Generalized Gradient Approximation Method Research Articles

High pressure, mechanical, and optical properties of ZrW2O8

In this paper, we present the high pressure studies of well known negative thermal expansion material ZrW2O8 on the basis of our density functional theory calculations. We reproduce the experimental α and γ crystal structures of ZrW2O8. We calculate the transition pressure (from α to γ) of this material to be ≈26 kbar. Our calculated positional and axial parameters are in an excellent agreement with the available experimental values for the both α and γ phases of this material. Then we study the mechanical properties of this material. We calculate the elastic constants, bulk, shear and Young’s moduli, Poisson’s ratio, and Debye temperature of α-ZrW2O8. In our study, we find that the generalized gradient approximation method fails to obtain the correct values while the local density approximation (LDA) method successfully reproduces the experimental bulk modulus of ZrW2O8. Our calculated values of the shear and Young’s moduli, Poisson’s ratio, and Debye temperature of α-ZrW2O8 with LDA method are also in good agreement with the experimental results. Finally, we predict and analyze the optical properties of this material.

Towards the Reduction of Optical Losses in Transition Metal-Based Nanomaterials

ABSTRACTEquilibrium geometries and cohesion energies of Ag0.94Cd0.06, Ag0.94In0.06, Au0.94Cd0.06, and Au0.94In0.06 solid alloys have been studied from the first principles within the Density Functional Theory using ab initio pseudopotentials. Equilibrium geometries are obtained by total energy minimization method using the Local Density Approximation and Generalized Gradient Approximation methods. Optical functions are calculated within the independent particles picture. We report essentially different behavior of Cd and In impurity atoms in Au- and Ag-based alloys: the aggregated (or quasi aggregated) phases in In-containing alloys are expected in contrast to the alloys with Cd atom where homogeneous impurity distribution over the bulk should dominate. Study of optical spectra in Ag0.94Cd0.06 and Au0.94Cd0.06 alloys indicate that optical losses in visible and near ultraviolet spectral range remarkably increase at bigger Cd concentrations. In ultraviolet spectral region redistribution of optical oscillator strengths results in both increase and decrease of optical losses in selected spectral regions.

A first-principles study of calcium-decorated, boron-doped graphene for high capacity hydrogen storage

Hydrogen adsorption and storage on calcium-decorated, boron-doped graphene was explored using density functional theory simulations based on local density approximation and generalized gradient approximation methods. The clustering problem for calcium-decorated graphene was investigated and it was shown that individual calcium atoms are not stable on pure graphene, and formation of aggregates is favorable. Substitutional boron doping can eliminate the clustering problem for Ca atoms on graphene. Up to four hydrogen molecules can stably bind to a Ca atom on a graphene plane with substitutional doping of a single boron atom. The average binding energy of ∼0.4 eV/H 2 is in the range that permits H 2 recycling at ambient conditions. Two binding mechanisms contribute to the adsorption of H 2 molecules: polarization of the H 2 molecule under the electric field produced by the Ca–graphene dipole, and hybridization of the 3d orbitals of Ca with the σ orbitals of H 2. Double-sided Ca-decorated graphene doped with individual boron atoms of 12 at.% can theoretically reach a gravimetric capacity of 8.38 wt.% hydrogen.

First-principles study of structural, electronic, and magnetic properties of double perovskiteHo2MnFeO6

Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of the designed double-perovskite-oxide ${\text{Ho}}_{2}{\text{MnFeO}}_{6}$ within generalized gradient approximation (GGA) and the $\text{GGA}+U$ methods. The calculations on electronic structure indicate the material to be half-metallic ferromagnets. Structural parameters are optimized using GGA method and different positional orderings and magnetic-coupling schemes are investigated. We also investigate the effect of Hubbard parameter $U$ on the ground-state structural parameters and electronic properties. We discuss the magnetic-coupling interactions for the ${\text{Mn}}^{3+}({d}^{4}){\text{-Fe}}^{3+}({d}^{5})$ system with special orbital ordering and calculate the magnetic nearest-neighbor coupling constants using Anderson's superexchange model and then obtain a mean-field approximation for the magnetic-ordering temperatures.

Ab initio calculations for properties of Ti 2AlN and Cr 2AlC

The electronic structure, dielectric function, Debye temperature and elastic properties of Ti 2AlN and Cr 2AlC were studied by means of pseudo-potential plane-waves method using the density function theory. The exchange-correlation approximation energy was determined by generalized gradient approximation method. The experimental lattice constants and the internal parameters were used in the calculation. The dielectric function was calculated by energy band theory, and the elastic constants were obtained using the static finite strain technique. The bulk and shear module, Young’s module and Poisson’s ratio for ideal polycrystalline Ti 2AlN and Cr 2AlC aggregates were derived in this paper. The Debye temperature of Ti 2AlN and Cr 2AlC from the average sound velocity was also estimated. The obtained results are in agreement with the available theoretical data.

