High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

Pressure-induced phase transition in defect Chalcopyrites HgAl2Se4 and CdAl2S4

Results of angle dispersive X-ray diffraction (ADXRD) measurements on the defect chalcopyrites (DCP), HgAl₂Se₄ and CdAl₂S₄ up to 22.2 and 34 GPa, respectively, are reported. The ambient tetragonal phase is retained in HgAl₂Se₄ and CdAl₂S₄ up to 13 and 9 GPa respectively. The values of the bulk modulus estimated from the Equation of State is 66(1.5) and 44.6(1) GPa for HgAl₂Se₄ and CdAl₂S₄ in the chalcopyrite phase. At higher pressure a disordered rock-salt structure and on pressure release a disordered zinc blende structure with broad X-ray diffraction lines are observed as is the case for several defect chalcopyrites.     

First and second harmonic generation of the XAl2Se4 (X=Zn,Cd,Hg) defect chalcopyrite compounds

The chemical bonding of the ZnAl₂Se₄, CdAl₂Se₄ and HgAl₂Se₄ defect chalcopyrites has been studied in the framework of the quantum theory of atoms in molecules (AIM). The GW quasi-particle approximation is used to correct the DFT-underestimation of energy gap, and as a consequence the linear and nonlinear optical properties are significantly enhanced. The second harmonic generation (SHG) displays certain dependence with the ionicity degree decrease through the dependency of the SHG on the band gap. The occurrence of the AIM saddle point is characterized and some clarifying features in relationship with the density topology are exposed, which enable to understand the relation with the second harmonic generation effect.     

Electronic and optical properties of defect chalcopyrite HgAl2Se4

The structural, electronic and optical properties of HgAl₂Se₄ are investigated using the full potential linear augmented plane wave method based on density functional theory. The calculated structural parameters using LDA are in excellent agreement with the available experimental result. The obtained energy band gap (2.24 eV) using EV-GGA approximation is in excellent agreement with experimental data (2.20 eV). Variation in the energy band gap as a function of the unit cell lattice parameter has been studied. The optical properties show a considerable anisotropy, which makes this compound very useful for various linear–nonlinear optical devices.     

New ring-like models and ab initio DFT study of the medium-range structures,energy and electronic properties of GeSe2 glass

Ab initio DFT calculations were performed on Ge_nSe_m nanoclusters (n?=?2, 3, 5, 6, 12; m?=?6–9, 14, 16, 30) that represent the local structure of GeSe₂ glass and on some ‘defect’ Ge_nSe_m clusters that are thought to be related to the inhomogeneity of the structure at the nanoscale. The optimal geometries, total energies and their derivatives as well as the electronic properties of Ge_nSe_m nanoclusters were calculated using traditional DFT method. In addition, the TD-DFT method has been applied to calculate the electronic band gaps of the clusters. The calculated physico-chemical properties of Ge_nSe_m nanoclusters and their couplings with the local-and medium-range order structure formations in GeSe₂ glass are analysed and discussed.     

二维GaTe/Bi2Se3异质结：一类有潜力的Z型光解水催化剂

本文基于第一性原理方法，计算了二维GaTe/Bi₂Se₃异质结的电子结构、界面电荷转移、静电势分布、吸收光谱及光催化性质. 计算结果表明异质结是一个小能隙的准直接半导体，能有效捕获太阳光. 由于相对较强的界面內建极化电场和带边轻微弯曲，导致异质结中的光生电子和空穴分别有效分离在GaTe单层和Bi₂Se₃薄片上，可用于析氢和产氧. 这些理论计算结果意味着二维GaTe/Bi₂Se₃异质结是一类有潜力的Z型太阳能全解水催化剂.     

Elastic,dynamical, and electronic properties of LiHg and Li<Subscript>3</Subscript>Hg: First-principles study

The elastic, dynamical, and electronic properties of cubic LiHg and Li₃Hg were investigated based on first-principles methods. The elastic constants and phonon spectral calculations confirmed the mechanical and dynamical stability of the materials at ambient conditions. The obtained elastic moduli of LiHg are slightly larger than those of Li₃Hg. Both LiHg and Li₃Hg are ductile materials with strong shear anisotropy as metals with mixed ionic, covalent, and metallic interactions. The calculated Debye temperatures are 223.5 K and 230.6 K for LiHg and Li₃Hg, respectively. The calculated phonon frequency of the T_2g mode in Li₃Hg is 326.8 cm^?1. The p states from the Hg and Li atoms dominate the electronic structure near the Fermi level. These findings may inspire further experimental and theoretical study on the potential technical and engineering applications of similar alkali metal-based intermetallic compounds.     

