Pseudopotential Density Functional Method Research Articles

Superconductivity in Diamond-like BC<sub>3</sub> Phase

Two possible phases of superhard material BC3 originating from the cubic diamond structure are investigated by ab initio pseudopotential density functional method using generalized gradient approximation (GGA). We calculate their elastic constants, electronic band structure, and density of states (DOS). Full phonon frequencies, electron-phonon coupling constant and possible superconducting Tc of the metallic phase with tetragonal symmetry (t-BC3, space group P-42m) have for the first time been investigated at 5 and 10 GPa. The calculated electron-phonon coupling (0.67) and the logarithmically-averaged frequency (862 cm-1) show superconductivity for the undoped t-BC3 with Tc = 20 K at 5 GPa, which decreases to 17.5 K at 10 GPa. Keywords: Superhard BC3; Band structure; Phonon spectra; Superconductivity. © 2010 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.DOI: 10.3329/jsr.v2i2.2638 J. Sci. Res. 2 (2), 203-213 (2010)

First-principles investigations of the band structure and optical properties of -boron

Abstract We investigate the electronic structures, optical properties and perform a population analysis of the high-pressure phase of boron (γ-B28) by the norm-conserving pseudopotential density functional method. We employ the Perdew–Burke–Eruzerhof (PBE) form of the generalized gradient approximation (GGA) with plane-wave basis sets to expand the periodic electron density. The dielectric function and optical property constants including the reflectivity, the absorption coefficient, the refractive index, the electron energy-loss function and the optical conductivity are presented in an energy range from 0 to 22 eV. The calculated optical properties indicate that γ -B 28 could be an indirect gap optical material with unique properties. The population analysis suggests that the chemical bonding in γ -B 28 has predominantly covalent character and mixed covalent–ionic character.

First-principles study of the structural and electronic properties of the cubic Zr3 N4 under high pressure

The structural and electronic properties of cubic zirconium nitride (c-Zr3N4) with the Th3P4 structure under high pressure were studied by the local density functional pseudopotential method. The obtained structural data at ambient pressure are in agreement with the experimental values. Analysis of the change of bond lengths of two different types of Zr–N bond with pressure suggests that the octahedral Zr–N is slightly less compressible than the tetrahedral ones. We find that c-Zr3N4 may have a very high hardness similar to that of γ-Si3N4. By fitting the calculated bandgap, the first- and second-order pressure coefficient for the direct bandgap of the c-Zr3N4 were determined to be 3.35 × 10−2 eV/GPa and −1.0 × 10−5 eV/(GPa)2, respectively. Based on the Mulliken population analysis, the c-Zr3N4 was found to have higher and higher covalent character with increasing pressure, and the charge of the N atom is more sensitive to pressure variation than that of the Zr atom.

Density functional calculations of the electronic structure and optical properties of magnesium oxide

The electronic structure and optical properties of magnesium oxide (MgO) are investigated at the structural phase transition pressure using the plane-wave pseudo-potential density functional method within the generalized gradient approximation (GGA). Good agreement between the calculated lattice parameters and experimental results is obtained, and a direct energy gap of 3.72 eV is estimated in the GGA for the NaCl structure (B1) of MgO. The electronic structure of the CsCl-type structure (B2) is predicted near the structural phase transition pressure for the first time and discussed in detail. The dielectric function and optical properties such as reflectivity, absorption coefficient, refractive index, and electron energy-loss function are presented in a wide energy range between 0 and 60 eV. It is found that the reflectivity spectrum and energy-loss spectrum of the B1 phase at 0 GPa are consistent with experimental results.

