Theoretical prediction of the structural, electronic and optical properties of SnB2O4 (B = Mg, Zn, Cd)

The structural, electronic and optical properties of the cubic spinels SnB₂O₄, with B = Mg, Zn and Cd, were studied by means of the full-potential (linear) augmented plane wave plus local orbitals method within the local density and generalized gradient approximations for the exchange-correlation potential. The Engel-Vosko form of the generalized gradient approximation (EV-GGA), which better optimizes the potential for the band structures, was also used. The results of bulk properties, including lattice constants, internal parameters, bulk moduli and their pressure derivatives are in good agreement with the literature data. The band structures show a direct band gap (Γ-Γ) for the three compounds. The computed band gaps using the EV-GGA show a significant improvement over the more common GGA. All the calculated band gaps increase with increasing pressure and fit well to a quadratic function. Analysis of the density of states revealed that the lowering of the direct gap (Γ-Γ) from SnMg₂O₄ to SnZn₂O₄ to SnCd₂O₄ can be attributed to the p-d mixing in the upper valence band of SnZn₂O₄ and SnCd₂O₄. We present calculations of the frequency-dependent complex dielectric function ?(ω). We find that the values of zero-frequency limit ?₁(0) increase with decreasing the energy band gap. The origin of the peaks and structures in the optical spectra is determined in terms of the calculated energy band structures.     

Synthesis and characterization of Mg2B2O5

Magnesium borate of the form Mg₂B₂O₅ has been prepared and its structural and thermal properties were studied using X-ray diffraction and differential thermal analysis. An investigation of the electrical and optical properties of Mg₂B₂O₅ system has been carried out. The electrical resistivity of the sample was measured in the temperature range of 170-400 K. The data analysis revealed an extrinsic nature of the conductivity with two impurity levels located at 0.13 and 0.71 eV in the temperature ranges of 170-230 K and 240-400 K, respectively. The optical transmission and reflection was recorded at 300 K in the incident photon energy range of 3.0-6.0 eV. The absorption coefficient data analysis revealed an indirect optical energy band gap of 4.73 eV. In addition, two impurity levels located at 3.43, and 4.49 eV were observed in the absorption spectra.     

Effect of substrate temperatures on the optical properties of evaporated Sc2O3 thin films

Scandium oxide (Sc₂O₃) films were deposited by electron beam evaporation with substrate temperatures varying from 50 to 350 °C. X-ray diffraction, scanning electron microscopy, spectrometer, and optical profilograph were employed to investigate the structural and optical properties of the films. The refractive index and extinction coefficient were calculated from the transmittance and reflectance spectra, and then the energy band gaps were deduced and discussed. Laser induced damage threshold of the films were also characterized. Optical and structural properties of Sc₂O₃ films were found to be sensitive to substrate temperature.     

Structural, elastic, electronic, chemical bonding and thermodynamic properties of CaMg2N2 and SrMg2N2: First-principles calculations

We report first-principles density functional theory calculations of the structural, elastic, electronic, chemical bonding and thermodynamic properties of the ternary alkaline earth metal nitrides CaMg₂N₂ and SrMg₂N₂. The calculated equilibrium structural parameters agree well with the experimental findings. Single-crystal and polycrystalline elastic constants and some related properties under pressure effect have been predicted. Both compounds exhibit a striking elastic anisotropy and a ductile behavior. Electronic properties and chemical bonding nature have been studied throughout the band structure, density of states and charge distribution analyses. It is found that these two materials have a direct band gap (Γ-Γ) and a transition to an indirect gap (Γ-M) occurs at about 8.63 and 5.16 GPa in CaMg₂N₂ and SrMg₂N₂, respectively. The chemical bonding has a mixture covalent-ionic character. Thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Influence of mixed alkali on the spectral properties of vanadyl ions doped xNa2O-(30 − x)K2O-60B2O3 glasses—an EPR and optical study

