First-principles study of structural,elastic, electronic and optical properties of perovskites XCaH3 (X = Cs and Rb) under pressure

The stability, structural parameters, elastic constants, electronic and optical properties of perovskites CsCaH₃ and RbCaH₃ were investigated by the density functional theory. The calculated lattice parameters are in agreement with previous calculation and experimental data. The energy band structures, density of states, born-effective-charge and Mulliken charge population were obtained. The perovskites CsCaH₃ and RbCaH₃ present a direct band gap of 3.15 eV and 3.27 eV at equilibrium. The top of the valence bands reflects the s electronic character for both structures. Furthermore, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss spectrum, and dielectric function were calculated. The origin of the spectral peaks was interpreted based on the electronic structures. The static dielectric constant and refractive index are indeed, inverse proportional to the direct band gap.     

Research on optical band gap of the novel GeSe2–In2Se3–KI chalcohalide glasses

The glass-forming region of the GeSe₂–In₂Se₃–KI system was reported firstly. The dependence of physical, thermal and optical properties on compositions as formula of (1 ? x)(0.8GeSe₂–0.2In₂Se₃)–xKI (x = 0, 0.1, 0.2, 0.3) chalcohalide glasses was investigated. The allowed direct transition and indirect transition, and Urbach energy of samples were calculated according to the classical Tauc equation. The results show that the glass system has good thermal stability and that there is an obvious blue-shift at the visible absorbing cutting-off edge. When the dissolved amount of KI increased from 0 to 30 mol%, the direct optical band gap and the indirect optical band gap were in the range from 1.617 to 1.893 eV and 1.573 to 1.857 eV. With the decrease of the molar refraction the refractive index decreases, optical band gap and metallization criterion increase. The relationship between energy band gap and metallization criterion was analyzed and the optical properties of chalcohalide glasses were summarized.     

Optical properties of a-Se90In10−x Snx chalcogenide thin films before and after gamma irradiation

Amorphous Se₉₀In_10?xSn_x (x=2, 4, 6, and 8) thin films of thickness 1000 Å were prepared on glass substrates by the thermal evaporation technique. Optical parameters of the films were investigated, in the wavelength range 400–700 nm, before and after irradiation by 4, 8, and 12 kGy doses of γ-ray. The optical absorption coefficient α for as-deposited and gamma irradiated films was calculated from the reflectance R and transmittance T measurements, which were recorded at room temperature. From the knowledge of α, at different wavelengths, the optical band gap E_g was calculated for all compositions of Se–In–Sn thin films before and after gamma irradiation. Results indicate that allowed indirect optical transition is predominated in as-deposited and irradiated films. Besides, it is found that the band gap decreases with increasing Sn concentration and this is attributed to the corresponding decrease in the average single bond energy of the films. The band gap, after irradiation at different doses of γ-ray, was found to decrease for all compositions of the studied films. This post-irradiation decrease in the band gap was interpreted in terms of a bond distribution model.     

Structural,elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn2P2

The present study explores the structural, elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn₂P₂ using a pseudopotential plane-wave method in the framework of density functional theory within the generalized gradient approximation. The calculated lattice constants and internal coordinates are in very good agreement with the experimental findings. Independent single-crystal elastic constants as well as numerical estimations of the bulk modulus, the shear modulus, Young's modulus, Poisson's ratio, Pugh's indicator of brittle/ductile behaviour and the Debye temperature for the corresponding polycrystalline phase were obtained. The elastic anisotropy of BaIn₂P₂ was investigated using three different indexes. The calculated electronic band structure and the total and site-projected l-decomposed densities of states reveal that this compound is a direct narrow-band-gap semiconductor. Under the influence of hydrostatic pressure, the direct D–D band gap transforms into an indirect B-D band gap at 4.08 GPa, then into a B–Γ band gap at 10.56 GPa. Optical macroscopic constants, namely, the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, absorption coefficient and energy-loss function, for polarized incident radiation along the [100], [010] and [001] directions were investigated.     

