Electronic structures and optical properties of TiO2:Improved density-functional-theory investigation

TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors.In fact,it has been widely used for a long time as white pigment and sunscreen because of its whiteness,high refractive index,and excellent optical properties.However,its electronic structures and the related properties have not been satisfactorily understood.Here,we use Tran and Blaha＇s modified Becke-Johnson（TB-mBJ） exchange potential（plus a local density approximation correlation potential） within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2.Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation（LDA） and generalized gradient approximation（GGA）,in contrast with substantially overestimated values from many-body perturbation（GW） calculations.As for optical dielectric functions（both real and imaginary parts）,refractive index,and extinction coefficients as functions of photon energy,our mBJ calculated results are in excellent agreement with the experimental curves.Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states.These results should be helpful to understand the high temperature ferromagnetism in doped TiO2.This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.     

Comparative study of structural,electronic, optical and thermoelectric properties of GaS bulk and monolayer

The structural, electronic, optical properties of GaS in bulk and monolayer forms have been studied by means of full-potential linearised augmented plane wave calculations within framework of the density functional theory. Generalised gradient approximation and Tran–Blaha modified Becke–Johnson exchange potential (mBJ) were employed for the treatment of exchange-correlation effect in calculations. Our calculated lattice parameters are in good agreement with previous theoretical results and available experimental data. The negative formation enthalpy and cohesive energy indicate that both bulk and monolayer GaS can be synthesised and stabilised experimentally. Our electronic results show that the band gap of GaS monolayer is higher than that of bulk counterpart and strong hybridisation between electronic states of constituent atoms is observed in both cases. The optical properties such as reflectivity, absorption coefficient, refractive index and optical conductivity were derived from calculated complex dielectric function for wide energy range up to 35?eV. Finally, the thermoelectric properties of GaS bulk and monolayer also were calculated using semi-classical Boltzmann theory within the constant relaxation time approximation for investigating their applicability in thermoelectric devices.     

The electronic structure,electronic charge density and optical properties of the diamond-like semiconductor Ag2ZnSiS4

The electronic structure, electronic charge density and optical properties of the diamond-like semiconductor Ag₂ZnSiS₄ compound with the monoclinic structure have been investigated using a full-relativistic version of the full-potential augmented plane-wave method based on the density functional theory, within local density approximation (LDA), generalized gradient approximation (GGA), Engel–Vosko GGA (EVGGA) and modified Becke Johnson (mBJ) potential. Band structures divulge that this compound is a direct energy band gap semiconductor. The obtained energy band gap value using mBJ is larger than those obtained within LDA, GGA and EVGGA. There is a strong hybridization between Si-s and S-s/p, Si-p and Zn-s, Ag-s/p and Zn-s, and Ag-s and Ag-p states. The analysis of the site and momentum-projected densities shows that the bonding possesses covalent nature. The dielectric optical properties were also calculated and discussed in detail.     

Effects of N-doping concentration on the electronic structure and optical properties of N-doped β-Ga₂O₃

The electronic structures and the optical properties of N-doped β-Ga2O3 with different N-doping concentrations are studied using the first-principles method.We find that the N substituting O(1) atom is the most stable structure for the smallest formation energy.After N-doping,the charge density distribution significantly changes,and the acceptor impurity level is introduced above the valence band and intersects with the Fermi level.The impurity absorption edges appear to shift toward longer wavelengths with an increase in N-doping concentration.The complex refractive index shows metallic characteristics in the N-doped β-Ga2O3.     

Electronic,structural and magnetic properties of TbO under pressure: FP-LAPW study

ABSTRACT

Using the framework of the density functional theory, we calculated electronic, magnetic and structural properties of terbium oxide (TbO) in rocksalt (RS), cesium chloride (CsCl) and zincblende (ZB). Full potential linearized augmented plane wave (FP-LAPW) method within the local spin density approximation (LSDA) and generalized gradient (PBE-GGA) approximations are used. Magnetic and non-magnetic calculations are performed and a modified version of Becke and Johnson (mBJ) exchange potential has been used to calculate the band gaps. We found that, although TbO is stable in a ferromagnetic state, it is stable in RS phase at ambient condition. Both LSDA and PBE-GGA calculations revealed that the three structures are metallic. However, using the mBJ calculation, it is clear that RS and CsCl phases of TbO compound are metallic, while ZB phase is found to be an insulator in the spin-up case and a semiconductor in the spin-down case at ambient pressure.     

First-principle calculations of structural, electronic and optical properties of BaTiO3 and BaZrO3 under hydrostatic pressure

A theoretical study of structural, electronic and optical properties of cubic BaTiO₃ and BaZrO₃ perovskites is presented, using the full-potential linear augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus, its pressure derivative, band structure, density of states, pressure coefficients of energy gaps and refractive indices. The results are compared with previous calculations and experimental data.     

