Electrical transport properties of potassium germanide tungstates (K10Ge18WO4): A theoretical study

The total and partial density of states, electronic charge density and optical properties of the monoclinic structure K₁₀Ge₁₈WO₄ compound 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) and Engel-Vosko GGA (EVGGA). Density of states discloses the semiconductor nature of the calculated compound. There exists a strong hybridization between K-p and K-s, W-d and O-p, W-f and K-p. The analysis of the chemical bonding shows that the bonding possesses strong covalent nature. The dielectric optical properties were also calculated and discussed in detail. The electrical transport coefficients of the under observation compound have been investigated using the density functional theory calculation within EVGGA.     

Ab-initio calculation of structural, electronic, and optical characterizations of the intermetallic trialuminides ScAl3 compound

We present first-principles study of the electronic and the optical properties for the intermetallic trialuminides ScAl₃ 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 L1₂ is found to be lower than that of D0₂₂ structure which confirms the stability of L1₂ structure. We found that the dispersion of the band structure of D0₂₂ is denser than L1₂ phase. The linear optical properties were calculated. The evaluations are based on calculations of the energy band structure.     

Full potential study of the elastic, electronic, and optical properties of spinels MgIn2S4 and CdIn2S4 under pressure effect

The structural, elastic, electronic, and optical properties of cubic spinel MgIn₂S₄ and CdIn₂S₄ 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 XIn₂S₄ aggregates. The Debye temperature is estimated from the average sound velocity. Electronic band structures show a direct band gap (Г-Г) for MgIn₂S₄ and an indirect band gap (K-Г) for CdIn₂S₄. 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.     

First Principle Calculation of the Geometrical and Electronic Structure of Impurity-Doped β-FeSi2 Semiconductors

The geometrical and electronic structures of impurity (Cr, Mn, Co, Ni)-doped β-FeSi₂ were investigated using first principles pseudopotential calculations based on generalized gradient approximation (GGA) density function theory. The calculated structural parameters depend strongly on the kinds of dopants and sites. The total energy calculations for substitution of dopants at the Fe_I and the Fe_II sites revealed that Mn prefers the Fe_I site, whereas Cr, Co, and Ni prefer the Fe_II site. The electronic structure is analyzed and discussed in terms of the atomic charges, bond overlap population, and total and partial densities of states (DOS).     

Ab-initio study of the structural, linear and nonlinear optical properties of CdAl2Se4 defect-chalcopyrite

The complex density functional theory (DFT) calculations of structural, electronic, linear and nonlinear optical properties for the defect chalcopyrite CdAl₂Se₄ 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 ε₁(0), refractive index n(ω), birefringence Δn(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 2ω and 1ω inter/intra-band to the imaginary part for the dominant component through the wide optical frequency range.     

Structural, optoelectronic, infrared and Raman spectra of orthorhombic SrSnO3 from DFT calculations

Orthorhombic SrSnO₃ was investigated using density functional theory (DFT) considering both the local density and generalized gradient approximations, LDA and GGA, respectively. The electronic band structure, density of states, complex dielectric function, optical absorption, and the infrared and Raman spectra were computed. Calculated lattice parameters are close to the experimental measurements, and an indirect band gap (2.27 eV) was obtained within the GGA (LDA) level of calculation. Effective masses for holes and electrons were estimated, being very anisotropic in comparison with similar results for orthorhombic CaSnO₃. The complex dielectric function and the optical absorption of SrSnO₃ were shown to be sensitive to the plane of polarization of the incident light. The infrared spectrum between 100 and 600 cm⁻¹ was obtained, with its main peaks being assigned, and a nice agreement between experimental and theoretical peaks of the Raman spectrum of orthorhombic SrSnO₃ was achieved.     