An indirect-to-direct band gap transition of NaSbS2 via minor Ga doping: A theoretical study

The efficiency of NaSbS₂ is limited by its wide indirect band gap. Alloying is a very effective strategy to tune the band gap over a wide range for the mixed-anion NaSb(S,Se)₂ alloys. However, these compounds are still indirect band gap semiconductors. The influence of Ga doping on the structure, electronic, and optical properties of NaSbS₂ is studied for the first time. Our calculated results show that NaSbS₂ is an indirect band gap semiconductor, and the difference between the indirect and direct band gaps is less than 0.1 eV. Moreover, the forbidden transition is discovered for the fundamental direct bandgap of NaSbS₂. The results indicate that the NaSb_1-xGa_xS₂ alloys are predicted to be synthesized in the proper conditions. An indirect-to-direct band gap transition is observed from NaSbS₂ to NaSb_1-xGa_xS₂. The minor Ga doping (less than10 %) has little effect on the electronic and optical properties of NaSbS₂. Importantly, the weak transition of the fundamental direct bandgap is allowed for NaSb_1-xGa_xS₂. This study can provide a route to explore the high efficiency of novel based-NaSbS₂ materials.     

A new processing method of CaZn2(OH)6·2H2O powders: Photoluminescence and growth mechanism

In this communication, we report on the formation of calcium hexahydroxodizincate dehydrate, CaZn₂(OH)₆·2H₂O (CZO) powders under microwave-hydrothermal (MH) conditions. These powders were analyzed by X-ray diffraction (XRD), Field-emission gum scanning electron microscopy (FEG-SEM), ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns confirmed that the pure CZO phase was obtained after MH processing performed at 130 °C for 2 h. FEG-SEM micrographs indicated that the morphological modifications as well as the growth of CZO microparticles are governed by Ostwald-ripening and coalescence mechanisms. UV-vis spectra showed that this material have an indirect optical band gap. The pure CZO powders exhibited an yellow PL emission when excited by 350 nm wavelength at room temperature.     

Investigation of the phonon band gap effect on Raman-active optical phonons in SrMoO4 crystal

The phonon dispersions of SrMoO₄ crystal are calculated using the lattice dynamical calculations approach. Spontaneous Raman spectra in the SrMoO₄ were measured in the temperature range from 10 K to 295 K, and the temperature dependence of the linewidth of the B_g (95 cm⁻¹) and A_g (888 cm⁻¹) Raman modes was analyzed using the lattice dynamical perturbative approach. We found that different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the phonon spectrum resulting in different anharmonic interactions. The calculated temperature dependence of the linewidth of A_g (888 cm⁻¹) mode was well accounted for the experimental one by including both down-conversion by the cubic term and the dephasing by quartic term. The dephasing processes are increased only at high temperatures and the effect of dephasing is related to the size of a large phonon band gap.     

Structural and optical properties of sprayed Zn1−xMnxO films

In recent years the dilute magnetic semiconductors have received much attention due to the complementary properties of semiconductor and ferromagnetic behaviour. Zn_1−xMn_xO thin films have been synthesized by chemical spray pyrolysis at a substrate temperature of 400 °C with different manganese compositions that vary in the range, 0.0 ≤ x ≤ 0.25, on Corning 7059 glass substrates. The X-ray diffraction studies revealed that all the films were strongly oriented along the (002) orientation corresponding to the hexagonal wurtzite structure. The crystalline quality of the layers was found to decrease with the increase of x, however, no structural changes were observed over the ‘Mn’ composition range investigated. The optical absorption studies revealed that the energy band gap of the films followed the Vegard's law. The optical band gap of the films prepared at x = 0.15 was found to be ∼3.35 eV. The photoluminescence characteristics of Zn_1−xMn_xO films showed an emission peak at around 390 nm with a broad band about 530 nm. The details of these results were reported and discussed.     

