES

While the As-rich 2 × 4 reconstruction of GaAs(001) is well explained by the so-called β2 structure, the atomic structure of the Ga-rich 4 × 2 phase has been discussed for a long time. In this review, the most important structural models for the GaAs(001) (4 × 2)/c(8 × 2) surface are compared from different theoretical and experimental points of view. The selected reconstructions include the recently proposed ζ model, a new mixed dimer model, and the well-known β, β2, Cerdà and Skala models. The different structures are compared on the basis of total energy calculations, simulations of STM experimental images and interpretation of X-ray diffraction data. Only the ζ model satisfies all criteria, and provides therefore a satisfactory explanation of the atomic structure of GaAs(001)-(4 × 2).

Theoretical investigation of sulfur solubility in pure copper and dilute copper-based alloys

Dilute Cu-based alloys containing S, P, and Ag impurities and also vacancies are studied theoretically on the basis of total energy calculations. This is done within the supercell approach by using the locally self-consistent Green's function (LSGF) method. The impurity solution energies, volume misfits, and interaction energies for these defects are calculated and used to study the microscopic mechanism behind the effect of these impurities on embrittlement of copper at intermediate temperatures. It is shown that the solubility of S in Cu is low due to precipitation of the highly stable Cu 2S phase. A large binding energy of a sulfur–vacancy defect pair in the first coordination shell (−0.46 eV) and a sulfur–sulfur defect pair in the second coordination shell (−0.12 eV) seem to favor this precipitation. The effect of phosphorus and silver impurities on the bulk S solubility has also been studied, and was found to depend on the competition of these impurities with sulfur for vacancies, as well as probably for other lattice defects.

Theoretical Investigation of The Defect Interactions in Dilute Copper Alloys Intended for Nuclear Waste Containers

AbstractApplication of pure, oxygen-free copper as a construction material having excllent corrosion resistance is limited because of the effect of intergranular emnibrittleinent at temperatures above 100–150°C. Dilute copper alloys containing S, P, and Ag inpirities and vacancies are studied theoretically on the basis of total energy calculations. The dissolution energies, volunie misfits, and defect interaction energies are calculated and used to study the microscopic miechanisin behind the effect of these impurities on the eiibrittlenieiit of colpper at interinedia temiperatures. A large linding energy of a sulfur-vacancy defect pair (–0.46 eV) is found. The sulfur-vacancy and sulftir-sulftir interactions in the copper matrix seeni to favor precipitation of copper sulphide Cu2S which is the most probable cause of the einhrittleiieiit. The effect of phosphorus and silver ilmpurities on the einbrittlement of sulfiir-contaminated copper can he related to their competition with sulfuir to attract to vacancies as well as to other lattice defects.

Stability of ferromagnetic monolayer of Fe in Cu(001) and Pd(001)

The electronic structure of a monolayer of Fe grown on Cu(001) and on Pd(001) surfaces is calculated from first principles. It is argued, on the basis of total energy calculations, that this is one possible way of stabilizing a ferromagnetic phase of γ-Fe. Comparison with experiments shows good agreement in the case of a monolayer of Fe in Cu. The reasons for this stability are shown to be mainly dominated by the size of the host.

Stability of ferromagnetic monolayer of Fe in Cu(001) and Pd(001)

The electronic structure of a monolayer of Fe grown on Cu(001) and on Pd(001) surfaces is calculated from first principles. It is argued, on the basis of total energy calculations, that this is one possible way of stabilizing a ferromagnetic phase of γ-Fe. Comparison with experiments shows good agreement in the case of a monolayer of Fe in Cu. The reasons for this stability are shown to be mainly dominated by the size of the host.