Electron paramagnetic resonance (EPR) and optical investigations have been performed in the mixed alkali borate xNa₂O-(30 − x)K₂O-60B₂O₃ (5 ≤ x ≤ 25) glasses doped with 10 mol% of vanadyl ions in order to look for the effect of ‘mixed alkalis’ on the spectral properties of the glasses. The observed EPR spectra have structures for x > 5 mol% which are characteristic of a hyperfine interaction arising from an unpaired electron with the ⁵¹V nucleus and it builds up in intensity as x increases. It is observed that the mixed alkali play a significant role in accommodating the vanadyl ions in these mixed alkali glasses and for x > 5 mol%, shows a well resolved hyperfine structure typical for isolated VO²⁺ ions. The spin-Hamiltonian parameters (g and A), the dipolar hyperfine coupling parameter (P) and Fermi contact interaction parameter (k) have been evaluated. It is observed that the spin-Hamiltonian parameters do not vary much with the change in composition. It is observed that with increase of x, an increase occurs in tetragonal distortion for VO²⁺. The number of spins (N) participating in resonance and the paramagnetic susceptibility (χ) have been calculated. It is observed that N and χ increase with x. The optical bandgap energies evaluated for these glasses slightly increase with x and reach a maximum around x = 20 and thereafter decrease showing the mixed alkali effect. Optical band gap energies obtained in the present work vary from 2.73 to 3.10 eV for both the direct and indirect transitions. The physical parameters of the glasses are also determined with respect to the composition.     

First-principles calculations of structural, electronic, and elastic properties of MgZrSi2O7

For the first time, the recently synthesized pyrochlore MgZrSi₂O₇ [J. Xu et al., Mater. Chem. Phys. 128 (2011) 410] has been analyzed using the first principles calculations. The electronic and elastic properties were predicted; in particular, the band gap is indirect and has the value of 6.75 eV. The bulk modulus equals to 186.51 ± 1.95 GPa. Anisotropy of elastic properties was analyzed by comparing the upper and lower estimates of the shear moduli. In addition, directional dependence of the Young's modulus was calculated and visualized; its value varies in the range from 249.7 GPa (along the a, b, c crystallographic axes) to 136.84 GPa (along the bisector direction in any of the ab, bc, ac planes).     

Nanocomposite thin films of Ga5Sb10Ge25Se60 chalcogenide glass for optical limiting applications

We report a low-cost, scalable method to fabricate optical grade composite thin films for nonlinear optical applications. The transmission and reflection spectra of prepared Ga₅Sb₁₀Ge₂₅Se₆₀/PVA composite films were investigated. Optical band gap of the thin films were calculated using Tauc’s extrapolation method. The band gap of the nanocomposite thin films were found to be tunable depending on the grain size of the films. Nonlinear optical characterization of samples were studied by the Z-scan technique using an Nd:YAG laser (532 nm, 7 ns, 10 Hz). Sequential Z-scan traces were made in four regimes of intensity for films with three different grain sizes. The Z-scan spectra reveal a strong nonlinear absorption depending on the grain size of the films suggesting that the new materials are promising candidates for the development of nonlinear optical devices and are extremely perspective as optical limiters of intense short pulse radiation.     

Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell

The phase formation, morphology and luminescent properties of ZnNb₂O₆ nanocrystals by the sol-gel method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb₂O₆ grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20-40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]⁷⁻. The optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV.     

Elastic properties and spectroscopic studies of fast ion conducting Li2OZnOB2O3 glass system

Glass systems of the composition xLi₂O-20ZnO-(80 − x)B₂O₃ where (x = 5, 10, 15, 20, 25 and 30 mol%) have been prepared by melt quenching technique. Elastic properties, ¹¹B MAS-NMR and IR spectroscopic studies have been employed to study the structure of Li₂O-ZnO-B₂O₃ glasses. Elastic properties have been investigated using sound velocity measurements at 10 MHz. Elastic moduli reveal trends in their compositional dependence. The bulk modulus and shear modulus increases monotonically with increase of BO₄ units, which increase the dimensionality of the network. ¹¹B MAS-NMR and IR spectra show characteristic features of borate network and compositional dependent trends as a function of Li₂O/ZnO concentration. The results are discussed in view of borate network and the dual structural role of Zn²⁺ ions. The results indicate that the Zn²⁺ are likely to occupy network-forming positions in this glass system.     

