Cohen Generalized Gradient Approximation Research Articles

Theoretical investigation of band gap and optical properties of ZnO1−xTex alloys (x = 0, 0.25, 0.5, 0.75 and 1)

In this study, the special quasi-random structure (SQS) approach has been considered for structural, electronic and optical properties of rock-salt (RS) and zinc-blende (ZB) phases of ZnO1−xTex (x=0, 0.25, 0.5, 0.75 and 1) using density functional theory. The Wu–Cohen generalized gradient approximation (GGA) has been employed for optimizing the lattice parameters (a0) and bulk moduli (B0) in both phases which show reasonable agreement with numerous theoretical and experimental results. To compute the band gaps with high degree of precision, we employed Engel–Vosko GGA and modified Becke and Johnson local density approximation (mBJLDA) functionals. In the RS phase, metallic nature of the compounds under investigation is evident for 0.25<x<1, whereas direct band gap appears at all concentrations in ZB phase. The density of states (total and partial) are presented to comprehensively analyze the electronic structure of all compounds. Contrary to earlier theoretical studies, the mBJLDA band gap values for the end binaries appear to be significantly improved showing overall better agreement with the experimental data. The optical properties of ZnO1−xTex alloys in the ZB phase are also discussed in terms of dielectric function which show their potential utilization in optoelectronic devices.

Structural, elastic and electronic structure of LiCu2Si, LiCu2Ge and LiAg2Sn intermetallic compounds

Structural, elastic and electronic properties of the intermetallic compounds, LiCu2Si, LiCu2Ge and LiAg2Sn, are investigated using ab initio technique as implemented in the (FP-(L) APW + lo) scheme based on the Wu–Cohen generalized gradient approximation. The computed equilibrium lattice parameters for the compounds are in good agreement with previous theoretical result and experimental data. The elastic constants reveal that the compounds are elastically stable, ductile and also exhibit large elastic anisotropy. The elastic wave velocities in the [100], [110] and [111] directions and their pressure dependence are presented and discussed. A set of isotropic elastic parameters and their related properties, namely bulk and shear moduli, Young’s modulus, hardness parameter, Poisson’s ratio, compressibility, Lame’s constant, average sound velocity, Debye temperature are predicted in the frame work of the Voigt–Reuss–Hill approximation for the polycrystalline aggregates of the compounds. Energy band structure calculations as well as the site and angular momentum projected density of states show that the compounds are metallic with mainly hybridized Cu or Ag d and Si, Ge or Sn p bands crossing the Fermi level.

Half-metallic ferromagnetism in V-doped ZnTe semiconductor at reduced dopant concentration

In this study, we examine the structural, electronic, magnetic and bonding properties of zincblende phase Zn 1-x V x Te (x = 0.0625, 0.125, 0.25) compounds to present them as suitable candidates for spintronic applications. Density functional theory calculations have been used by implementing the accurate full-potential linear-augmented-planewave plus local-orbital method. Structural properties have been computed using Wu–Cohen generalized gradient approximation, whereas the modified Becke and Johnson local (spin) density approximation (mBJLDA) function has been employed for the evaluating ground state electronic properties and ferromagnetic behavior. The half-metallic (HM) ferromagnetism in Zn 1-x V x Te is analyzed in terms of V -3d states and it is shown that mBJLDA predicts wide HM gaps which promise the possibility of achieving V -doped ZnTe with high Curie temperature. The spin exchange splittings Δx(d) and Δx(pd) have been estimated and the contribution of conduction band (CB) and valence band (VB) in exchange splitting is calculated in terms of the exchange constants N0α and N0β. Furthermore, spin-polarized charge density calculation is presented for elucidating the bonding nature, while pressure dependence of total magnetic moment for three concentrations of V -doped ZnTe is also discussed.

