Engel Vosko Generalized Gradient Approximation Formalism Research Articles

Two symmetric n-type interfaces SrTiO3/LaAlO3 in perovskite: Electronic properties from density functional theory

The first principles study of the (001) two symmetric n-type interfaces between two insulating perovskites, the nonpolar SrTiO3 (STO), and the polar LaAlO3 (LAO) was performed. We have analyzed the formation of metallic interface states between the STO and LAO heterointerfaces by using the all-electron full-potential linearized augmented plane-wave approach based on the density functional theory, within the local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), and the Engel-Vosko GGA (EVGGA) formalism. It has been found that some bands cross the Fermi energy level (EF), forming a metallic nature of two symmetric n-type 6.5STO/1.5LAO interfaces with density of states at EF, N(EF) of about 3.56 (state/eV/unit cell), and bare electronic specific heat coefficient (γ) of about 0.62 mJ/(mol cell K2). The electronic band stature and the partial density of states in the vicinity of EF are mainly originated from Ti1,2,3,4-3dxy orbitals. These bands are responsible for the metallic behavior and the forming of the Fermi surface of the two symmetric n-type 6.5STO/1.5LAO interfaces. To obtain a clear map of the valence band electronic charge density distribution of the two symmetric n-type 6.5STO/1.5LAO interfaces, we have investigated the bond's nature and the interactions between the atoms. It reveals that the charge is attracted towards O atoms as it is clear that the O atoms are surrounded by uniform blue spheres which indicate the maximum charge accumulation.

First-principles study of the electronic structure, charge density, Fermi surface and optical properties of zintl phases compounds Sr2ZnA2 (A=P, As and Sb)

We present first-principles calculations of the electronic structure, Fermi surface, electronic charge density and optical properties of Sr2ZnA2 (A=P, As and Sb) based on density-functional theory using the local density approximation (LDA), generalized-gradient approximation (GGA) and the Engel–Vosko GGA formalism (EV-GGA). Additionally, modified Becke–Johnson (mBJ) is also used to improve the band splitting results. The calculated band structure and density of states show that Sr2ZnA2 compounds are metallic. The total DOS at Fermi level N(EF) is 72.92, 73.06 and 33.47 states/eV and the bare electronic specific heat coefficient (γ) is 12.64, 5.805 and 12.67mJ/mol-K2 for Sr2ZnP2, Sr2ZnAs2 and Sr2ZnSb2, respectively. The Fermi surface of Sr2ZnA2 compounds is composed of two bands crossing along the Γ−A direction of Brillouin zone. There exists a strong hybridization between Zn-p/s and Sb-d, Sb-p and Sr-d and also between Sr-s and Sr-p states. The bonding features are analyzed by using the electronic charge density contour in the (101) crystallographic plane. We found that Sr forms an ionic bond with Zn, whereas Zn forms a strong covalent interaction with P/As/Sb atoms. For further insight information about the electronic structure, the optical properties are derived and analyzed.

Ab initio study of structural, electronic and thermodynamic properties of tungstate double perovskites Ba2MWO6 (M = Mg, Ni, Zn)

The structural and electronic properties of the double perovskite Ba2MWO6 with M=Mg, Ni, Zn have been studied using the full-potential linearized augmented plane wave (FP-LAPW) method by employing both the local density approximation (LDA) and the generalized gradient approximation (GGA), which are 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 calculations. However, we have evaluated the ground state quantities such as lattice parameter, bulk modulus and its pressure derivative. Also, we have presented the results of the band structures and densities of states. These results were in favorable agreement with previous theoretical works and the existing experimental data. To complete the fundamental characteristics of these compounds we have analyzed the thermodynamic properties such as thermal expansion coefficient, heat capacities and other structural parameters in the whole pressure range from 0 to 20GPa and temperature range from 0 to 1000K.

First principal studya of structural, electronic and thermodynamic properties of KTaO3-perovskite.

1 ,R.Ascri 1 , L.Ghalouci Abstract. The results of first-principles theoretical study of structural, elastic, electronic and thermodynamic properties of KTaO3 compound, have been performed using the full-potential linear augmented plane-wave method plus local orbitals (FP-APW+lo) as implemented in the Wien2k code. The exchange-correlation energy, is treated in generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE96) and PBEsol, Perdew 2008 parameterization. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. The calculated equilibrium parameter is in good agreement with other works. The elastic constants were calculated by using the Mehl method. The electronic band structure of this compound has been calculated using the Angel-Vosko (EV) generalized gradient approximation (GGA) for the exchange correlation potential. We deduced that KTaO3-perovskite exhibit an indirect from R topoint. To complete the fundamental characterization of KTaO3 material we have analyzed the thermodynamic properties using the quasi-harmonic Debye model.