Cr、Sn、Co掺杂对层状二维材料MoSi2N4的电子和光学性能的影响(英文)

基于新合成的二维材料MoSi₂N₄(MSN),我们建立了一系列MSN的掺杂模型进行了第一原理计算。首先,我们计算了本征MSN的电子特性,包括其能带结构和态密度。然后我们研究了Cr、Sn和Co掺杂对MSN的电子和光学性质的影响。结果表明,在3种掺杂体系中,Co掺杂体系表现出最低的形成能,这表明Co掺杂体系是最稳定的。通过带隙计算表明,尽管3种掺杂模型都降低了MSN的固有带隙,但却表现出3种不同的电子特性。态密度图也显示,Cr和Co掺杂体系都在导带底(CBM)和价带顶(VBM)附近产生局部尖峰。此外,光学性质的计算中表明,掺杂后体系的光学性质也得到了改善。     

First principles calculations of electronic and optical properties of Ag2S

A theoretical study of structural, electronic and optical properties of Ag₂S is presented using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, the modified Becke Johnson (MBJ) potential coupled with Local Density Approximation (LDA) was used for the exchange-correlation potential calculation. Ground state properties are determined for the bulk material in monoclinic phase. Band structure reveals that this compound is a direct energy band gap semiconductor. MBJLDA results for the band gap of this compound are much better than those obtained using LDA, Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and Engel–Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gap and corresponding experimental values as compared to other calculations. Optical constants including the dielectric function, refractive index, extinction coefficient, electron energy loss function, reflectivity and absorption coefficient are obtained and discussed.     

Band structure calculations on the monoclinic bulk and nano-SrAl2O4 crystals

The electronic structure of SrAl₂O₄ is calculated by density functional method and exchange and correlation have been treated by the generalized gradient approximation within the scheme due to Perdew-Burke-Ernzerhof. The bond length and bond covalency are also calculated by chemical bond method. Compared with the SrAl₂O₄ bulk crystal, the bond covalency of nanocrystal has an increasing trend; its band gap also is wider; the bond lengths of SrAl₂O₄ nanocrystal become shorter, which is responsible for the change of the covalency and band gap.     

A DFT study on the electronic structure for iridium nitride under high pressure

We present density functionary theory (DFT) calculations on the structural parameters and electronic structure for iridium nitride by using the generalized gradient approximation (GGA) and the Perdew–Burke–Ernserhof (PBE) exchange-correlation functional. The lattice parameters and bulk modulus (B ₀) for the ground state are obtained, and the energy band structure and electron densities of states (DOS) of IrN₂ are presented. It is found that IrN₂ has a very close indirect energy gap. There is a strong covalent bond between the two nearest N atoms. This gives rise to a very high elastic modulus of IrN₂ and reveals the quasimolecular nature of the N₂ in IrN₂ crystal. Lattice parameters, bulk modulus, and the electronic structure of IrN₂ under high pressure have also been investigated based on DFT. The compressibility along three cell vectors is very close to each other. The band gap increases a little with the pressure even when the pressure is up to 100 Gpa.     

Linear and nonlinear optical susceptibilities for a novel borate oxide BaBiBO4: Theory and experiment

The linear and nonlinear optical susceptibilities for different tensor components of BaBiBO₄ single crystals have been calculated using the full-potential linear augmented plane wave method. The results of these calculations are verified by our measurements of linear and nonlinear optical properties using Nd-YAG laser at fundamental wavelength 1064 nm. The calculated energy gap is in a good agreement with experimental energy gap data obtained by optical absorption. We present results for the imaginary and real parts of the frequency-dependent dielectric constant. The calculated birefringence of BaBiBO₄ is positive in agreement with the experimental data. Calculations are reported for the frequency-dependent complex part of second-order nonlinear optical susceptibilities . The linear and nonlinear optical susceptibilities are scissors corrected to match the value of the energy gap from the local density approximation calculations with the experimental value. The second harmonic generation efficiency of this compound is about five times larger than KDP (KH₂PO₄). It is crucial that we have obtained a large anisotropy of the second-order susceptibilities for three main second-order tensor components both experimentally and theoretically. The possible origin of the obtained anisotropy is discussed within a framework of the energy band calculations.     

Pt原子在γ-Al2O3(001)表面的吸附及迁移

采用密度泛函理论(DFT)中广义梯度近似(GGA)方法, 对Pt原子与γ-Al₂O₃(001)面的相互作用及迁移性能进行了研究. 分析了各种可能吸附位及吸附构型的松弛和变形现象, 吸附能和迁移能垒的计算结果表明: Pt团簇能够稳定吸附在该表面. Pt原子在表面O位的吸附能明显较高, 这主要是由Pt向基底O原子转移了电子所致. 电荷布居分析表明, Pt原子显电正性, Pt和Al原子之间存在排斥作用, 导致与Al原子产生较弱相互作用. 计算的平均吸附能大小依赖于Pt团簇的大小和形状, 总体趋势是随着Pt原子数增多, 吸附能降低. Pt原子在γ-Al₂O₃(001)表面迁移过程所需克服的迁移能垒最高值为0.51 eV. 随着吸附的Pt原子数增多,更倾向于形成Pt团簇. 因此, Pt原子在γ-Al₂O₃(001)表面的吸附演变不可能形成光滑、均匀平铺的吸附构型, 而在一定条件下容易出现团聚.     