Ionic layered BaFCl andBa1−xSrxFClcompounds: Physical- and chemical-pressure effects

The effect on crystal structure and vibrational frequencies of physical pressure in BaFCl and chemical pressure in ${\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}\text{FCl}$ solid solutions is studied using periodic density-functional theory (DFT) calculations performed within the local-density approximation (LDA) and the generalized gradient approximation (GGA). These results are compared with previously published experimental data for BaFCl in conjunction with new experimental data for ${\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}\text{FCl}$ and show overall a good agreement with experiment. The GGA method outperforms the LDA method for the description of BaFCl under pressure. However, the two DFT methods perform equally well for the description of the solid solutions, which have been studied within the virtual-crystal approximation. They also give consistent values of the energy of formation of ${\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}\text{FCl}$, which can be correlated with the experimentally observed melting points. The comparison of the calculated mode Gr\"uneisen parameters shows that, for the investigated systems, the effect of the chemical pressure and that of the physical pressure are not identical.

GGA and GGA+ U calculations of the ground state of SrMnO 3

A very recent experimental report suggests the ground state of SrMnO 3 is orthorhombic with space group C222 1. I perform spin-polarized generalized gradient approximation (GGA) calculations with/without on-site Coulomb interaction included (GGA+ U) for this ground state of SrMnO 3. In contrast to the latest theoretical report of cubic and hexagonal SrMnO 3 where GGA method is considered to be able to reproduce the insulating behavior of SrMnO 3, I find that for the orthorhombic SrMnO 3, GGA method is not enough to reproduce its insulating behavior. When the on-site Coulomb interaction is included by means of GGA+ U, the insulating character is recovered. The results show that correlation interaction plays an important role in the ground state of SrMnO 3.

Study of Co2MnAl Heusler alloy as half metallic ferromagnet

We present the study of half metallacity of Co2MnAl as half-Heusler alloy in L21 structure which consists of four inter-penetrating fcc sub-lattices. Density functional theory based electronic structure calculations will be performed by using the full-potential linear augmented plane wave (FP-LAPW). We will use the general gradient approximation method (GGA) and local density approximation method called LDA+U. Density of states and band structure results will be presented in this paper.

First-principles study of electronic structure and magnetic properties of [formula omitted

Electronic structures of Mg 1− x Cr x O ( x = 0 , 0.25, 0.5, 0.75) were investigated based on first-principles calculations using generalized gradient approximation (GGA) and GGA + U . Different magnetic and electronic properties are predicted from the calculated spin-dependent density of states near the Fermi surface. Using the GGA method, Mg 1− x Cr x O is calculated to be ferromagnetic at x = 0.25 and antiferromagnetic at x = 0.5 and 0.75. Alternatively, the ferromagnetic state is stable for all dopant levels when using the GGA + U method. Mg 1− x Cr x O is also half-metallic and its relevant metallic nature is calculated to be highly dependent on the spin state and the dopant levels: the DOS distributions exhibit metallic characteristics for spin-up electrons at x = 0.25 , or for spin-down electrons at x = 0.5 , or even for both at x = 0.75 .

First principles studies of an Si tip on anSi(100)2 × 1 reconstructed surface

We present a systematic study of the interaction between a silicon tip and a reconstructedSi(100)2 × 1 surface by means of total energy calculations using density functional theory. We performgeometry optimization to obtain the reconstructed Si surface using the local densityapproximation and the generalized gradient approximation methods and compare ourresults with those obtained experimentally. We then study the effects of the tip of ascanning probe of an atomic force microscope (AFM) on the behaviour of atoms on thereconstructed surface when the tip translates at distances close to it. Our results show thatat certain positions of the tip relative to the surface and depending on the direction of thescan, the Si dimer on the surface flips, resulting in a local reconstruction of the surface intop(2 × 2) or c(4 × 2) configurations. These configurations exhibit energies lower by0.05 eV/dimer than theSi(100)2 × 1 structure.