Ab-initio study of fundamental properties of ternary ZnO1−xSx alloys by using special quasi-random structures

We employed the special quasi-random structure (SQS) approach to investigate the structural, electronic and optical properties of the binary compounds (ZnO and ZnS) and their alloys ZnO1−xSx (x=0.25, 0.50, 0.75) in the rock salt (RS) and zinc blende (ZB) phases. All the calculations were performed using full-potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the frame work of density functional theory (DFT). The DFT functionals, Wu–Cohen generalized gradient approximation (WC-GGA), Perdew–Burke–Ernzerhof GGA (PBE–GGA) and modified Becke–Johnson GGA (mBJ–GGA) were applied to calculate the exchange–correlation energy and band gap energies. In structural optimization, the predicted results of lattice constant and bulk modulus have shown a non-linear behavior with the selected doping concentrations. From the study of band structures, it has been noticed that the compounds and their alloys under study has metallic behavior in RS phase except at x=0 and 0.25 concentrations (indirect band gaps), whereas semiconducting behavior is obvious in ZB phase with increasing doping order (direct band gaps). The total and partial densities of states (TDOS and PDOS) have been examined to deeply study the electronic properties. Finally, by incorporating the basic optical properties, we discussed the dielectric function, refractive index, optical reflectivity, the absorption coefficient and optical conductivity in terms of incident photon energy up to 20eV.

Structural properties and relative stability of silver-doped gold clusters AgAun−1 (n = 3–13): Density functional calculations

Density functional calculations using Wu and Cohen generalized gradient approximation functional are performed to investigate the structural properties and relative stability of silver doped gold clusters AgAun−1 (n=3–13). Low-lying energy structures include two dimensional and three dimensional geometries. Especially, for the lowest energy structures, the transition from planar to three dimensional structures is found to occur at n=13 and the impurity Ag atom prefers to occupy higher coordination sites. The calculated binding energies, second-order differences in energies, dissociation energies and HOMO–LUMO energy gaps show pronounced odd–even oscillating behaviors, indicating that clusters with even number of atoms keep a higher relative stability than their neighboring odd-numbered ones. Particularly, the cluster AgAu5 shows the strongest stability. Moreover, vertical ionization potential, vertical electronic affinity, and chemical hardness are discussed and compared in depth. The same odd–even oscillations are found.

First principle study of MF2 (M = Mg, Ca, Sr, Ba, Ra) compounds

Dialkali halides have been widely studied because of their large transparency range. The structural, elastic, electronic and optical properties of MF2 (M=Mg, Ca, Sr, Ba, Ra) compounds in fluorite structure have been studied by using self consistent full potential linearized augmented plane wave (FLAPW) method with Wu–Cohen generalized gradient approximation (WC-GGA) and the modified Becke–Johnson potentials. Lattice constant varies inversely to bulk modulus by changing the cation from Mg to Ra in MF2. All the compounds are found to have wide indirect band gap. Refractive index, reflectivity and optical conductivity are calculated to explain the optical nature of these compounds.

Theoretical prediction of the structural, electronic, and thermal properties of Al1−xBxAs ternary alloys

First-principles calculations are performed to study the structural, electronic, and thermal properties of the AlAs and BAs bulk materials and Al1−xBxAs ternary alloys using the full potential-linearized augmented plane wave method within the density functional theory. The structural properties are investigated using the Wu–Cohen generalized gradient approximation that is based on the optimization of total energy. For band structure calculations, both Wu–Cohen generalized gradient approximation and modified Becke-Johnson of the exchange-correlation energy and potential, respectively, are used. The dependence of the lattice constant, bulk modulus, and band gap on the composition x was analyzed. The lattice constant for Al1−xBxAs alloys exhibits a marginal deviation from the Vegard's law. A small deviation of the bulk modulus from linear concentration dependence was observed for these alloys. The composition dependence of the energy band gap was found to be highly nonlinear. Using the approach of Zunger and coworkers, the microscopic origins of the gap bowing were detailed and explained. The quasi-harmonic Debye model was used to determine the thermal properties of alloys up to 500K.

STRUCTURAL, ELECTRONIC, MAGNETIC AND OPTICAL PROPERTIES OF FERROMAGNETIC Pb1-xEuxSe AND Pb1-xEuxTe ALLOYS (x = 0, 0.25, 0.50, 0.75 AND 1)