Structural, elastic, electronic, optical and thermal properties of c-SiGe2N4

We have investigated the structural, elastic, electronic, optical and thermal properties of c-SiGe2N4 by using the ultrasoft pseudopotential density functional method within the generalized gradient approximation. The calculated structural parameters, including the lattice constant, the internal free parameter, the bulk modulus and its pressure derivative are in agreement with the available data. The independent elastic constants and their pressure dependence, calculated using the static finite strain technique, satisfy the requirement of mechanical stability, indicating that c-SiGe2N4 compound could be stable. We derive the shear modulus, Young’s modulus, Poisson’s ratio and Lame’s coefficients for ideal polycrystalline c-SiGe2N4 aggregate in the framework of the Voigt-Reuss-Hill approximation. We estimate the Debye temperature of this compound from the average sound velocity. Band structure, density of states, Mulliken charge populations and pressure coefficients of energy band gaps are investigated. Furthermore, in order to understand the optical properties of c-SiGe2N4, the dielectric function, refractive index, extinction coefficient, optical reflectivity and electron energy loss are calculated for radiation up to 40 eV. Thermal effects on some macroscopic properties of c-SiGe2N4 are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account. We have obtained successfully the variations of the primitive cell volume, volume expansion coefficient, heat capacities and Debye temperature with pressure and temperature in the ranges of 0–40 GPa and 0–2000 K. For the first time, the numerical estimates of the elastic constants and related parameters, and the thermal properties are performed for c-SiGe2N4.

First-principles study of the electronic, optical, and lattice dynamics properties of LiInSe2 polymorph

First-principles calculations of the electronic, optical, and lattice dynamics properties of LiInSe2 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of electronic structure reveal that the different coordinated structure for α-NaFeO2-type LiInSe2 leads to the broadening of density of states and the smaller band gap. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated and are found to be in good agreement with experiments. We also presented phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties, such as constant volume heat capacity and Debye temperature. The results show that the α-NaFeO2-type LiInSe2 does not only have special optical properties but also demonstrates special vibrational properties and thermodynamic properties, which may lead to significance research meaning and application value.

First‐principles calculations on elasticity and the thermodynamic properties of TaC under pressure

AbstractFirst‐principles calculations on the elastic and the thermodynamic properties of TaC have been carried out with the plane‐wave pseudopotential density functional method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The dependence of the elastic constants cij, the aggregate elastic moduli (B, G, E), and the elastic anisotropy on pressure have been investigated. Moreover, the variation of the Poisson ratio and Debye temperature with pressure have been investigated for the first time. Through the quasi‐harmonic Debye model, the thermodynamic properties were also obtained successfully. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic property of Na-doped epitaxial graphenes on SiC

The electronic property of epitaxial graphenes with Na adsorption or intercalation is studied with the use of pseudopotential density functional method. It is found that the charge transfer and the Na binding energy show strong coverage dependence. Calculated energetics shows that Na prefers the intercalation between the buffer and top graphene layers to the adsorption on top graphene layer. The buffer layer is inert to Na adsorption on top graphene layer but it is charged when Na atoms are intercalated. This indicates that the conduction of epitaxial graphenes can be affected significantly by Na intercalation.

First-principles calculations on the electronic structure and optical properties of BaSi2

The electronic structure, densities of states and optical properties of the stable orthorhombic BaSi2 have been calculated using the first-principle density function theory and pseudopotential method. The results show that BaSi2 is an indirect semiconductor with the band gap of 1.086 eV, the valence bands of BaSi2 are mainly composed of Si 3p, 3s and Ba 5d, and the conduction bands are mainly composed of Ba 6s, 5d as well as Si 3p. The static dielectric function ɛ 1(0) is 11.17, the reflectivity n 0 is 3.35, and the biggest peak of the absorption coefficient is 2.15×105 cm−1.

A study on electronic structure and optical properties of Zn1-xBexO

In the paper，electronic structure and optical properties of Zn1-xBexO are calculated by CASTEP program based on density functional theory and plane-wave pseudopotential method. The band gap increases in the range from 0.963 eV to 7.293 eV when Be content changes from 0 to 1. The lattice strain and band repulsion affecting band gap is investigated. The variation of the band gap is dominated by a/b-axis strain for x=0.125，0.25，0.375，0.5，0.625 and 0.75. The variation of the band gap is dominated by c-axis strain for x=0.875 and 1. In the paper，p-d and Γ1v-Γ1c repulsion are used to investigate the top of valence band and the bottom of conduction band，respectively. In addition，we also investigate the imaginary part of the dielectric function，ε2 of Zn1-xBexO.

