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

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 ε(ω) and its zero-frequency limit ε₁(0).     

First-principles study of the optical properties of PbFX (X = Cl,Br, I) compounds in its matlockite-type structure

We present the results of the ab initio theoretical study of the optical properties for PbFX (X = Cl, Br, I) compounds in its matlockite-type structure using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed 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 Z resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε( ω) and its zero-frequency limit ε₁ ( 0 ). We find that the values of ε₁ ( 0 ) increases with decreasing the energy gap. The reflectivity spectra and absorption coefficient has been calculated and compared with the available experimental data. The optical properties are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure.     

Structural, electronic and optical properties of AgI under pressure

We report results of first-principles total-energy calculations for structural properties of the group I-VII silver iodide (AgI) semiconductor compound under pressure for B1 (rocksalt), B2 (cesium chloride), B3 (zinc-blende) and B4 (wurtzite) structures. Calculations have been performed using all-electron full-potential linearized augmented plane wave plus local orbitals FP-LAPW + lo method based on density-functional theory (DFT) and using generalised gradient approximation (GGA) for the purpose of exchange correlation energy functional. In agreement with experimental and earlier ab initio calculations, we find that the B3 phase is slightly lower in energy than the B4 phase, and it transforms to B1 structure at 4.19 GPa. Moreover, we found AgI has direct gap in B3 structure with a band gap of 1.378 eV and indirect band gap in B1 phase with a bandgap around 0.710 eV. We also present results of the effective masses for the electrons in the conduction band (CB) and the holes in the valence band (VB). To complete the fundamental characteristics of this compound we have analyzed their linear optical properties such as the dynamic dielectric function and energy loss function for a wide range of 0-25 eV.     

Theoretical study of CuxAg1−xI alloys

We have performed first-principles calculations using full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT) to investigate the fundamental properties of Cu_xAg_1−xI alloys. We used both GGA96 [J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.] and EVGGA [E. Engel, S.H. Vosko, Phys. Rev. B. 47 (1993) 13164.] generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential. Quantities such as lattice constants, bulk modulus, band gap, density of occupied states and effective mass were calculated as a function of copper molar fraction x. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter, which follows Vegard's law. The microscopic origins of the gap bowing were explained using the approach of Zunger and co-workers; we have concluded that the band-gap energy bowing was mainly caused by the chemical charge-transfer effect and the volume deformation , while the structural relaxation contribute to the gap bowing parameter at smaller magnitude. The calculated phase diagram shows a broad miscibility gap for this alloy with a high critical temperature.     

First-principles calculations of structural, elastic, electronic and optical properties of the antiperovskite AsNMg3

The density functional theory (DFT) calculations of structural, elastic, electronic and optical properties of the cubic antiperovskite AsNMg₃ has been reported using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). The equilibrium lattice, bulk modulus and its pressure derivative have been determined. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus and Poisson's ratio for ideal polycrystalline AsNMg₃ aggregate. We estimated the Debye temperature of AsNMg₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of AsNMg₃ compound, and it still awaits experimental confirmation. Band structure, density of states and pressure coefficients of energy gaps are also given. The fundamental band gap (Γ-Γ) initially increases up to 4 GPa and then decreases as a function of pressure. Furthermore, the dielectric function, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. The all results are compared with the available theoretical and experimental data.     

Improved calculation of band gap of Sr2Bi2O5 crystal using modified Becke–Johnson exchange potential

The electronic structure of Sr₂Bi₂O₅ is calculated by the scalar-relativistic full potential linearized augmented plane wave (FLAPW+lo) method using the modified Becke–Johnson potential combined with the local density approximation correlation (MBJ–LDA). Both the valence band maximum (VBM) and conduction band minimum (CBM) exist at the Γ-point, indicating that Sr₂Bi₂O₅ is a direct-band-gap material. The band gap is calculated to be 3.17 eV, which is very close to the experimental value. This result is in great contrast to the underestimation based on the GGA calculation. On the other hand, there is only a small difference in the effective masses of holes and electrons photogenerated near the VBM and CBM for the MBJ–LDA and GGA approaches. The optical properties of Sr₂Bi₂O₅ are calculated from the complex dielectric function ε(ω)=ε₁(ω)+iε₂(ω). A highly polarized peak is observed at 3.5 eV in the ε₂(ω) function. Furthermore, the absorption coefficient estimated from the MBJ–LDA is very similar to that from the experimental result.     

The optical properties of HgS under high pressures

The DOS (density of states) and the optical properties of HgS under high pressure are studied with the first-principle computations. The change of the imaginary part, ε₂(ω), of the dielectric function shows that HgS tends to metallization with increasing pressure, and this well agrees with the band gap calculations and the conductivities measurement results in the previous work. Under the pressures below 15 GPa, ε₂(ω) is relatively anisotropic and tends to be more anisotropic with increasing pressure; while under the pressures above 15 GPa, the anisotropy decreases and finally becomes almost absolutely isotropic after the phase transition. The behavior of ε₂(ω) is strongly related to the structure change in the cinnabar to rocksalt phase transition process under high pressure.     