Theoretical investigation on structural, magnetic and electronic properties of ferromagnetic GdN under pressure

The tight-binding linear muffin tin orbital (TB-LMTO) method within the local density approximation is used to calculate structural, electronic and magnetic properties of GdN under pressure. Both nonmagnetic (NM) and magnetic calculations are performed. The structural and magnetic stabilities are determined from the total energy calculations. The magnetic to ferromagnetic (FM) transition is not calculated. Magnetically, GdN is stable in the FM state, while its ambient structure is found to be stable in the NaCl-type (B₁) structure. We predict NaCl-type to CsCl-type structure phase transition in GdN at a pressure of 30.4 GPa. In a complete spin of FM GdN the electronic band picture of one spin shows metallic, while the other spin shows its semiconducting behavior, resulting in half-metallic behavior at both ambient and high pressures. We have, therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for GdN in the B₁ and B₂ phases. The magnetic moment, equilibrium lattice parameter and bulk modulus is calculated to be 6.99 μ_B, 4.935 Å and 192.13 GPa, respectively, which are in good agreement with the experimental results.     

Electronic and thermal properties of B2-type AlRE intermetallic compounds: A first principles study

The ground state electronic structure and thermal properties of B₂-type intermetallic compounds AlRE (RE: Pm, Sm, Eu, Tb, Gd and Dy) have been studied using a self-consistent tight-binding linear muffin-tin orbital (TB-LMTO) method at ambient as well as at high pressure. These compounds show metallic behavior under ambient condition. The band structure, total energy, density of states and ground state properties like lattice parameter, bulk modulus are calculated in the present work. The Debye-Grüneisen model is used to calculate the Debye temperature and the Grüneisen constant. The calculated results are in good agreement with the reported experimental and other theoretical results. The variation in the Debye temperature with pressure has also been reported. We present a detailed analysis of the role of f electrons of RE in the AlRE system.     

Investigation of excitonic states effects on optoelectronic properties of Sb2Se3 crystal for broadband photo-detector by highly accurate first-principles approach

The rapid demand of photodetector is increasing day by day due to its versatility of applications that affect our lives. However, it is still very challenging to produce low-cost high-performance broadband photo-detector that can detect light from near infrared to the ultraviolet frequency range for medical diagnosis and visible light communication applications. Regarding this, low-cost antimony selenide (Sb₂Se₃), with direct energy gap and strong light absorption over a wider range from near infrared to ultraviolet frequency, is considered a promising candidate material for such kind of applications. Therefore, to expose its hidden potential, detailed analysis of its structural, electronic and optical properties is very essential. To accomplish this purpose, different schemes of the first-principles calculations are used in this study. Structural properties of Sb₂Se₃ are calculated by first-principles methods realized within density functional theory (DFT) framework. Whereas, to compute the quasiparticle (QP) band structure, excitonic and optical properties, many-body perturbation theory (MBPT) based on one-shot GW (G₀W₀) and Bethe-Salpeter equation (G₀W₀-BSE) approaches are used. Our DFT calculations show that Wu-Cohen GGA (WC-GGA) reproduces lattice parameters of Sb₂Se₃ material consistent with the experimental measurements. Similarly, G₀W₀ calculations confirm the Sb₂Se₃ a direct bandgap energy material of 1.32 eV and show good agreement with the experimental results. Similarly, the results on the optical properties of Sb₂Se₃ with the inclusion of electron-hole interaction show that the exciton energy of the material is 1.28eV while its corresponding plasma energy is 10.86 eV. These values show that the investigated material can absorb photons from near infrared to ultraviolet wavelengths. It is, therefore, anticipated that this material will be useful for new-generation optoelectronic applications from near infrared to ultraviolet wavelengths.     