AB INITIO STUDY OF STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF MgxCd1-xX (X = S, Se, Te) ALLOYS

This study describes structural, electronic and optical properties of Mg x Cd 1-x X (X = S, Se, Te) alloys in the complete range 0≤x ≤1 of composition x in the zinc-blende (ZB) phase with the help of full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within density functional theory (DFT). In order to calculate total energy, generalized gradient approximation (Wu–Cohen GGA) has been applied, which is based on optimization energy. For electronic structure calculations, the corresponding potential is being optimized by Engel–Vosko GGA formalism. Our calculations reveal the nonlinear variation of lattice constant and bulk modulus with different concentration for the end binary and their ternary alloys, which slightly deviates from Vegard's law. The calculated band structures show a direct band gap for all three alloys with increasing order in the complete range of the compositional parameter x. In addition, we have discussed the disorder parameter (gap bowing) and concluded that the total band gap bowing is substantially influenced by the chemical (electronegativity) contribution. The calculated density of states (DOS) of these alloys is discussed in terms of contribution from various s-, p- and d-states of the constituent atoms and charge density distributions plots are analyzed. Optical properties have been presented in the form of the complex dielectric function ε(ω), refractive index n(ω) and extinction coefficient k(ω) as function of the incident photon energy, and the results have been compared with existing experimental data and other theoretical calculations.

Theoretical study of structural, electronic and thermal properties of Zn1−xBexS ternary alloy

The structural, the electronic and the thermal properties of Zn1−xBexS ternary alloy have been calculated using the full-potential linearized-augmented plane wave (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) parameterization. Moreover, the Engel–Vosko GGA formalism is also applied to optimize the corresponding potential for band structure calculations. The ground state properties such as lattice constants and bulk modulus are in good agreement with numerous experimental and theoretical data. The thermodynamic stability of this alloy was explained by calculating the phase diagram. The quasi harmonic Debye model, using a set of total energy versus volume calculations obtained with FP-LAPW method is applied to study the thermal and vibrational affects. Temperature effect on the lattice parameter, thermal expansion coefficient, heat capacity and Debye temperature is determined from the non-equilibrium Gibbs function.

Ab initio study of the structural, electronic and thermodynamic properties of PbSe1−xSx, PbSe1−xTex and PbS1−xTex ternary alloys

The structural, electronic and thermodynamic properties of PbSe1−xSx, PbSe1−xTex and PbS1−xTex ternary alloys have been calculated using the full-potential linearized-augmented plane wave method. The exchange and correlation potential is treated by the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof parameterization. Moreover, the Engel–Vosko GGA formalism is also applied to optimize the corresponding potential for band structure calculations. A nonlinear dependence of the effect of the concentration (x) on the lattice constants, bulk modulus and band gaps is found. The microscopic origins of the band gap bowing parameter have been discussed. Moreover, the thermodynamic stability of the studied alloys is investigated by means of the miscibility critical temperature.

Calculated optical properties of GaX (X=P, As, Sb) under hydrostatic pressure

The hydrostatic pressure dependence of the principal energy gaps and of the optical properties of GaX (X = P, As and Sb) has been calculated using the full potential-linearized augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) for the exchange and correlation potential is applied. Also, we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band-structure and the optical properties calculations. Structural properties such as equilibrium lattice constants, the bulk modulus, and its pressure derivatives were calculated for GaP, GaAs, and GaSb in the zinc-blende structure (ZB). We have found that the results of the structural properties calculations are in agreement with those of ab initio and experimental data. In general, the pressure dependence of the principal energy gaps is compared to other values. The same is for the pressure coefficient. However, for the same structure, the comparison of our results with those of experimental and theoretical calculations shows good agreement. On the other hand, the effect of the applied pressure is clearly seen in the optical properties especially near the energy transition regions.

Full potential study of the elastic, electronic, and optical properties of spinels MgIn 2S 4 and CdIn 2S 4 under pressure effect

The structural, elastic, electronic, and optical properties of cubic spinel MgIn 2S 4 and CdIn 2S 4 compounds have been calculated using a full relativistic version of the full-potential linearized-augmented plane wave with the mixed basis FP/APW+lo method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA). Moreover, the Engel–Vosko GGA formalism is also applied to optimize the corresponding potential for band structure calculations. The ground state properties, including the lattice constants, the internal parameter, the bulk modulus, and the pressure derivative of the bulk modulus are in reasonable agreement with the available data. Using the total energy-strain technique, we have determined the full set of first-order elastic constants C ij and their pressure dependence, which have not been calculated or measured yet. The shear modulus, Young’s modulus, and Poisson’s ratio are calculated for polycrystalline XIn 2S 4 aggregates. The Debye temperature is estimated from the average sound velocity. Electronic band structures show a direct band gap (Г–Г) for MgIn 2S 4 and an indirect band gap (K–Г) for CdIn 2S 4. The calculated band gaps with EVGGA show a significant improvement over the GGA. The optical constants, including the dielectric function ε( ω), the refractive index n( ω), the reflectivity R( ω), and the energy loss function L( ω) were calculated for radiation up to 30 eV.