Optical,structural and electrochromic studies of molybdenum oxide thin films with nanorod structure

The MoO₃ thin films were prepared via sol–gel dip coating method on glass and FTO glass substrate. The optical and other properties of multilayered MoO₃ films with 2–10 layers were investigated. The MoO₃ films were studied using UV–Visible transmission, XRD, SEM, FTIR and Cyclic Voltammetry (CV) measurements. The band gap value for MoO₃ films was evaluated and in the range of 3.2 eV–3.72 eV. The XRD spectrum reveals that the crystallinity increases along the (020) and (040) planes with the increase in thickness. The SEM images showed the formation of nanorods upto six layers. The FTIR spectrum confirms the formation of MoO₃. The 6 layered films show the maximum anodic (spike)/cathodic (peak) diffusion coefficient of 18.84/1.701 × 10^?11 cm²/s. The same film exhibits the change in optical transmission of 49% with the bleached/coloured state transmission of 62/13%.     

Optical,piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications

Single crystals of pure and xylenol orange (XO; C₃₁H₃₂N₂O₁₃S) dye doped (0.01, 0.05 and 0.1 mol%) ammonium dihydrogen phosphate (ADP; NH₄H₂PO₄) were grown by slow evaporation method with the vision to improve the properties of pure ammonium dihydrogen phosphate crystal. The theoretical morphology of the grown crystals was drawn using Bravais–Friedel–Donnay–Harker (BFDH) law. The selective nature of xylenol orange dye to selectively stain the particular growth sectors of ADP crystal was studied. The structural analysis of as grown crystals was carried out using powder XRD study. The identification of the functional groups present in the ADP material was done using Fourier transform infrared (FTIR) spectroscopy. The linear optical study on pure and dye doped crystals was carried out using UV–vis–NIR spectroscopy. The optical band gap, extinction coefficient, refractive index and optical conductivity were calculated using the transmittance spectra for all the samples. In photoluminescence studies, the blue emission intensity got quenched and an orange emission at 597 nm was seen as a result of XO doping. The thermal stability and decomposition temperature of ADP crystal were found to decrease as an effect of dye doping. The piezoelectric charge coefficient, SHG conversion efficiency, mechanical strength and wettability were also enhanced as a result of XO dye doping.     

Structural,elastic, electronic and thermal properties of the cubic perovskite-type BaSnO3

First-principles calculations are performed to investigate the structural, elastic, electronic and thermal properties of the cubic perovskite-type BaSnO₃. The ground-state properties are in agreement with experimental data. The independent elastic constants, C₁₁, C₁₂ and C₄₄, are calculated from direct computation of stresses generated by small strains. A linear pressure dependence of the elastic stiffnesses is found. From the theoretical elastic constants, we have computed the elastic wave velocities along [100], [110] and [111] directions. The shear modulus, Young's modulus, Poisson's ratio, Lamé’s coefficients, average sound velocity and Debye temperature are estimated in the framework of the Voigt-Reuss-Hill approximation for ideal polycrystalline BaSnO₃ aggregate. Using the sX-LDA for the exchange-correlation potential, the calculated indirect fundamental band gap value is in very good agreement with the measured one. The analysis of the site-projected l-decomposed density of states, charge transfer and charge density shows that the bonding is of ionic nature. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the temperature effect on the lattice constant, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature is calculated.     

Thickness dependent physical properties of close space evaporated In2S3 films

In recent years, In₂S₃ is considered as a promising buffer layer in the fabrication of heterojunction solar cells. Film thickness is one of the important parameters that alters the physical characteristics of the grown layers significantly. The effect of film thickness on the structural, morphological, optical and electrical properties of close space evaporated In₂S₃ layers has been studied. In₂S₃ thin films with different thicknesses in the range, 100–700 nm were deposited on Corning glass substrates at a constant substrate temperature of 300 °C. The films were polycrystalline exhibiting strong crystallographic orientation along the (103) plane. The deposited films showed mixed phases of both cubic and tetragonal structures up to a thickness of 300 nm. On further increasing the film thickness, the layers showed only tetragonal phase. With increase of film thickness, both the crystallite size and surface roughness in the films were found to be increased. The optical constants such as refractive index and extinction coefficient of the as-grown layers have been calculated from the optical transmittance data in the wavelength range, 300–2500 nm. The optical transmittance of the films was decreased from 82% to 64% and the band gap varied in the range, 2.65–2.31 eV with increase of film thickness. The electrical resistivity as well as the activation energy was evaluated and found to decrease with film thickness. The detailed study of these results was presented and discussed.     