AgGa(S1-xSex)2固溶体的电子结构和光学性质

采用基于赝势平面波基组的密度泛函理论方法, 对一系列具有黄铜矿结构的AgGa(S_1-xSe_x)₂固溶体的构型、电子结构、线性和二阶非线性光学性质进行了系统研究. 结果表明, 各固溶体具有类似的能带结构, 体系带隙随x值增加而逐渐减小. 当所引入的Hartree-Fock交换项贡献为22.56%时, 对应的杂化PBE泛函得到的带隙值与实验结果相近. 固溶体的各种光学性质, 包括折射率、双折射率、反射率、吸收系数和二阶倍频系数等均随着组成的改变呈现出有规律的变化趋势, 变化范围介于AgGaS₂和AgGaSe₂二者之间. 因此, 利用固溶体光学性质的变化规律, 可从中寻找出具有特定光学性能的晶体材料.     

Structural,elastic, electronic and magnetic properties of perovskite-like Co3WC,Rh3WC and Ir3WC from first principles calculations

We have performed ab initio total energy calculations using the full-potential linearized augmented plane wave method (FP-LAPW) with the generalized gradient approximation (GGA) for the exchange-correlation potential to predict the structural, elastic, cohesive, electronic and magnetic properties of perovskite-like phases Co₃WC, Rh₃WC and Ir₃WC. The optimized lattice parameters, density, independent elastic constants (C_ij), bulk moduli, shear moduli, tetragonal shear moduli, compressibility, and Cauchy pressure, as well as electronic densities of states, cohesive and formation energies, atomic magnetic moments have been obtained and analyzed for the first time.     

The reflectivity spectra of ZnXP2 (X=Si, Ge, and Sn) compounds

Full Potential Augmented Plane Wave plus local orbital method ( FAPW+lo) calculations were performed for ZnSiP₂, ZnGeP₂, and ZnSnP₂ in the chalcopyrite structure in order to investigate the optical properties and to show the origin of the different optical transitions and their correspondence in the band structure. It is found that the most important features of the band gap is pseudo-direct for ZnSiP₂, indirect for ZnGeP₂, and direct for ZnSnP₂. Then the contribution of the different transitions peaks are analyzed from the imaginary part of the dielectric function and the reflectivity spectra.     

Evolution and characterization of fluorite-like nano-SrBi2Nb2O9 phase in the SrO-Bi2O3-Nb2O5-Li2B4O7 glass system

Transparent glasses of various compositions in the system (100−x)Li₂B₄O₇−x(SrO-Bi₂O₃-Nb₂O₅) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi₂Nb₂O₉ phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple-Didomemenico and Gladstone-Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active.     

Electronic Structure and Formation Heat of Pu3M and PuM3 (M=Ga, In, Sn and Ge) Compounds

The equilibrium structure, electronic structure, and formation heat of Pu₃M (PuM₃) (M=Ga, In, Sn, and Ge) compounds with AuCu₃ structure have been calculated using full potential linear augmented plane wave (FPLAPW) method within generalized-gradient approximation (GGA) including spin-orbit coupling (SOC) and spin polarization (SP). The calculated lattice parameters are in good agreement with the experimental values. Density of state analysis shows hybridization effects between Pu and M are governed by the competitions depending on the M amount: Pu 6d-Pu 5f, M p-Pu 6d, and M sp-M sp interactions. Electronegativity difference and electronic hybridization effect are two important factors to influence the formation heat and stability of Pu₃M (PuM₃) compounds. The larger is the electronegativity difference and the lower is M s-band or p-band center relative to the Fermi energy, the more negative is the formation heat and the more stable are Pu₃M (PuM₃) compounds.     