立方晶相HfO2电子结构与光学性质的第一性原理计算

利用密度泛函理论框架下的第一性原理平面波超软赝势方法计算了立方晶相二氧化铪(c-HfO2)的电子结构,得到了c-HfO2的总念密度、分波态密度和能带结构.经带隙校正后,计算了c-HfO2的光学线性响应函数随光子能最的变化关系,包括复介电函数,反射率、复折射率以及光学吸收系数,并从理论上给出了c-HfO2材料光学性 质与电子结构的关系.经比较发现,对c-HfO2的电子结构和光学性质的计算结果与已有的实验数据和其它理论研究吻合得较好,从而为c-HfO2光电材料的设计与应用提供了理论依据.同时,计算结果也表明采用密度泛函理论的广义梯度近似来计算和预测c-HfO2材料的电子结构和光学性质是比较可靠的.     

First principles study on structural,electronic and optical properties of Ga1−xBxP ternary alloys (x = 0, 0.25, 0.5, 0.75 and 1)

The structural, electronic and optical properties of GaP, BP binary compounds and their ternary alloys Ga_1?xB_xP ($x=0.25$, 0.5 and 0.75) have been studied by full-potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory (DFT) as implemented in WIEN2k package. Local density approximation (LDA) and generalized gradient approximation (GGA) as proposed by Perdew–Burke–Ernzerhof (PBE), Wu–Cohen (WC) and PBE for solid (PBESol) were used for treatment of exchange-correlation effect in calculations. Additionally, the Tran–Blaha modified Becke–Johnson (mBJ) potential was also employed for electronic and optical calculations due to that it gives very accurate band gap of solids. As B concentration increases, the lattice constant reduces and the energy band gap firstly decreases for small composition x and then it shows increasing trend until pure BP. Our results show that the indirect–direct band gap transition can be reached from $x=0.33$. The linear optical properties, such as reflectivity, absorption coefficient, refractive index and optical conductivity of binary compounds and ternary alloys were derived from their calculated complex dielectric function in wide energy range up to 30 eV, and the alloying effect on these properties was also analyzed in detail.     

Structural,electronic and optical properties of cubic fluoroelpasolite Cs2NaYF6 by density functional theory

First-principles full-potential linearized augmented plane-wave method based on density functional theory is used to investigate the structural, electronic and optical properties of the cubic fluoroelpasolites Cs₂NaYF₆ within the local density approximation (LDA) and generalized gradient approximation (GGA) for potential exchange correlation. The modified Becke–Johnson (mBJ) potential approximation is also used for calculating the electronic and optical properties of the material. We have analyzed the structural parameters, total and partial densities of states, dielectric functions, absorption and reflectivity. The results show that the band structure of the fluoroelpasolites Cs₂NaYF₆ has an insulating behavior for the two directions of spin and as a result there is no net magnetic moment. A wide band gap of 9.6 eV is obtained with mBJ-GGA, which allows the application of this material as X-ray storage phosphor materials and scintillators.     

First-principles calculations of the structural, electronic and optical properties of cubic BxGa1−xAs alloys

Density functional calculations are performed to study the structural, electronic and optical properties of technologically important B_xGa_1−xAs ternary alloys. The calculations are based on the total-energy calculations within the full-potential augmented plane-wave (FP-LAPW) method. For exchange-correlation potential, local density approximation (LDA) and the generalized gradient approximation (GGA) have been used. The structural properties, including lattice constants, bulk modulus and their pressure derivatives, are in very good agreement with the available experimental and theoretical data. The electronic band structure, density of states for the binary compounds and their ternary alloys are given. The dielectric function and the refractive index are also calculated using different models. The obtained results compare very well with previous calculations and experimental measurements.     

First-principles calculations of the stibnite at the level of modified Becke–Johnson exchange potential

The demand for cheaper, nontoxic and earth-abundant materials as absorbing layer for solar cell is immensely needed to replace scarce, toxic and expensive one. In this regard, chalcogenide materials have considerably attracted the attention of a lot of researchers because of showing a great potential for different applications. Stibnite (Sb₂S₃), a chalcogenide binary material is considerably investigated for exploiting its potential for different energy technologies being a less toxic, abundantly available, stable and efficient, which are the fundamentals for sustainability as well as to realize the dream of green energy. In this study, theoretical calculations of the structural, electronic and optical properties of stibnite (Sb₂S₃) crystal structure are presented using the full potential (FP) linearized augmented plane wave (LAPW) framed within density functional theory (DFT). To incorporate the exchange-correlation part in the total energy functional, besides the local density approximation (LDA), Wu-Cohen parameterized generalized gradient approximation (WC-GGA), Perdew–Burke–Ernzerhof parameterized generalized gradient approximation (PBE-GGA), and Perdew–Burke–Ernzerhof generalized gradient approximation for solids and surfaces (PBEsol-GGA) are used for the calculations of the structural parameters, where the Trans-Blaha approach of the modified Becke–Johnson (TB-mBJ) potential is used to get more reliable results for the fundamental band gap energy value. These calculations are performed by involving spin-orbit coupling (SOC) contribution. Additionally, optical properties, such as imaginary and real parts of the dielectric function, optical conductivity, absorption coefficient, refractive index, reflectivity, and electron energy loss function are analyzed. Our first-principles calculations show that Wu-Cohen GGA (WC-GGA) reproduces results for lattice parameters comparable to the experimental measurements. The obtained results of the band gap energy and optical properties with TB-mBJ potential are also closer to the experimental data and, endorse its potentiality for the photovoltaics applications.     