Ab-initio calculation of structural, electronic, and optical characterizations of the intermetallic trialuminides ScAl 3 compound

We present first-principles study of the electronic and the optical properties for the intermetallic trialuminides ScAl 3 compound using the full-potential linear augmented plane wave method within density-functional theory. We have employed the generalized gradient approximation (GGA), 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 calculating the electronic band structure and optical properties. The electronic specific heat coefficient ( γ), which is a function of density of states, can be calculated from the density of states at Fermi energy N( E F). The N( E F) of the phase L1 2 is found to be lower than that of D0 22 structure which confirms the stability of L1 2 structure. We found that the dispersion of the band structure of D0 22 is denser than L1 2 phase. The linear optical properties were calculated. The evaluations are based on calculations of the energy band structure.

First principles calculations of structural, electronic and optical properties of BAs1−xPx alloy

We present first principles calculations of the structural, electronic and optical properties of boron ternary alloy BAs1−xPx, using a hybrid full-potential (linear) augmented plane wave plus local orbitals (L/APW+l0) method within the density-functional theory (DFT). The generalized gradient approximation (GGA) was used as well as the Engel-Vosko GGA formalism to calculate the band gap. We investigated the effect of composition on lattice constants, bulk modulus and band gap. Deviations of the lattice constants from Vegard’s law and the bulk modulus from linear concentration dependence (LCD) were observed for the alloy. Using the approach of Zunger and co-workers, the microscopic origins of the gap bowing were explained. For the optical properties, the compositional dependence of the refractive index and the dielectric constant was studied.

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.

Optical properties of alkaline-earth metal oxides from first principles

This study reports the results of an ab initio electronic and optical calculation of alkaline-earth metal oxides (MgO, CaO, SrO and BaO) in the NaCl crystal structure using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory. The exchange-correlation potential is treated by the generalized gradient approximation within the Perdew et al scheme. Moreover, the Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The real and imaginary parts of the dielectric function ɛ( ω), the optical absorption coefficient I( ω), the reflectivity R( ω) and the energy loss function are calculated by random phase approximation (RPA). The calculated results show a qualitative agreement with the available experimental results in the sense that we can recognize some peaks qualitatively, those due to single particle transitions. Furthermore the interband transitions responsible for the structures in the spectra are specified. It is shown that the oxygen 2 p states and metal d states play the major role in optical transitions as initial and final states respectively. The effect of the spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low energy region. The dielectric constants are calculated and compared with the available theoretical and experimental results.

X-ray Photoelectron Spectrum and Electronic Properties of a Noncentrosymmetric Chalcopyrite Compound HgGa2S4: LDA, GGA, and EV-GGA

An all electron full potential linearized augmented plane wave method has been applied for a theoretical study of the band structure, density of states, and electron charge density of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4) using three different approximations for the exchange correlation potential. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma resulting in a direct energy gap of about 2.0, 2.2, and 2.8 eV for local density approximation (LDA), generalized gradient approximation (GGA), and Engel-Vosko (EVGGA) compared to the experimental value of 2.84 eV. We notice that EVGGA shows excellent agreement with the experimental data. This agreement is attributed to the fact that the Engel-Vosko GGA formalism optimizes the corresponding potential for band structure calculations. We make a detailed comparison of the density of states deduced from the X-ray photoelectron spectra with our calculations. We find that there is a strong covalent bond between the Hg and S atoms and Ga and S atoms. The Hg-Hg, Ga-Ga, and S-S bonds are found to be weaker than the Hg-S and Ga-S bonds showing that a covalent bond exists between Hg and S atoms and Ga and S atoms.

FP-APW+lo calculations of the electronic and optical properties of alkali metal sulfides underpressure

The electronic and optical properties ofM2S (M = Li,Na, K and Rb) compounds in the cubic antifluorite structure have been calculated, using afull relativistic version of the full-potential augmented plane-wave plus local orbitalsmethod based on density functional theory, within both the local density approximation(LDA) and the generalized gradient approximation (GGA). Moreover, the Engel–VoskoGGA formalism (EV-GGA) is applied so as to optimize the corresponding potential forband structure calculations. The calculated equilibrium lattices and bulk moduli arein good agreement with the available data. Band structure, density of states,electron charge density and pressure coefficients of energy gaps are given. Resultsobtained for band structure using EV-GGA are larger than those with LDA andGGA. It is found that the spin–orbit coupling lifts the triple degeneracy at theΓ point and the double degeneracy at the X point. The analysis of the electron charge densityshows that the M–S bonds have a significant ionic character. The complex dielectric functionsε2(ω) for alkali metal sulfides were calculated for radiation up to 30 eV and the assignment of thecritical points to the band structure energy differences at various points of the Brillouinzone was made. The pressure and volume dependence of the static dielectric constant andthe refractive index are calculated.