Structural, electronic, elastic and optical properties of fluoro-perovskite KZnF3

We determine the structural, electronic, elastic and optical properties of fluoro-perovskite KZnF₃ using the full potential linear augmented plane wave approach (FP-LAPW) based on the density functional theory (DFT). The exchange-correlation potential is treated by the local density approximation (LDA) and the generalized gradient approximation (GGA). The calculated structural parameters are in good agreement with the available data. We have obtained an indirect band gap. The effect of the pressure on the band gaps is investigated. We evaluate the elastic constants (C_ij), elastic moduli and the Debye temperature. The imaginary and the real parts of the dielectric function ε(ω) and some optical constants are also calculated.     

First principles calculations of structural,electronic and optical properties of various phases of CaS

First principles calculations have been performed within the framework of density functional theory to investigate the structural, electronic and optical properties of all four possible B1, B2, B3 and B4 phases of CaS. Apart from the standard local density approximation (LDA) and GGA (PBE), a more accurate nonempirical density functional generalized gradient approximation (GGA), as proposed by Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] for the exchange-correlation energy, E_XC, has been attempted in these calculations. Calculated electronic structure and the density of states are analyzed in terms of the contribution of Ca d states and S s and p states in determining the nature of the fundamental band gap in various phases. Reflectivity, R (ω), the real and imaginary part of the dielectric functions, ε(ω), have been calculated for all the phases and the results have been discussed and compared with the existing experimental data.     

Theoretical investigation of the conduction and valence band offsets of GaAs1−x Nx/GaAs1−yNy heterointerfaces

Theoretical investigations of the conduction band offset (CBO) and valence band offset (VBO) of the relaxed and pseudo-morphically strained GaAs_1−xN_x/GaAs_1−yN_y heterointerfaces at various nitrogen concentrations (x and y) within the range 0-0.05 and along the [0 0 1] direction are performed by means of the model-solid theory combined with the empirical pseudopotential method under the virtual crystal approximation that takes into account the effects of the compositional disorder. It has been found that for y < x, the CBO and VBO have negative and positive signs, respectively, whereas the reverse is seen when y > x. The band gap of the GaAs_1−xN_x over layer falls completely inside the band gap of the substrate GaAs_1−yN_y and thus the alignment is of type I (straddling) for y < x. When y > x, the alignment remains of type I but in this case it is the band gap of the substrate GaAs_1−yN_y which is fully inside the band gap of the GaAs_1−xN_x over layer. Besides the CBO, the VBO and the relaxed/strained band gap of two particular cases: GaAs_1−xN_x/GaAs and GaAs_1−xN_x/GaAs_0.98N_0.02 heterointerfaces have been determined.     

A better ferrimagnetic half-metal LuCu3Mn4O12: Predicted from first-principles investigation

Electronic structure calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) and GGA+U for manganite cuprate compound LuCu₃Mn₄O₁₂ have been performed, using the full-potential linearized augmented plane wave method. The calculated results indicate that LuCu₃Mn₄O₁₂ is ferrimagnetic and half-metallic in both GGA and GGA+U calculations. The minority-spin band gap is 0.7 eV within GGA, which is larger than that of LaCu₃Mn₄O₁₂ (0.3 eV), indicating its better half-metallicity. Further, the minority-spin gap enlarges from 0.7 to 2.8 eV with U taken into account, and simultaneously the Fermi level being shifted to the middle of the gap, making the half-metallic energy gap to be 1.21 eV. These results demonstrate that electronic correlation effect enhances the stability of half-metallic property. These facts make this system interesting candidates for applications in spintronic devices.     

Electronic structure calculations of lead chalcogenides PbS, PbSe, PbTe

In this work, we have extended our study of the mechanical properties and the electronic structure of PbTe to include other Pb chalcogenide compounds (PbSe, PbS). The calculations were performed self-consistently using the scalar-relativistic full-potential linearized augmented plane wave method. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) to density-functional theory were applied.The equilibrium lattice constants and the bulk modulus of a number of structures (NaCl, CsCl, ZnS) were calculated as well as the elastic constants for the structures (NaCl, CsCl). The NaCl structure is found to be the most stable one among all the three phases considered. We have found that the GGA predicts the elastic constants in good agreement with experimental data.Both the LDA and GGA were successful in predicting the location of the band gap at the L point of the Brillouin zone but they are inconclusive regarding the value of the band-gap width. To resolve the issue of the gap, we performed Slater-Koster (SK) tight-binding calculations, including the spin-orbit coupling in the SK Hamiltonian. The SK results that are based on our GGA calculations give the best agreement with experiment.Results are reported for the pressure dependence of the energy gap of these compounds in the NaCl structure. The pressure variation of the energy gap indicates a transition to a metallic phase at high pressure. Band structure calculations in the CsCl structure show a metallic state for all compounds. The electronic band structure in the ZnS phase shows an indirect band gap at the W and X point of the Brillouin zone.     