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.     

Orientation effects on the bandgap and dispersion behavior of 0.91Pb(Zn_1/3Nb_2/3)O₃-0.09PbTiO₃ single crystals

0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO 3(PZN-9%PT) single crystals with different orientations are investigated by using a spectroscopic ellipsometer,and the refractive indices and the extinction coefficients are obtained.The Sellmeier dispersion equations for the refractive indices are obtained by the least square fitting,which can be used to calculate the refractive indices in a low absorption wavelength range.Average Sellmeier oscillator parameters E o,λ o,S o,and E d are calculated by fitting with the single-term oscillator equation,which are related directly to the electronic energy band structure.The optical energy bandgaps are obtained from the absorption coefficient spectra.Our results show that the optical properties of [001] and [111] poled crystals are very similar,but quite different from those of the [011] poled crystal.     

Structural,elastic, electronic and optical properties of the cubic perovskites CaXO3 (X=Hf and Sn)

The structural, elastic, electronic and optical properties of CaXO₃ compounds with the cubic perovskites structure have been investigated, by employing a first principles method, using the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA). The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal monocrystalline and for polycrystalline CaXO₃ aggregates which we have classified as ductile in nature. Band structures reveal that these compounds are indirect energy band gap (R-G) semiconductors; the analysis of the site and momentum projected densities, valence charge density bond length, bond population and Milliken charges, shows that bonding is of covalent–ionic nature. We have found that the elastic constants C₁₁, C₁₂, C₄₄ are in good correlation with the bonding properties. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV.     

Density functional study of optical properties of beryllium chalcogenides compounds in nickel arsenide B8 structure

The structural, electronic and optical properties of beryllium chalcogenides BeS, BeSe and BeTe using the full-potential linear augmented plane wave (FP-LAPW) method are investigated. The exchange-correlation energy within the local density approximation (LDA) and the generalized gradient approximation (GGA) are described. The Engel-Vosko (EVGGA) formalism is applied for electronic and optical properties. The structural parameters of our model and the transition pressure from zinc-blende (B3) to the NiAs (B8) phase are confirmed. It is found that these compounds have indirect band gaps except for BeTe in NiAs (B8) phase. The results of reflectivity, refractive index and optical dielectric functions of Be compounds are investigated. An agreement is found between our results and those of other theoretical calculations and the experimental data.     

Pressure effect on the structural,electronic and optical properties of the cubic triangular quaternary BNxAsyP1−x−y alloys: First principle study

Predicted results of the structural, electronic and optical properties of the cubic zinc-blende phase of BN, BAs and BP binary compounds and their related ternary and quaternary alloys are presented. The density functional theory (DFT) within full potential linearized augmented plan wave (FP-LAPW) is employed. Different exchange correlation approximations were used to calculate the structural properties as well as the total energies, lattice parameters, bulk modulus and its first pressure derivative. The electronic band structures were treated with the local density approach and Tran Blaha modified Beck-Johnson (TB-mBJ) approximation. A quadratic fit of the lattice parameter, bulk modulus and band gap was performed, where a nonlinear variation with the composition x and y is found. Moreover, the optical properties have been investigated, where the dielectric behavior, the refractive index variations and the loss energy were studied. Furthermore, the electronic and optical properties were computed under hydrostatic pressure. Our results showed great agreement with the previous available experimental and theoretical data found in the literature.     

高压下氧化镉弹性性质、电子结构和光学性质的第一性原理研究

 运用基于密度泛函理论的平面波赝势方法（PWP）,计算研究了氧化镉NaCl结构（B1结构）和CsCl结构（B2结构）在不同压力条件下的几何结构、弹性性质、电子结构和光学性质。交换关联能分别采用广义梯度近似（GGA）和局域密度近似（LDA）。通过比较计算和实验得到的晶格常数和体模量不难发现,LDA的计算结果更符合实验值。在高压的作用下,两种结构的导带能级有向高能级移动的趋势,而价带能级有向低能级移动的趋势,因此直接带隙变大。同时,对照态密度分布图及高压下能级的移动情况,分析了CdO两种结构在高压作用下的光学性质。