Electronic and optical properties of ZnSc2S4 and CdSc2S4 cubic spinels by the modified Becke–Johnson density functional

Structural, electronic and optical properties of the ZnSc₂S₄ and CdSc₂S₄ cubic spinels have been investigated by means of the full-potential (linearized) augmented plane wave plus local orbitals based on density functional theory. The exchange-correlation potential is treated by the GGA–PBEsol [J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett. 100 (2008) 136406] and the recently proposed modified Becke–Johnson potential approximation (mBJ) [F. Tran, P. Blaha, Phys. Rev. Lett. 102 (2009) 226401], which successfully corrects the band-gap problem found with GGA for a wide range of materials. The obtained structural parameters are in good agreement with the available experimental data. This gives support for the predict properties for ZnSc₂S₄ and CdSc₂S₄. The band structures reveal that both compounds are semiconductor with a direct gap. The obtained gap values show that mBJ is superior for estimating band gap energy. We have calculated the electron and hole effective masses in different directions. The density of states has been analyzed. Based on our electronic structure obtained using the mBJ method we have calculated various optical properties, including the complex dielectric function ɛ(ω), complex index of refraction n(ω), reflectivity coefficient R(ω), absorption coefficient α(ω) and electron energy-loss function L(ω) as functions of the photon energy. We find that the values of zero-frequency limit ɛ₁(0) increase with decreasing the energy band gap in agreement with the Penn model. The origin of the peaks and structures in the optical spectra is determined in terms of the calculated energy band structures.     

Structural, thermodynamic and optical properties of MgF2 studied from first-principles theory

A detailed theoretical study of structural, electronic, elastic, thermodynamic and optical properties of rutile type MgF₂ has been carried out by means of first-principles Density Functional Theory (DFT) calculations using plane wave pseudo-potentials within the local density approximation and generalized-gradient approximation for the exchange and correlation functionals. The calculated ground state properties and elastic constants agree quite well with experimental values. From the calculated elastic constants we conclude that MgF₂ is relatively hard when compared to other alkaline-earth fluorides and ductile in nature. The thermodynamic properties such as heat capacity, entropy, free energy, phonon density of states and Debye temperatures are calculated at various temperatures from the lattice dynamical data obtained through the quasi-harmonic Debye model. From free energy and entropy it is found that the system is thermodynamically stable up to 1200 K. The imaginary part of the calculated dielectric function ε₂(ω) could reproduce the six prominent peaks which are observed in experiment. From the calculated ε(ω), other optical properties such as refractive index, reflectivity and electron energy-loss spectrum are obtained up to the photon energy range of 30 eV.     

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.     

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.     

Structural phase transition and 5f-electrons localization of PuSe explored by ab initio calculations

An investigation into the structural phase transformation, electronic and optical properties of PuSe under high pressure was conducted by using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method, in the presence and in the absence of spin-orbit coupling (SOC). Our results demonstrate that there exists a structural phase transition from rocksalt (B 1) structure to CsCl-type (B 2) structure at the transition pressure of 36.3 GPa (without SOC) and 51.3 GPa (with SOC). The electronic density of states (DOS) for PuSe show that the f-electrons of Pu are more localized and concentrated in a narrow peak near the Fermi level, which is consistent with the experimental studies. The band structure shows that B 1-PuSe is metallic. A pseudogap appears around the Fermi level of the total density of states of B 1 phase PuSe, which may contribute to its stability. The calculated reflectivity R(ω) shows agreement with the available experimental results. Furthermore, the absorption spectrum, refractive index, extinction coefficient, energy-loss spectrum and dielectric function were calculated. The origin of the spectral peaks was interpreted based on the electronic structures.     

AC conductivity and dielectric properties of 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl)acetonitrile thin films

The dark AC conductivity and dielectric properties of thermally evaporated 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl)acetonitrile (DOPNA) thin films in sandwich structure employing symmetrical gold ohmic contacts have been investigated as function of temperature (303–443 K) and frequency (100 Hz–5 MHz). The AC conductivity, σ_AC(ω), is found to obey Jonscher’s universal power law, σ_AC(ω)=Aω^s (ω is the angular frequency). The AC conductivity of DOPNA thin films has been analyzed with reference to various theoretical models. The correlated barrier hopping is found to be the dominant conduction mechanism for charge carrier transport; the maximum barrier height, hopping length and the density of localized states are estimated. The temperature dependence of the AC conductivity shows Arrhenius type with two thermal activation energies. The activation energies are determined as a function of frequency. The behavior of the real and imaginary parts of the dielectric constant as a function of both temperature and frequency is discussed.     