First principles study of the electronic and optical properties of SbTaO4

The electronic density of states (DOS), band structure and optical properties of orthorhombic SbTaO₄ are studied by first principles full potential-linearized augmented plane wave (FP-LAPW) method. The calculation is done in the framework of density functional theory with the exchange and correlation effects treated using generalized gradient approximation (GGA). We find an indirect band gap of 1.9 eV at the R→Γ symmetry direction of the Brillouin zone in SbTaO₄. It is observed that there is a strong hybridization between Ta-5d and O-2p electronic states which is responsible for the electronic properties of the system. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of SbTaO₄. The real and imaginary parts of the dielectric function of SbTaO₄ are calculated, which correspond to electronic transitions from the valence band to the conduction band. The band gap obtained is in close agreement with the experimental data.     

Synthesis and properties of In2(Se1-xTex)3 thin films: a new semiconductor compound

In₂(Se_1-xTe_x)₃ polycrystalline films were prepared by a dual-source thermal evaporation technique. The depositions onto glass and SnO₂-coated glass substrates were carried out in a vacuum chamber and followed by an annealing in neutral ambient (Ar or N₂). The structural, morphological and compositional studies of the films were made by X-ray diffraction, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, Raman scattering and optical transmission. Optimum conditions are investigated for the formation of the ternary compound In₂(Se_1-xTe_x)₃ in order to tune the band gap by changing the Te concentration. The film properties as a function of Te amount are discussed. It is shown that single-phase, textured and homogeneous layers of In₂(Se_1-xTe_x)₃ can be grown with x≤0.2 at optimal deposition and heat treatment conditions. For x≅0.17 these films showed an energy band gap of about 1.45 eV and an electrical conductivity at room temperature six orders of magnitude higher than that of the binary γ-In₂Se₃ thin films. Received: 9 July 1999 / Accepted: 25 November 1999 / Published online: 13 July 2000     

Electrical properties of a-Ge<Subscript>x</Subscript>Se<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>

In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of Ge_xSe_100-x in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of Ge_xSe_{100-x (x} = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (Ge_xSe_100-x ) have been successfully explained by correlated barrier hopping (CBH) model.     

First principles study of the electronic structure and phonon dispersion of naphthalene under pressure

The structural, electronic and vibrational properties of crystalline naphthalene has been investigated within the framework of density functional theory including van der Waals interactions. The computed lattice parameters and cohesive energy have good agreement with experimental data. We study on the structural and electronic properties of the naphthalene under the hydrostatic pressure of 0–20 GPa. The isothermal equations of state calculated from the results show good agreement with experiment in the pressure intervals studied. The phonon dispersion curves have been computed at ambient and hydrostatic pressure of 10 and 20 GPa. We have also calculated the quasiparticle band structure of naphthalene with the G₀W₀ approximation.     

The electronic structure,elastic and optical properties of Cu2ZnGe(SexS1 − x)4 alloys: density functional calculations

The electronic structure, elastic and optical properties of Cu₂ZnGe(Se_xS_{1 ? x})₄ alloys are systematically analysed using first-principles calculations. The lattice parameters agree well with the theoretical and experimental values which are searched as complete as possible indicating our calculations are reliable. The elastic properties are investigated first and are compared with the similar compounds CZTS and CZTSe due to the unavailable experimental data currently. The variation of the optical properties caused by the increase of Se/S ratio is discussed. The static optical constants are calculated and the corrected values are also predicted according to the available experimental data.     

Electronic and optical properties of ZnS x Se1−x alloys

A series of calculations from first principles have been carried out to study structural, electronic, and optical properties of ZnS_xSe_1−x alloys. Our results show that the lattice constant scales linearly with sulfur composition. The imaginary parts of the dielectric function are calculated, which are in good agreement with the experimental data. We have also interpreted the origin of the spectral peaks on the basis of band structure and density of states. Additionally, we find that no bowing effect in the absorption edge is observed, unlike other II-VI semiconductor alloys.      