Electronic and optical properties of the orthorhombic compounds FeX2 (X = P,As and Sb)

First principles calculations, by means of the full-potential linearized augmented plane wave (FP-LAPW) method within the local density approximation (LDA), were carried out for the structural, electronic and optical properties of the orthorhombic compounds FeP₂, FeAs₂ and FeSb₂. The structural properties are determined through the total energy minimization and the relaxation of the internal parameters. The modified Becke–Johnson (mBJ) method is applied for the electronic structure of FeSb₂. Our LDA-calculation shows that the first two compounds are indirect-gap semiconductors, while for the third one it predicts a small hole-pocket at the R point. The mBJ gives a semiconducting state with an indirect energy gap of 0.248 eV for FeSb₂. The overall shape of the calculated imaginary parts of the dielectric tensor is similar for the three compounds. The assignment of the structures in the optical spectra and band structure transitions are investigated. The electronic dielectric constant along (0 1 0) direction is the largest for the three compounds. For FeAs₂, the calculated components of reflectivity have the same trend of variation as the measured ones in the energy range 1.54–3.1 eV.     

FP-APW + lo study of the elastic, electronic and optical properties of the filled skutterudites CeFe4As12 and CeFe4Sb12

Using a full-relativistic version of the full-potential augmented plane wave plus local orbitals (FP-APW + lo) method within the local density approximation (LDA), we have studied the elastic, electronic and optical properties of the filled skutterudites CeFe₄As₁₂ and CeFe₄Sb₁₂. Structural parameters, including lattice constant, internal free parameters and, bulk modulus and its pressure derivative were calculated. We have determined the full set of first-order elastic constants, Young’s modulus, Poisson’s ratio and the Debye temperature of these compounds. Band structures, density of states, pressure coefficients of energy band gaps are also given. It is found that both CeFe₄As₁₂ and CeFe₄Sb₁₂ are indirect band gap semiconductors. The valence band maximum (VBM) is located at Γ point, whereas the conduction band minimum (CBM) is located at N point. Optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss were calculated for radiation up to 30 eV. This is the first quantitative theoretical prediction of the elastic and optical properties for these compounds, and it still awaits experimental confirmation.     

Research on the novel GeSe2-In2Se3-KBr chalcohalide optic glasses

(1 − x)(0.75GeSe₂-0.25In₂Se₃) − x(1/3In₂Se₃-2/3KBr) (x = 0, 0.15, 0.3, 0.45, 0.525) chalcohalide glasses were prepared by traditional melt-quenching method and its glass-forming region was determined. The physical, thermal and optical properties of the GeSe₂-In₂Se₃-KBr ternary glass system are reported. The results show that the GeSe₂-In₂Se₃-KBr glass system has relatively high glass transition temperature (T_g = 286-335 °C) and good thermal stability. With increasing KBr content from 0 to 35 mol%, a blue-shift from 750 to 620 nm at the visible absorbing cutting-off edge is observed. The red-shifting of the transmission cutting-off edge at the long-wave IR band occurs linearly with increasing KBr. The allowed direct transition and indirect transition of samples were calculated according to the classical Tauc equation. The direct optical band gaps and indirect optical band gaps were in the range from 1.649 to 1.931 eV and 1.513 to 1.863 eV, respectively.     

Structural, optical and electrical properties of reactively sputtered Ag2Cu2O3 films

Ag₂Cu₂O₃ thin films were deposited on glass substrates by RF magnetron sputtering of an equiatomic silver-copper target (Ag_0.5Cu_0.5) in reactive Ar-O₂ mixtures. The reactive sputtering was done at varying power, oxygen flow rate and deposition temperature to study the influence of these parameters on the deposition of Ag₂Cu₂O₃ films. The film structure was determined by X-ray diffraction, while the optical properties were examined by spectrophotometry (UV-vis-NIR) and photoluminescence. Furthermore, the film thickness and resistivity were measured by tactile profilometry and 4-point probe, respectively. Additional mobility, resistivity and charge carrier density Hall effect measurements were done on a few selected samples. The best films in terms of stoichiometry and crystallography were achieved with a sputtering power of 100 W, oxygen and argon flow rates of 20 sccm (giving a deposition pressure of 1.21 Pa) and a deposition temperature of 250 °C. The optical transmittance and photoluminescence spectra of films deposited with these parameters indicate several band gaps, most prominently, a direct one of around 2.2 eV. Electrical characterization reveals charge carrier concentrations and mobilities in the range of 10²¹-10²² cm^− 3 and 0.01-0.1 cm²/Vs, respectively.     