Investigation of structural,optical and luminescent properties of sprayed N-doped zinc oxide thin films

N-doped ZnO (NZO) thin films are synthesized via spray pyrolysis technique in aqueous medium treating zinc acetate and N,N-dimethylformamide as precursors. Influence of N doping on structural, optical and luminescence properties have been investigated. Films are nanocrystalline having hexagonal crystal structure. Raman analysis depicts an existence of NZnO structure in NZO thin film. XPS spectrum of N 1s shows the 400 eV peak terminally bonded, well screened molecular nitrogen (γ-N₂). Lowest direct band gap of 3.17 eV has been observed for 10 at% NZO thin film. The UV, blue, and green deep-level emissions in photoluminescence of NZO films are due to Zn interstitials and O vacancies.     

Synthesis,spectroscopic, structural and thermal characterizations of [(C7H6NO4)2TeBr6·4H2O]

Tellurium (IV) complexes with pyridine-2,6-dicarboxylate ligand were synthesized by slow evaporation from aqueous solutions yielding a new compound: [(C₇H₆NO₄)₂TeBr₆·4H₂O]. The structure of this compound was solved and refined by single-crystal X-ray diffraction. The compound is centrosymmetric P2₁/c (N°: 14) with the parameters a = 8.875(5) Å, b = 15.174(5) Å, c = 10.199(5) Å, β = 94.271° (5) and Z = 2. The structure consists of isolated H₂O, isolated [TeBr₆]^2? octahedral anions and (pyridine-2,6-dicarboxylate) [C₇H₆NO₄]⁺ cations. The stability of the structure was ensured by ionic and hydrogen bonding contacts (N–H?Br and O–H?Br) and Van-Der Walls interaction. The thermal decomposition of the compound was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The FTIR and Raman spectroscopy at different temperatures confirm the existence of vibrational modes that correspond to the organic, inorganic and water molecular groups. Additionally, the UV–Vis diffuse reflectance spectrum was recorded in order to investigate the band gap nature. The measurements show that this compound exhibits a semiconducting behavior with an optical band gap of 2.66 eV.     

Role of nitrogen in evolution of sp2/sp3 bonding and optical band gap in hydrogenated carbon nitride

Drastic changes in the bonding are found in amorphous hydrogenated carbon nitride (a-CNx:H) film as a function of nitrogen concentration (or N/C ratio). The total C-sp³ fraction and hardness shows a sharp decrease (at N/C = 0.40) whereas optical band gap and resistivity shows a gradual increase as nitrogen concentration increases from 0.07 to 0.58. Raman spectrum of a-CNx:H film is fitted with both Gaussian (integrated intensity ratios are used instead of the height ratios of the Lorentzian (D mode)) and Breit–Wigner–Fano (BWF, G Mode) method for a comparative study of optical properties and crystalline size of the graphite domain. Visible Raman (488 nm) spectroscopy finds that the in-plane crystalline size of graphite domains (L_a) is increased (from 34 to 38 Å) with nitrogen incorporation. Optical band gap of a-CNx:H solid measured by means of ellipsometry differs from the one obtained from Raman spectroscopy. In addition, we propose a simple extension of the existing band gap model to obtain the optical band gap of a-CNx:H film from Raman spectrum. Our estimation agrees well with the experimental value.     