First-principles study of the electronic and optical properties of rutile TiO2

Abstract The electronic, structural properties and optical properties of the rutile TiO2 have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), 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 calculations. Our results including lattice parameter, bulk modulus, density of states, the reflectivity spectra, the refractive index and band gap are compared with the experimental data. We present calculations of the frequency-dependent complex dielectric function e ( ω ) and its zero-frequency limit e 1 ( 0 ) .

Phase transition of LaX (X = P, As, Sb and Bi) at high pressure: Theoretical investigation of the structural and electronic properties

The electronic and structural properties of the Lanthanum pnictides LaX (X = P, As, Sb and Bi) in the two phases: rock-salt (B1), and primitive tetragonal (PT) have been studied using the full-potential linearized augmented plane wave (FP-LAPW) method. We used both the local-density approximation (LDA) and the generalized gradient approximation (GGA), 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 calculations. We notice that below 19 GPa and around 17 GPa, 9 GPa and 8 GPa, respectively, LaP, LaAs, LaSb and LaBi maintain the rock-salt structure (B1). At high pressure there occurs a crystallographic phase transition. In all compounds, the high pressure phases are tetragonal structure. The ground state properties of these compounds such as equilibrium lattice constant, bulk modulus are calculated and compared with available literature. The calculated band structure for B1 phase of LaX (X = P, As, Sb and Bi), using EVGGA, show that these compounds are semiconductors with an indirect band gap ( Γ → X ) of about 0.56 eV, 0.43 eV, 0.15 eV and 0.07 eV, respectively. We note that for the same phase (B1) the LDA leads to close the gap at Fermi energy making all compounds as metallic in agreement with the previous calculation. For PT phase both LDA and EVGGA show these compounds as metallic.

X-ray photoelectron spectra and the electronic band structure for non-centrosymmetric Bi 2ZnB 2O 7 nonlinear single crystal

We have synthesized and characterized the bismuth borate Bi 2ZnB 2O 7 single crystals. The results of the experimental measurements of the X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS) along with an ab initio theoretical study of the electronic band structure, and density of states, are reported. The theoretical calculations are based on crystal structure built from our experimentally determined atomic parameters. The theoretical calculations have been done by using the full potential linearized augmented plane wave (FP-LAPW) method. We applied the Perdew–Burke–Ernzerhof generalized gradient approximation (GGA) and the Engel–Vosko GGA formalism. The observed XPS pattern is in good agreement with the theoretical one, confirming the pure phase of Bi 2ZnB 2O 7. The purity and composition of the as-prepared sample are studied by XPS analysis. XPS measurements show that the bismuth possesses +3 valence state, and Zn and B species exist only in the traditional valence states of Zn 2+ and B 3+. The O 1s spectra are broad and asymmetric and can be deconvoluted into two peaks. The lower and higher binding energy peaks (around 530.1 and 531.5 eV) are assigned to originate from O 1s(1) and O 1s(2) terms, respectively, which are attributed to non-bridging oxygen (B–O − bonds) and bridging oxygen (B–O–B bonds), respectively. Our calculations show that Bi 2ZnB 2O 7 has an indirect energy gap and that Bi–O bonds are of strong covalent character.

Structural, electronic and optical properties of SrCl2 under hydrostatic stress

The results of first-principles theoretical study of the structural, electronic and optical properties of SrCl2 in its cubic structure, have been performed using the full-potential linear augmented plane-wave method plus local orbitals (FP-APW+lo) as implemented in the WIEN2k code. In this approach both the local density approximation (LDA) and the generalized gradient approximation (GGA) are used for the exchange-correlation (XC) potential. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. We performed these calculations with and without spin-orbit interactions. Including spin-orbit coupling cause to lifts the triple degeneracy at Γ point and a double degeneracy at X point. Results are given for structural properties. The pressure dependence of elastic constants and band gaps are investigated. The dielectric function, reflectivity spectra and refractive index are calculated up to 30 eV. Also we calculated the pressure and volume dependence of the static optical dielectric constant.

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), 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 calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε ( ω ) and its zero-frequency limit ε 1 (0). We find that the value of ε 1 (0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.