Self-Assembled Metal Atom Chains on Graphene Nanoribbons

Electronic and magnetic properties of alkali and alkaline-earth metal doped graphene nanoribbons (GNRs) are studied by the pseudopotential density functional method. Strong site dependence is observed in metal adsorption on GNRs, and the adsorbed metal atoms are found to spontaneously form atomic chains in a particular form of GNRs. Such doped GNRs exhibit intriguing magnetic properties such as hysteresis and spin compensation as metal atoms switch from one edge to another at alternating gate voltages. Our study shows that the metal atoms can be used as reagents that can identify the edge atomic structures of GNRs and also as gate-driven spin valves that control the spin current in GNRs.

Quantum-mechanical study of elastic properties of nanoparticles and their agglomeration processes

The response of nanoparticles of different nature (metals, covalent materials, and oxide dielectrics) to tension, compression, and adhesion was studied using the electron density functional theory and pseudopotential methods. It was shown that the elastic modulus of nanoparticles, as a rule, exceeds the corresponding values of a bulk material by several times. Bringing together nanoparticles causes, as a rule, their spontaneous adhesion (agglomeration) accompanied by a noticeable change in the initial shape and electronic structure.

Optimization of metal dispersion in doped graphitic materials for hydrogen storage

The noncovalent hydrogen binding on transition-metal atoms dispersed on carbon clusters and graphene is studied with the use of the pseudopotential density-functional method. It is found that the presence of acceptorlike states in the absorbents is essential for enhancing the metal adsorption strength and for increasing the number of hydrogen molecules attached to the metal atoms. Particular configurations of boron substitutional doping are found to be very efficient for providing such states and thus enhancing storage capacity. Optimal doping conditions are suggested based on our calculations for the binding energy and ratio between metal and hydrogen molecules.

Optical Properties of Metal Free and Metal Phthalocyanine by Molecular Band Calculation

The optical properties of metal-free and metal phthalocyanine were calculated by using density functional theory with various metals including copper, zinc, cobalt, iron and manganese. The polymorphic form of these crystals was employed only for β from. The molecules were optimized with the symmetry of D4h. For the alignments of the molecule in the crystal structures of this polymorphic form which have not been reported in detail, the variation of total energy was examined as a function of the align angles. The align angle at minimum total energy was used for the band calculation. The density functional theory and plane-wave pseudopotential method were used to calculate the energy band structure and electron density of state. The calculated band structures of various metal phthalocyanines can be divided in two groups according to the peak wavelength of the maximum absorption. The first group with the peak wavelength at about 230 nm consists of β-CuPc, β-H2Pc, and β-MnPc while the wavelength of another group for β-CoPc, β-FePc, and β-ZnPc occurs at 350 nm. From the density of state calculation, it indicates that these two transitions originate from the different band and the ratio of the absorption between these states depending on the type of metal in phthalocyanine. The optical absorption was derived to examine the absorption spectra for various metal compositions while the variation in intrinsic electrical conductivity can be estimated from the shape of the band. The phonon and infrared spectra were also determined in order to investigate the vibration mode of molecule in the crystals

Energetics of adsorbed hydrogen and surface germanium on steppedSixGe1−x(100)−(2×1)surfaces

The energetics of adsorbed hydrogen and surface germanium on vicinal silicon-germanium surfaces is important in understanding the growth of a silicon-germanium film at the atomic scale. By using plane wave pseudopotential density functional theory methods, we have calculated the molecular hydrogen adsorption energy at silicon-germanium sites at and near four different types of steps that are important. We have also calculated the relative energies for a germanium atom that is found at various sites in the vicinity of these steps. It is known that adsorbed hydrogen is more likely to be found at step sites than on terraces for pure silicon surfaces. However, our calculations show that the distribution of adsorbed hydrogen is considerably more uniform on silicon-germanium surfaces. Our calculations also show that for clean surfaces, germanium substitution is energetically favored at step sites, while for hydrogenated surfaces, the distribution of surface germanium atoms is likely to be uniform. These results show that the equilibrium distributions of adsorbed hydrogen and surface germanium atoms are correlated to each of their concentrations.