Mechanism of molecular hydrogen dissociation on gold chains and clusters as model prototypes of nanostructures

The reactivity of H(2) on several gold clusters is studied using density functional theory with generalized gradient approximation methods, as model systems designed to study the main effects determining their catalytic properties under controlled conditions. Border effects are studied in finite linear gold chains of increasing size and compared with the corresponding periodic systems. In these linear chains, the reaction can proceed with no barrier along the minimum energy path, presenting a deep chemisorption well of approximately 1.4 eV. The mechanism presents an important dependence on the initial attacking site of the chain. Linear Au(4) chains joined to model-nanocontacts, formed by 2 or 3 gold atoms, in a planar triangle or in a pyramid, respectively, are also studied. The reaction barriers found in these two cases are approximately 0.24 and 0.16 eV, respectively, corresponding to H(2) attacking the more coordinated edge atom of the linear chain. The study is extended to planar clusters with coordinations IV and VI, for which higher H(2) dissociation barriers are found. However, when the planar gold clusters are folded, and the Au-Au distances elongated, the reactivity increases considerably. This is not due to a change of coordination, but to a larger flexibility of the gold orbitals to form bonds with hydrogen atoms, when the planar sd-hybridization is broken. Finally, it is concluded that the major factor determining the reactivity of gold clusters is not strictly the coordination of gold atoms but their binding structure and some border effects.

First-principles study of atomic and electronic structures of the silicon oxide clusters on Si(001) surfaces

The possible models of the silicon oxide clusters on Si（001） surface, including the regular symmetric structure （A）, the periodic asymmetric structure （B）, the periodic asymmetric structure （C） and the irregular structure （D）, have been fully optimized using the first-principles general gradient approximation method based on density-functional theory. The results show that all the optimized surface structures are amorphous. The optimized surface structures of the B, C and D models have the similar geometric character as the tetrahedron structure of SiO2. Furthermore, the coalescence between the Si and O atoms of the silicon oxide clusters includes significant ionic bond and certain covalent bond as shown by employing the Mulliken population analysis and the graphics of electron localization function.

Electronic structure and thermodynamic properties of LiBC under high pressure

This paper investigates the electronic structure and thermodynamic properties of LiBC in the hexagonal structure by using the generalized gradient approximation (GGA) and local density approximation correction scheme in the frame of density functional theory. The geometric structure of LiBC under zero pressure, and the dependences of the normalized lattice parameters a/a0 and c/c0, the ratio c/a, the normalized primitive volume V/V0 on pressure are given. The thermodynamic quantity (including the heat capacity Cv, Debye temperature ΘD, thermal expansion α and Grüneisen parameter γ) dependences on temperature and pressure are obtained through the GGA method and the quasi-harmonic Debye model. The band structures and density of state of LiBC under different pressures have also been analysed.

First-principles investigation on extraction of lithium ion from monoclinic LiMnO 2

Changes with Li extraction in the crystal structure and electronic structure of monoclinic LiMnO 2 have been investigated by first-principle calculations using spin-polarized generalized gradient approximation method. It is found that the Li extraction has changed the oxidation state of Mn ion from 3+ to 4+ and thereby induced a phase transition from the monoclinic structure to rhombohedral symmetry, which opens channels for the migration of Mn ions from their own octahedral sites to the interlayer Li vacancies. It is the migration of Mn ions that responds for the phase transition to a spinel-like structure and the severe capacity loss during the electrochemical cycles of monoclinic LiMnO 2.

Electronic properties of bilayered manganiteCa2.5Sr0.5GaMn2O8from first-principles calculations

By means of first-principles calculations using spin-polarized generalized gradient approximation method and ultrasoft pseudopotentials, the electronic and magnetic structure of the recently synthesized orthorhombic phase of bilayer manganite ${\text{Ca}}_{2.5}{\text{Sr}}_{0.5}{\text{GaMn}}_{2}{\text{O}}_{8}$ compound is obtained. Our calculations have shown that the specific antiferromagnetic ordering, in accordance with experimental findings, is the most stable one. The total energies calculated for the several possible magnetic states of the title compound in the orthorhombic phase enable us to estimate spin exchange interactions for nearest ${J}_{nn}=\ensuremath{-}1.60\text{ }\text{meV}$ and next\penalty1000\-\allowhyphens{} nearest neighbors ${J}_{nnn}=\ensuremath{-}0.21\text{ }\text{meV}$. Calculated Curie--Weiss temperature in the mean-field approximation is in excellent agreement with the measured magnetic phase transition temperature.