Structural, electronic and optical calculations of CaxZn1−xO alloys: A first principles study

First principles density functional calculations, using full potential linearized augmented plane wave (FP-LAPW) method, have been performed in order to investigate the structural, electronic and optical properties of Ca_xZn_1−xO alloy in B1 (NaCl) phase. Dependence of structural parameters as well as the band gap values on the composition x have been analyzed in the range 0?x?1. Calculated electronic structure and the density of states of these alloys are discussed in terms of the contribution of Zn d, O p and Ca p and d states. Furthermore, optical properties such as complex dielectric constants ε(ω), refractive index including extinction coefficient k(ω), normal-incidence reflectivity R(ω), absorption coefficient α(ω) and optical conductivity σ(ω) are calculated and discussed in the incident photon energy range 0-45 eV.     

First-principles calculations on TiO2 doped by N, Nd, and vacancy

First-principles density functional theory calculations have been carried out to investigate electronic structures of anatase TiO₂ with substitutional dopants of N, Nd, and vacancy, which replace O, Ti, and O, respectively. The calculation on N-doped TiO₂ with the local density approximation (LDA) demonstrates that N doping introduces some states located at the valence band maximum and thus makes the original band gap of TiO₂ smaller. Examining the effect of the strong correlation of Nd 4f electrons on the electronic structure of Nd-doped TiO₂, we have obtained the half-metallic ground state with the LDA and the insulating ground state with the LDA+U (Hubbard coefficient), respectively. In addition, the calculation on vacancy-doped TiO₂ with the LDA shows that a vacancy can induce some states in the band-gap region, which act as shallow donors.     

Theoretical investigations on KClxBr1−x, KClxI1−x and KBrxI1−x: A first-principles study

Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl_xBr_1−x, KCl_xI_1−x and KBr_xI_1−x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

Density functional calculations of Pb1−xCaxSySe1−y alloys lattice matched to different substrates

Density functional calculations are performed to study the structural and electronic properties of technologically important Pb_1?xCa_xS_ySe_1?y quaternary alloys. The calculations are based on the total-energy calculations within the full-potential augmented plane-wave (FP-LAPW) method. For exchange-correlation energy and corresponding potential, the generalized gradient approximation (GGA) by Perdew–Burke–Ernzerhof (PBE) and Engel–Vosko (EVGGA) have been used. We investigated the effect of composition on lattice constant, bulk modulus and band gap for pseudobinary as well as for quaternary alloys, which showed non-linear dependence on the composition x and y. The presented contour maps of energy band gap and lattice constants versus concentrations could be useful for designing new structures with the desired optical properties. In addition, the energy band gap and natural band offset of simple cubic Pb_1?xCa_xS_ySe_1?y quaternary alloys lattice matched to PbS and SrS substrates are investigated. The obtained results show that the quaternary alloys of interest could be appropriate materials for designing heterostructures with desired optical and interfacial properties.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

Full potential calculation of electronic properties of rutile RO2 (R=Si, Ge, Sn and Pb) compounds via modified Becke Johnson potential

The electronic properties of RO₂ (R=Si, Ge, Sn and Pb; a group IVA element) compounds in rutile structure have been calculated using WIEN2k implementation of full potential linearized augmented plane wave (FPLAPW) method. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke Johnson (MBJ) potential as coupled with Local Density Approximation (LDA) for all the compounds except for PbO₂ where only Generalized Gradient Approximation (GGA) is considered for the same. We predict a direct band gap in all these compounds with continuous decrease as the atomic size of IVA element increases such that there is an appearance of semimetallic band structure for the last compound, PbO₂. The largest band gap (7.66 eV) has been found for SiO₂, which governs its insulating nature. We observe that MBJLDA results for band gaps of these compounds are far better than those obtained using GGA and Engel-Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gaps with corresponding experimental values as compared to other calculations. The electronic band structures are also analyzed in terms of contributions from various electrons.     

Fcc-hcp phase coexistence in Al at high pressures: Explanation in terms of density of states

The density of states (DOS) of fcc and hcp structures of Al has been calculated for normal and high pressures. It has been found that the DOS of both structures, near the Fermi level, is similar over a range of compressed volumes close to the fcc-hcp transition volume (V/V₀∼0.53). This similarity is the reason for the reported coexistence of fcc-hcp phases over a wide range of pressures near the fcc-hcp phase transition. All calculations have been performed using the FP-LAPW method with GGA.