Adsorption Configurations and Reactions of Boric Acid on a TiO2 Anatase (101) Surface

This study investigates the adsorption and reactions of the monomer and dimer of B(OH)3 on a TiO2 anatase (101) surface by first-principles calculations based on the density functional theory and pseudopotential method. On the clean surface, the most stable adsorption structure for B(OH)3 is a molecular monodentate configuration with one hydrogen bonded to a neighboring surface bridging oxygen. The adsorbed B(OH)3 molecule can dissociate into the bidentate adsorption configuration, Ti−OB(OH)O−Ti(a), in which the −OB(OH)O− moiety binds to the surface through two Ti−O bonds with the two dissociated H atoms on neighboring bridged surface oxygen atoms following two successive H migrations. The overall exothermicity is 10.8 kcal/mol; significantly the adsorption energy for Ti−OB(OH)O−Ti(a) with 2 H’s on two O2c surface atoms is 140.1 kcal/mol. In the case of the dimer, there are two identical molecules like the monodentate configuration of B(OH)3 adsorbed on two 5-fold-coordinated Ti atoms of the surface. The ...

First-principles study of the mechanical properties of NiAl microalloyed by M (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd)

Structural, electronic and elastic properties for NiAl with 4d alloying elements M (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd) have been studied using the first-principles pseudopotential density functional method within a generalized gradient approximation. From the elastic constants, C11, C12, C44, bulk modulus B0, Young's modulus E, the shear modulus G, the ratios of shear modulus to bulk modulus G/B0, negative Cauchy pressure parameter (C12 − C44) and Poisson's ratio ν calculated after structural full relaxation, M (Tc, Ru, Rh, Pd) alloying addition in NiAl has been shown to increase the stiffness of NiAl and improve its ductility. The density of states and charge density contour involving alloying additions of Ru were further investigated to clarify the electronic causes of the alloying additions.

Ab initiostudies of structural and electronic properties of the crystallineGe2Sb2Te5

We study the atomic structure and the electronic and optical properties of ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$ in two different crystalline states of cubic and hexagonal structures with the use of ab initio pseudopotential density functional method. It is found that electronic and atomic structures are very sensitive to the layer sequence in the two phases. The proximity of vacancy layer to Ge layer leads to the splitting of Ge-Te bond length, which, in turn, affects the electronic and optical properties. The effect of $\text{Te}\text{ }d$ orbitals is also investigated with respect to structural properties.

Optical properties of rocksalt and zinc blende AlN phases: First-principles calculations

We investigate the electronic structures and optical properties of the wurtzite, zinc blende, and rocksalt AlN phases at their structural phase transition pressures by the ultrasoft pseudopotential density functional method. We employ the Perdew–Burke–Eruzerhof form of the generalized gradient approximation available in the CASTEP code with the plane-wave basis sets to expand the periodic electron density. The dielectric function and optical properties such as reflectivity, absorption coefficient, refractive index, and electron energy-loss function are presented in a wide energy range between 0 and 50eV. Our results reveal that for rocksalt AlN phase, the intensity of N 2p orbitals in the conduction bands obviously decreases when the pressure increases from 15to17GPa. The calculated optical properties indicate that the zinc blende and wurtzite AlN phases have some similar features but the rocksalt AlN phase has other characteristics.

Re-emergent direct–indirect band gap transitions in carbon nanotubes under shear strains

Electronic structures of single-walled carbon nanotubes under varying shear strains are studied with the use of pseudopotential density functional method. We present a new scheme to induce shear strains in one-dimensional materials like nanotubes without introducing artificial edge atoms. It is found that the band gap of semiconducting zigzag nanotubes exhibits reemerging direct–indirect transitions as they are twisted. The breaking of the three-fold rotational symmetry and the electron–hole symmetry of corresponding graphitic band structures under shear is shown to be the origin of the behavior in the band gap.