FIRST-PRINCIPLES STUDY OF GAMMA-Si3N4 WITH GENERALIZED GRADIENT APPROXIMATION AND LOCAL DENSITY APPROXIMATION

The electronic structure, and optical and mechanical properties of gamma- Si 3 N 4 are calculated by means of the plane-wave pseudo-potential method (PWP) with generalized gradient approximation (GGA) and local density approximation (LDA). The calculated values are in good agreement with experimental results. A comparative investigation reveals that the band gap calculated by GGA agrees better with the experimental value than by the other methods, while the calculated bulk modulus by LDA is comparable with that by the method of orthogonalized linear combinations of atomic orbitals (OLCAO). Our results indicate that a satisfactory calculation of the structures and properties of gamma- Si 3 N 4 could be achieved by combining GGA and LDA methods.

Studies of magnetic interactions in Mn-doped β-Ga 2O 3 from first-principles calculations

The magnetic behavior of Mn-doped beta-Ga2O3 is Studied from first-principles calculations within the generalized gradient approximation method. Calculations show that ferromagnetic ordering is always favorable for configurations in which two Mn ions substitute either tetrahedral or octahedral sites, and the ferromagnetic ground state is also sometimes favorable for configurations where one Mn ion substitutes a tetrahedral site and another Mn ion substitutes an octahedral site. However, the configurations of the latter case are less stable than those of the former. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

First-principles study on electronic structure and optical properties of Al and Mg doped GaN

The electronic structure and optical properties of pure and Al_and Mg-doped cubic-blende GaN have been studied based on the density functional theory and the generalized gradient approximation method. The relationship between the distribution of the electronic states and their structures has been analyzed.The dielectric function and the complex refractive index function on the doping GaN system have been obtained. The calculation results indicate that the hole concentration of Mg-doped GaN is increased and the Mg-doping markedly enhanced the conductivity of the material.But the carrier concentration of Al-doped GaN is not changed and optical bandgap is broadened.The mechanism of luminescence has been explained in terms of an analysis of the dielectric function and the complex refractive index function. It offers theoretical data for the design and application of GaN optoelectronic materials. By comparison, our results are in good agreement with that published in the literatures.

Geometry and electronic and magnetic properties ofMPd12clusters (M=Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pd) from first principles

The atomic, electronic, and magnetic structures of $M{\mathrm{Pd}}_{12}$ clusters have been investigated by using the density functional theory formalism under spin polarized generalized gradient approximation method. The results showed icosahedron configuration for all $M{\mathrm{Pd}}_{12}$ clusters with transition metal elements $(M)$ occupying the central position. Moreover, it is found that the substitution of these elements improves the average binding energy of ${\mathrm{Pd}}_{13}$ clusters. Unlike bulk Pd metal, for ${\mathrm{Pd}}_{13}$ cluster, each Pd atom carries a magnetic moment of about $0.6{\ensuremath{\mu}}_{B}$, which orders ferromagnetically with a resultant magnetic moment of $8{\ensuremath{\mu}}_{B}$. Expectedly, the magnetic characteristics of ${\mathrm{Pd}}_{13}$ clusters are significantly influenced by the $M$ element. While the substitution of Pd by Ti, V, Cr, Cu, and Zn reduces the total magnetic moment through antiferromagnetic coupling between the moments of Pd and the central $M$ atom, Mn, Fe, and Co show an opposite effect, i.e., they enhance the magnetic moment through ferromagnetic coupling. In particular, the magnetic moment of ${\mathrm{Pd}}_{13}$ cluster shows maximum difference for Cr and Mn substitutions. While Cr substitution quenches the total moment up to $2{\ensuremath{\mu}}_{B}$, Mn substitution leads to enhancement of magnetic moment up to $11{\ensuremath{\mu}}_{B}$.

Optical properties of cadmium telluride in zinc-blende and wurzite structure

The optical properties of cadmium telluride including the linear optical dielectric function, the refractive index, the extinction coefficient, the reflectivity and the plasmon energy have been calculated by density functional theory (DFT). The full potential linearized augmented plane wave (FL-LAPW) method was used with the generalized gradient approximation (GGA) including the orbital dependence of the self-energy, i.e. the orbital-dependent potentials of Coulomb and exchange interactions (GGA+ U). Using only LDA or GGA methods underestimates the electronic parameters (band gap and band dispersion). Applying orbital-dependent potentials splits the Te-5s state and shifts the binding energies of the Cd-4d levels towards the experimentally determined position. The calculated results indicated that although Te-5s and Cd-4d overlap, Cd-4d plays an important role in absorption and reflectivity constants. The optical constants of CdTe in hexagonal structure exhibit anisotropy (birefringence) in two directions (in basal-plan and c-axis) but the difference is very small in the static limit.