Electronic structure and optical properties of ordered compounds potassium tantalate and potassium niobate and their disordered alloys

The electronic energy band structure, site and angular momentum decomposed density of states (DOS) of cubic perovskite oxides KNbO₃ and KTaO₃ have been obtained from a first principles density functional based full potential linearized augmented plane wave (FLAPW) method within a generalized gradient approximation (GGA). The total DOS in valence region is compared with the experimental photo-emission spectra (PES). The calculated DOS is in good agreement with the experimental energy spectra and the features in the spectra are interpreted by comparison with the projected density of states (PDOS). The valence band PES is mainly composed of Nb-4d/Ta-5d and O 2p states in KNbO₃ and KTaO₃, respectively. Using the PDOS and the band structure we have analyzed the inter-band contribution to the optical properties of these materials. The real and imaginary parts of the dielectric function have been calculated and compared with experimental data. They are found to be in a reasonable agreement. The role of band structure on the optical properties have been discussed.     

Tunable electronic structure in stained two dimensional van der Waals g-C2N/XSe2 (X = Mo,W) heterostructures

The electronic structure of the strained g-C₂N/XSe₂ (X=Mo, W) van der Waals heterostructures are investigated by first-principles calculations. The g-C₂N/MoSe₂ heterostructure is an indirect band gap semiconductor at a strain from 0% to 8%, where its band gap is 0.66, 0.61, 0.73, 0.60 and 0.33 eV. At K point, the spin splitting is 186, 181, 39, 13 and 9 meV, respectively. For g-C₂N/WSe₂ heterostructures, the band gap is 0.32, 0.37, 0.42, 0.45 and 0.36 eV, and the conduction band minimum is shifted from Г-M region to K-Г region as the strain increases from 0% to 8%. Its spin splitting monotonically decreases as a strain raises to 8%, which is 445, 424, 261, 111 and 96 meV, respectively. Moreover, at a strain less than 4%, the conduction band mainly comes from g-C₂N, but it comes from XSe₂ (X=Mo, W) above 6%. Our results show that the g-C₂N/XSe₂ heterostructures have tunable electronic structures, which makes it a potential candidate for novel electronic devices.     

The structural, electronic, elastic and optical properties of AlCu(Se1−xTex)2 compounds from first-principle calculations

The structural, electronic structure, elastic and optical properties of the AlCu(Se_1−xTe_x)₂ compounds have been investigated by using a first-principles method based on density functional theory. The lattice constants of the quaternary compounds AlCu(Se_1−xTe_x)₂ increase with the increasing of Te composition. The calculated lattice constants for the ternary compounds i.e. AlCuSe₂ and AlCuTe₂ are in good agreement with the experimental data. The band structures show that the compounds have direct band gap and the band gaps are found to vary nonlinearly with composition. The total and part density of states of the quaternary AlCu(Se_1−xTe_x)₂ compounds are discussed. The calculated elastic constants indicate that all of the AlCu(Se_1−xTe_x)₂ compounds are mechanically stable. The bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν can be obtained by using the Voigt-Reuss-Hill averaging scheme. The B/G ratios of the AlCu(Se_1−xTe_x)₂ compounds indicate that AlCu(Se_0.8Te_0.2)₂ is ductile and the others are brittle. The Debye temperature of the AlCu(Se_1−xTe_x)₂ compounds decreases a little with increasing Te content except the compound with x = 0.4. The dielectric functions, refractive index, extinction coefficient, absorption spectrums and energy-loss function of the AlCuSe₂ and AlCuTe₂ are also calculated and discussed in this work.     

First principles study of structural,elastic and electronic structural properties of strontium chalcogenides

Structural, elastic and electronic properties of strontium chalcogenides SrX (X = O, S and Se) in the B1 (NaCl) and B2 (CsCl) phases were investigated in the present work. The calculations were performed using density functional theory (DFT) within generalized gradient approximation (GGA) using scalar relativistic Vanderbilt-type ultrasoft pseudopotentials. Results for structural properties of both phases, the pressure at which transition from B1 to B2 phase occurs and the volume compression ratio for each compound were reported. Elastic properties of the B1 phase of these compounds, such as elastic constants C₁₁, C₁₂, and C₄₄, shear modulus (G), Young's modulus (E), Poisson's ratio (σ), Kleinman parameter (ξ), and anisotropy factor (A) were also calculated at ambient conditions. The band gaps and density of states were studied too for the B1 structure of these compounds. The present results were compared with the available experimental and other theoretical results, and found to be in satisfactory agreement with them.