Calculating Permittivity and Dielectric Loss Frequency Spectra for Aqueous Electrolyte Solutions

Analytic expressions for dielectric permittivity factor ε₁(ω) and dielectric dissipation factor ε₂(ω) of electrolyte solutions are obtained, based on the ratio between complex factors of dielectric permittivity and specific conductivity. The range of frequency dispersion of dynamic factors ε₁(ω) and ε₂(ω) for aqueous solutions of LiCl, NaCl, KCl, and CsCl is considered. Numerical calculations are performed for friction coefficients β _a and β _b ; relaxation times τ _a , τ _b , and τ _ab ; and factors ε₁(ω) and ε₂(ω) in a wide range of variation for ρ; concentration c; temperature T; and frequencies ω. The resulting theoretically calculated ε₁(ω) and ε₂(ω) values and the Cole–Cole diagram are in quantitative agreement with experimental data.     

Theoretical investigations of the elastic,electronic, optical and thermodynamics properties of SrSi

The crystal structural, electronic, optical and thermodynamic properties of SrSi are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). We have calculated the ground states properties and they are in good agreement with the available experimental data and other theoretical results. We have obtained the electronic structure and density of states, and the results showed that both of Immm and Cmcm phases are metal material. The elastic properties such as elastic constants, shear modulus, Young's modulus and Poisson's ratio are obtained for the first time. Furthermore, the optical properties are reported for radiation up to 30 eV. Finally, the thermodynamic properties of Cmcm phase such as free energy, entropy, enthalpy, heat capacity and Debye temperature are given for reference.     

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.     

Calculations of the frequency dependences of viscosity coefficients and elastic moduli for solutions of electrolytes over broad ranges of thermodynamic parameter variations

The dynamic equations obtained earlier for the coefficients of bulk η_V(ω) and shear η_S(ω) viscosities and bulk K(ω) and shear μ(ω) elastic moduli were used to calculate these values for a certain model of solution structure in the approximation of the theory of osmotic solution properties. The interparticle interaction potential was written as the sum of the potential of hard spheres and Coulomb attraction. The corresponding thermodynamic parameter values for the density ρ, temperature, concentration C, and adiabatic bulk modulus K _S were taken from experimental works. The results of numerical calculations of the viscoelastic properties of solutions over broad ranges of thermodynamic parameter and frequency variations were plotted and tabulated. The calculation results are compared with the available experimental data. The theoretical viscoelastic properties of solutions of electrolytes were in satisfactory agreement with the published experimental data and the results of numerical experiments for classic liquids.     

Equation of state for molten alkali metal alloys

Results on the density of binary and ternary alkali metal alloys of Cs–K, Na–K, Na–K–Cs, at temperatures from the freezing point up to several hundred degrees above the boiling point are presented. The theoretical equation of state is that of Ihm, Song, and Mason. The second virial coefficients, B(T), are calculated by using the corresponding states correlation of Boushehri and Mason. Calculation of the other two temperature-dependent parameters, α(T) and b(T), are performed by scaling rules with the latent heat of vaporization and the freezing point density as scaling constants. The results are within 5%.     

A MS-CASPT2 study of the low-lying electronic excited states of CH2BrCl

The low-lying singlet excited states of CH₂BrCl have been calculated using multiconfigurational CASSCF, second-order perturbation theory CASPT2 and its multistate extension MS-CASPT2. The CASSCF method shows spurious valence–Rydberg mixing and a wrong order of states. Inclusion of dynamical correlation by single root CASPT2 lowers dramatically the energy of the valences states but does not lead to a complete separation between valence and Rydberg states. This situation is improved by the MS-CASPT2 calculations, which gives two valence states for both A^′ and A″ symmetries below the lowest Rydberg state, corresponding to n(Br)→σ^*(C–Br) and n(Cl)→σ^*(C–Cl) transitions at 6.1 eV (203 nm) and 7.2 eV (173 nm), and being repulsive along C–Br and C–Cl coordinates.     