First-principles total energy calculations have been performed using full potential linear-augmented-plane-wave method within the framework of density functional theory to study the structural, electronic, magnetic and optical properties of the Pb 1-x Eu x Se and Pb 1-x Eu x Te (0 ≤ x ≤1) alloys in the ferromagnetic (FM) ordering. The calculations have been extended to treat the strongly localized f electrons of Eu atom by the self-interaction correction (SIC) approach. For structural optimization, the Wu and Cohen generalized gradient approximation (GGA) functional has been used, whereas for calculating electronic properties, the GGA parameterization scheme formulated by Engel and Vosko (EV) has also been utilized. It has been observed that the use of experimental value of Coulomb parameter (Uf- expt. ) within the SIC does not yield an accurate EuSe and EuTe energy band structure. The improvement in the electronic band structures of nonmagnetic PbSe / PbTe and ferromagnetic EuSe / EuTe have been achieved by considering the effects of spin–orbit coupling for Pb atoms, by a suitable choice of U and by treating the U values for Eu atom's f and d electrons as parameters. The electronic and optical properties of FM Pb 1-x Eu x Se in agreement with experiments can be achieved by combining EV GGA with a Hubbard U &lt; Uf- expt. , however, a stronger and stable AFM coupling in EuTe leaves the above scheme unable to provide good electronic structure of FM Pb 1-x Eu x Te . In case of Pb 1-x Eu x Se the nonlinear behaviour of electronic structure is reflected in the optical properties of Eu -doped PbSe that have been studied in terms of incident photons' energy dependent complex dielectric function.

Study of half-metallic ferromagnetism in V-doped CdTe alloys by using first-principles calculations

For investigating the structural, electronic and magnetic properties of zincblende (ZB) Cd1−xVxTe (0≤x≤1), we have employed the Wu–Cohen generalized gradient approximation (WC-GGA) and the modified Becke and Johnson local density approximation (mBJLDA) functionals within the frame-work of spin polarized density functional theory (DFT). The former exchange-correlation parameterization scheme has been used for optimizing the equilibrium structural properties, while the electronic band structures, electron density of states (DOS) and charge densities are computed using both functionals and their performances are compared. Our results show that the electron spin polarization in unfilled V-3d orbitals gives rise to spin exchange splittings (Δx(d) and Δx(pd)) which is the cause of half metallic (HM) ferromagnetism in the V-doped CdTe as revealed by the computed DOS. Consequently, the nature of effective potential is more attractive in spin-down case rather than that in spin-up case. The total magnetic moment for each of the compounds under study is 3μB where the main contribution comes from the V atom, while the nonmagnetic sites Cd and Te are also bestowed with minor atomic magnetic moments due to Te-5p–V-3d hybridization. Furthermore, we calculated the exchange constants N0α and N0β to determine the conduction and valence band contributions in exchange splitting process. A comparison of the two functionals, considered in this work, shows that the mBJLDA provides a better description of the electronic structure especially of the ferromagnetic Cd1−xVxTe (0.75≤x≤1). Moreover, as compared to WC-GGA, the mBJLDA predicts high Curie temperatures in V-doped CdTe by providing significantly larger values of Δx(d), Δx(pd), N0α, N0β and, importantly, a quite wide HM gap.

Density functional investigation of structural and electronic properties of small bimetallic silver–gold clusters

Structural and electronic properties of bimetallic silver–gold clusters up to eight atoms are investigated by the density functional theory using Wu and Cohen generalized gradient approximation functional. By substitution of Ag and Au atoms, in the optimized lowest energy structures of pure gold and silver clusters, we determine the ground state conformations of the bimetallic silver–gold ones. We reveal that Ag atoms prefer internal positions whereas Au atoms prefer exposed ones favoring charge transfer from Ag to Au atoms. For each size and composition, binding energy, HOMO–LUMO gap, magnetic moment, vertical ionization potential, electron affinity and chemical hardness were calculated. On increasing the size of the cluster by varying number of Ag atoms with fixed number of Au ones, vertical ionization potential and electron affinity show obvious odd–even oscillations consistent with the pure Ag and Au clusters. Au atoms inclusion in the cluster increases the binding energy and vertical ionization potential, indicating higher stability as the number of Au atoms grows. The variation of chemical hardness with the composition in a cluster with the same size shows peaks when the number of Ag atoms is greater than or equal to Au ones, corresponding to transition from planar to tri-dimensional structures. For clusters with even number of atoms, the peaks indicate that the clusters with the same number of Ag and Au atoms are the most stable ones. Analyzing the density of states, we found that increasing the concentration of Ag atoms affects the energy separation between the HOMO and the low lying occupied states.

Ab initio study of structural, electronic and optical properties of Be-doped CdS, CdSe and CdTe compounds

Full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within density functional theory (DFT) has been utilized to calculate structural, electronic and optical properties of Be-doped CdS, CdSe and CdTe compounds with the dopant concentration x in the range 0≤x≤1. For the contribution of exchange-correlation potential, we used Wu–Cohen generalized gradient approximation (GGA) to calculate structural parameters, whereas both Wu–Cohen and Engel–Vosko GGA have been applied to calculate electronic structure of the materials. Only a slight deviation from Vegard's law has been observed for the calculated lattice constants and bulk moduli of the alloys. Structural and chemical factors that affect the band-gap bowing of these semiconductor alloys have been estimated and discussed. Density of states curves and charge density maps have been calculated and analyzed. Lastly, optical properties of both binary and their related ternary alloys have been discussed in terms of the calculated dielectric function. The resultant optical parameters are compared with the available experimental data and other calculations.