Electronic,elastic and thermal properties of lutetium intermetallic compounds

The electronic, structural, elastic, thermal and mechanical properties of Lutetium intermetallic compounds LuX (X = Mg, Cu, Ag, Au, Zn, Cd and Hg) have been studied using ab-initio full potential linear augmented plane wave (FP-LAPW) with the generalized gradient approximation (GGA) in their non magnetic phase. The ground state properties such as lattice constant, bulk modulus, pressure derivatives of bulk modulus are reported in CsCl-(B₂ phase) structure. We also report the band structure and density of states at equilibrium lattice constant. The calculated band structures indicate that these intermetallics are metallic in nature. The second order elastic constants of these compounds are also predicted for the first time. The ductility of these compounds is determined by calculating the bulk to shear ratio B/G_H.     

稀土金属掺杂对锐钛矿型TiO2光催化活性影响的理论和实验研究

从理论和实验两方面探讨稀土金属掺杂对锐钛矿型TiO2光催化活性的影响.理论上采用基于密度泛函理论(DFT)的第一性原理,对稀土掺杂TiO2前后的几何结构、能带结构、态密度及电子结构进行了系统的研究.结果表明,Y,La,Gd,Lu,Ce,Eu,Yb和Tb掺杂有助于TiO2光催化活性的提高;而对于Pr,Nd,Pm,Sm,Dy,Ho,Er和Tm掺杂,由于在价带顶和导带底之间形成了较多的可能成为光生电子和空穴的复合中心的杂质能级,故此类稀土的掺杂浓度需要控制在较小的范围内.另一方面,采用溶胶-凝胶法制备了9种稀土金属(RE=Y,Ce,Pr,Sm,Gd,Dy,Ho,Er,Yb)掺杂的TiO2粉体,运用X射线衍射(XRD)和紫外-可见光谱法(UV-Vis)分别表征其晶体结构和光学吸收性质.结果表明,掺杂前后的TiO2均为锐钛矿相,且Ho,Pr,Ce,Sm,Y,Yb和Gd掺杂使TiO2在可见光区的吸收有不同程度的提高.理论预测与实验结果基本一致,且理论研究结果与周期表中稀土元素外层电子轨道排布规律一致,从而揭示了稀土元素掺杂的本质规律,指明了适量的稀土掺杂有利于TiO2光催化活性的提高.     

Thermal and photochemical effects on the structure,morphology, thermal and optical properties of PVA/Ni0.04Zn0.96O and PVA/Fe0.03Zn0.97O nanocomposite films

Ni_0.04Zn_0.96O and Fe_0.03Zn_0.97O with average diameter of 23 and 19 nm, respectively, have been synthesized by a modified sol–gel method to be used in the preparation of (100 − x)/x poly(vinyl alcohol)/oxide nanocomposite films, with x = 0, 1, 3 and 5 (in wt.%). A 125 W-Hg vapor lamp with emission above 254 nmwas used to irradiate PVA/Ni_0.04Zn_0.96O and PVA/Fe_0.03Zn_0.97O films. The effect on their structural, thermal, morphological and optical properties was studied by TG, DSC, DRX, AFM, UV–vis and PL spectrophotometry. The Ni_0.04Zn_0.96O addition on PVA films decreases the thermal stability of the polymer in inert and in oxidative atmosphere. In contrast, the Fe_0.03Zn_0.97O presence in the PVA films seems to increase the thermal stability of the polymer. The characteristic peak of the crystalline phase of PVA and wurtzite phase of the zinc oxide were identified through X-ray diffraction in both films. The crystallinity of the PVA film increases with UV irradiation and with the presence of Ni_0.04Zn_0.96O and Fe_0.03Zn_0.97O. The roughness of the PVA film was not modified by the addition of the doped oxides; however, it increases after UV irradiation, more significantly in the films containing the oxides. The PVA film exhibits absorption around 280 nm characteristic of π–π^∗ transitions related to carbonyl groups from residuals acetate, while the 95/05 PVA/Ni_0.04Zn_0.96O and 95/05 PVA/Fe_0.03Zn_0.97O nanocomposite films show absorption at the visible region which is characteristics of the band gap reduction of the doped oxides. The photoluminescence of PVA was modified by the presence of the oxides in the film. These nanocomposite films are interesting due to their thermal, mechanical (flexible) properties and low cost of production. In addition they are also able to exhibit peculiar optical properties showing potential to be used in photonic devices, gas sensors and organic solar cell applications.