First-principles study of structural,elastic, electronic and vibrational properties of BiCoO3

We used density functional theory (DFT) to study the structural, elastic, electronic, and lattice dynamical properties of tetragonal BiCoO₃ applying the “norm-conserving” pseudopotentials within the local spin density approximation (LSDA). The calculated equilibrium lattice parameters and atomic displacements are in agreement with the available experimental and theoretical results. Moreover, the structural stability of tetragonal BiCoO₃ were confirmed by the calculated elastic constants. In addition, the elastic properties of polycrystalline aggregates including bulk, shear and Young's moduli, and Poisson's ratio are also determined. The electronic band structure, total and partial density of states (DOS and PDOS) with ferromagnetic spin configuration are obtained. The results show that tetragonal BiCoO₃ has an indirect band gap with both up- and down-spin configurations and its bonding behavior is of covalent nature. We compute Born effective charge (BEC) which is found to be quite anisotropic of Bi, Co and O atoms. The infrared and Raman active phonon mode frequencies at the Г point are found. The phonon dispersion curves exhibit imaginary frequencies which lead from the high-symmetry tetragonal phase to low-symmetry rhombohedral phase in BiCoO₃. The six independent elastic constants, including bulk, shear and Young's moduli, and Poisson's ratio, complete BEC tensor and phonon dispersion relations in tetragonal BiCoO₃ are predicted for the first time. Results of the calculations are compared with the existing experimental and theoretical data.     

Electronic structure and Raman spectroscopy study of dibarium magnesium orthoborate,Ba2Mg(BO3)2

The samples of dibarium magnesium orthoborate Ba₂Mg(BO₃)₂ were synthesized by solid-state reaction. The X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Electronic structure and vibrational spectroscopy of Ba₂Mg(BO₃)₂ were systematically investigated by first principle calculation. A direct band gap of 4.4 eV was obtained from the calculated electronic structure results. The top valence band is constructed from O 2p states and the low conduction band mainly consists of Ba 5d states. Raman spectra for Ba₂Mg(BO₃)₂ polycrystalline were obtained at ambient temperature. The factor group analysis results show the total lattice modes are 5E_u + 4A_2u + 5E_g + 4A_1g + 1A_2g + 1A_1u, of which 5E_g + 4A_1g are Raman-active. Furthermore, we obtained the Raman active vibrational modes as well as their eigenfrequencies using first-principle calculation. With the assistance of the first-principle calculation and factor group analysis results, Raman bands of Ba₂Mg(BO₃)₂ were assigned as E_g (42 cm⁻¹), A_1g (85 cm⁻¹), E_g (156 cm⁻¹), E_g (237 cm⁻¹), A_1g (286 cm⁻¹), E_g (564 cm⁻¹), A_1g (761 cm⁻¹), A_1g (909 cm⁻¹), E_g (1165 cm⁻¹). The strongest band at 928 cm⁻¹ in the experimental spectrum is assigned to totally symmetric stretching mode of the BO₃ units.     

Isobaric specific heat capacity of {xCH3OH + (1 − x)H2O} with x = (1.0000, 0.7943, 0.4949, 0.2606, 0.1936, 0.1010, and 0.0496) at T = (280, 320, and 360) K in the pressure range from (0.1 to 15) MPa

Measurements of the isobaric specific heat capacities for {xCH₃OH + (1 − x)H₂O} with x = (1.0000, 0.7943, 0.4949, 0.2606, 0.1936, 0.1010, and 0.0496) were carried out by the calorimeter with the thermal relaxation method, which we have developed, at T = (280, 320, and 360) K in the pressure range from (0.1 to 15) MPa. The present c_p measurements for (methanol + water) show mole fraction dependence at constant temperature with the maximum, and the maximum shifts to greater values of mole fraction with increasing temperature. Pressure dependence of the present measurements is insignificant. Temperature dependence increases with increasing mole fraction.     

Ab initio study of the bonding and elastic properties of Ti2CdC

The chemical bonding and elastic properties of Ti₂CdC were investigated by means of a first-principles pseudopotential total energy method. The calculated results for the lattice constants and internal coordinate agree with experimental values very well. Ti₂CdC is conducting, and the Cd-d states have little effect on the chemical bonding. The elastic properties were estimated from the individual elastic constants by Voigt approximation. The calculated shear-modulus of Ti₂CdC, 70 GPa, is the lowest value among all MAX phases. The lower shear-modulus and shear-modulus-to-bulk-modulus ratio are related to the weaker Ti–Cd bond, which indicates the lower coefficient of friction. This suggests that Ti₂CdC would be a potential electrical frictional material.