Evidence of Coulomb correction and spin–orbit coupling in rare-earth dioxides CeO2, PrO2 and TbO2: An ab initio study

The current study investigates the structural, elastic, electronic and optical properties of CeO2, PrO2 and TbO2 using the full potential (linearized) augmented plane wave plus local orbital method within the Wu–Cohen generalized gradient approximation (GGA) with Hubbard (U) correction and spin–orbit coupling (SOC). The GGA+U implementation lead us to describe correctly the relativistic effect on 4f electrons for CeO2. We clarify that the inclusion of the Hubbard U parameter and the spin–orbit coupling are responsible for the ferromagnetic insulating of PrO2 and TbO2. The magnetic description is achieved by the spin-density contours and magnetic moment calculations, where we show the polarization of oxygen atoms from the rare earth atoms. The mechanical stability is shown via the elastic constants calculations. The optical properties, namely the dielectric function and the reflectivity are calculated for radiation up to 12eV, giving interesting optoelectronic properties to these dioxides.

Magnetic ordering and electronic properties of Pd‐doped ZnO

AbstractWe carried out first‐principles spin‐polarized calculations in order to provide comprehensive information regarding the structural, electronic and magnetic properties of the PdxZn1–xO (with x = 0.0625) semiconductor. The highly accurate all‐electron full‐potential linearized augmented plane wave method was used to calculate total energy and electronic structure of the Pd‐doped ZnO semiconductor. The recently developed Wu–Cohen generalized gradient approximation to the exchange correlation functional was adopted. The results indicate that the ferromagnetic ground state originates from the strong hybridization between Pd‐4d and O‐2p states, which is in agreement with previous studies on 4d doping in wide gap semiconductors. This article shows that 4d transition metals such as palladium may also be considered as candidates to explore new half‐metallic ferromagnetism in semiconductors. This study gives new clues to the fabrication of diluted magnetic semiconductors.

Ab-initio study of the structural, linear and nonlinear optical properties of CdAl 2Se 4 defect-chalcopyrite

The complex density functional theory (DFT) calculations of structural, electronic, linear and nonlinear optical properties for the defect chalcopyrite CdAl 2Se 4 compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code. We employed the Wu and Cohen generalized gradient approximation (GGA-WC), which is based on exchange–correlation energy optimization to calculate the total energy. Also we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band structure, density of states and the spectral features of the linear and nonlinear optical properties. This compound has a wide direct energy band gap of about 2.927 eV with both the valence band maximum and conduction band minimum located at the center of the Brillouin zone. The ground state quantities such as lattice parameters ( a, c, x, y and z), bulk modulus B and its pressure derivative B′ are evaluated. We have calculated the frequency-dependent complex ε ( ω ) , its zero-frequency limit ε 1 ( 0 ) , refractive index n ( ω ) , birefringence Δ n ( ω ) , the reflectivity R ( ω ) and electron energy loss function L ( ω ) . Calculations are reported for the frequency-dependent complex second-order nonlinear optical susceptibilities. We find opposite signs of the contributions of the 2 ω and 1 ω inter/intra-band to the imaginary part for the dominant component through the wide optical frequency range.

Pressure induced phase transitions inPbTiO3

Recent theoretical simulations using density functional theory (DFT) andnovel low temperature high energy x-ray diffraction experiments clearly showthe existence of a high pressure morphotropic phase boundary (MPB) in purePbTiO3.The experiments show a richer phase diagram than the simulations, with multiple monoclinic phases(Pm and Cm) in the MPB region. In this paper we examine the MPB region in more detail usinghigh precision DFT calculations within the local-density approximation (LDA)and the Wu–Cohen generalized gradient approximation. Our results support thepolarization rotation theory and open up fresh possibilities for applying chemicalpressure to engineer novel electromechanical materials. We also explain why thezone-boundary mode is more likely to be stable only at higher pressures above∼25 GPa and not atmoderate pressures of ∼10 GPa, using the LDA.