Ca3Co4O9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties

Ca₃Co₄O₉ (349) thermoelectric (TE) oxide ceramics were successfully prepared by Spark Plasma Sintering process. The effects of the uniaxial pressure (30-100 MPa), the dwell temperature (700-900 °C) and the cooling rate were investigated. Microstructure analyses have revealed strong enhancements of the bulk density as the pressure level and the applied temperature during the SPS process are increased. Mechanical properties were investigated by using instrumented nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness were shown to improve drastically and depend on the processing parameters. Thermal expansion measurements reveal a noticeable anisotropy induced by unidirectional hot pressing. The mechanical, thermal and thermoelectric properties were correlated to the microstructure and crystallographic texture of the resulting ceramics.     

Elastic properties and Vickers hardness of optically transparent glass-ceramics with fresnoite Ba2TiSi2O8 nanocrystals

Optically transparent glass-ceramics (40BaO-20TiO₂-40SiO₂ (mol%)) consisting of nonlinear optical fresnoite Ba₂TiSi₂O₈ (BTS) nanocrystals (diameter: 100-200 nm) are fabricated, and their elastic properties and deformation behavior are examined as a function of the volume fraction (f) of BTS nanocrystals using cube resonance and Vickers indentation techniques. The elastic properties such as Young's modulus (E) increases linearly with increasing the volume fraction of nanocrystals, e.g., E = 84 GPa for f = 0% (glass) to E = 107 GPa for f = 54.5%. The Vickers hardness (H_v) and indentation fracture toughness (K_c) increase from 5.0 to 6.0 GPa for H_v and 0.48 to 1.05 MPa m^−1/2 for K_c with increasing the volume fraction (from f = 0% to f = 54.5%), but they do not change linearly against the volume fraction of nanocrystals. It is suggested that BTS nanocrystals themselves induce a high resistance against deformation during Vickers indenter loadings.     

Observation of surface plasmon resonance of silver particles and enhanced third-order optical nonlinearities in AgCl doped Bi2O3-B2O3-SiO2 ternary glasses

Bi₂O₃-B₂O₃-SiO₂ ternary glasses embedded with Ag nanoparticles were prepared by introducing AgCl into the bismuthate glasses using conventional melt quenching method and characterized by several experimental techniques. Scanning electron microscopic studies indicated the formation of Ag contained nanoclusters which crack and become regular with increase of AgCl content in these composites. Optical absorption spectra of the nanocomposites showed the presence of absorption band of surface plasmon resonance (SPR) due to Ag nanoparticles at ∼600 nm. Z-scan measurement with femtosecond laser was used to investigate third-order optical nonlinearities of the nanocomposites. The results show that the nonlinear refraction γ was dramatically increased up to 30 times by the appearance of Ag nanoparticles when excited within its SPR region, while nonlinear absorption due to two-photon absorption exhibited opposite tendency or even saturated behavior. The calculation of figure of merit suggests that the Ag particle embedded Bi₂O₃-B₂O₃-SiO₂ glass composites are promising candidates for optoelectronic devices.     

Formation and properties of AgInSe2 thin films by co-evaporation

The electrical and optical properties of silver indium selenide thin films prepared by co-evaporation have been studied. X-ray diffraction indicates that the as prepared films were polycrystalline in nature. The lattice parameters were calculated to be a=0.6137 and b=1.1816 nm. Composition was determined from energy dispersive analysis of X-ray. Silver indium selenide thin films were also prepared by bulk evaporation of powdered sample for comparative study. They have an optical band gap (E_g) of 1.25 eV and it is a direct allowed transition. Refractive index (n) and extinction coefficient (k) were calculated from absorption and reflection spectra. Steady-state photoconductivity was measured from 300 to 400 K. Carrier lifetime was calculated from transient photoconductivity measurements at room temperature at different intensities of illumination.     

Structural,thermodynamic, electronic,and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3 GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3 ± 0.9 GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature Θ_D, heat capacity C_V, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700 K and pressure ranges from 0 to 32 GPa and 33 to 100 GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ?(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.