Numerical Calculation of Permittivity and Dielectric Loss of Aqueous KF Solution Depending on State Parameters

Based on the analytical expressions for permittivity ε₁(ω) and dielectric loss ε₂(ω) are obtained by the kinetic equation method, the frequency spectra of these coefficients are analyzed for an aqueous KF solution in a wide variation range of the density ρ, the concentration C, and the temperature T. With a certain choice of the solution model, the potential interaction energy Φab(|r|), and the radial distribution function g_ab(|r|) of a- and b-type ions, ε₁(ω) and ε₂(ω) of an aqueous KF solution are numerically calculated depending on ρ, C, T, and ω.     

Quantum dynamics of vibration–vibration energy transfer for vibrationally excited HF colliding with H2

The rate constants for H₂–HF energy transfer processes, especially for those in vibrationally excited states, are very demanding in astrophysics and chemical laser engineering, especially for those in vibrationally excited states. Based on our recent potential energy surface, we used the coupled-states approximation including the nearest neighboring Coriolis couplings with energy-based local basis set to perform dynamics calculation for the H₂–HF energy transfer system. Rate constants for vibrational transitions (1; 3) → (0; 4), (1; 3) → (2; 2), and (0; 3) → (1; 2) were obtained. For state-to-state rate constants, transitions that have no internal angular momentum gap dominate at high temperatures. The vibrational-resolved rate constant for (1; 3) → (0; 4) initially decreases and then increases with the temperature, while those for (1; 3) → (2; 2), and (0; 3) → (1; 2) transitions monotonically increase. The calculated rate constants are in good agreement with the available experimental results. These dynamical data can be further applied to the numerical simulation of hydrogen fluoride chemical laser. © 2018 Wiley Periodicals, Inc.     

Structural,electrical and optical properties of Lin@C20 (n = 1–6) nanoclusters

The current research was undertaken to investigate the structural, electrical, and optical properties of C₂₀ fullerene decorated with different numbers of lithium (Li) atoms on its surface. The stability of the structure increased as the number of lithium atoms increased. Increasing the number of lithium atoms around C₂₀ from one to four slightly increased the E_g (energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital). Increasing the number to five or six narrowed the E_g. The electrical properties such as ionization potential (I), electron affinity (A), chemical potential (μ), global hardness (η), global softness (γ), global electrophilicity (ω), and electronegativity (χ) were also calculated. The polarizability (α) and first hyperpolarizability (β₀), which correspond to the linear optical and nonlinear optical properties, respectively, were also calculated. An intense increase in β₀ was recorded as the effect of five Li atoms adsorbed onto the C₂₀ surface. The results of this study can be used to design and fabricate nanomaterials with adjustable electro-optical properties.     

Density functional theory studies on covalent functionalization of single-walled carbon nanotubes with benzenesulfonic acid

The calculated Raman spectra of the covalently functionalized zigzag single-walled carbon nanotubes, f-(n,0)-SWCNTs (n = 6–10), with benzenesulfonic acid showed that the radial breathing modes (RBMs) frequencies (ω_RBM) of the sidewall functionalized tubes are red shifted in comparison with the frequencies of their non-functionalized counterparts, (n,0)-SWCNTs. It is also observed that this ω_RBM red-shift is tube-diameter (d_t) dependent, where almost no redshifts are observed for the (10,0)-SWCNT. Moreover, many new Raman bands, which results from the functional group, appear in the low energy region, ranging from 100 to 600 cm⁻¹. In the high frequency region, resulting from the functional group, the Raman spectra of the f-(n,0)-SWCNTs exhibited a few new peaks above the tangential bands of the SWCNTs (ranging from 1580 to 1650 cm⁻¹), which entirely belongs to the CC stretching, including bending deformation of the CCC bonds, and rock of CH in phenyl group of the benzenesulfonic acid as well as many new Raman peaks dispersed through spectrum. The calculated IR spectra also exhibited many new peaks through spectra of the f-(n,0)-SWCNT when comparing these with corresponding IR spectrum of the isolated nanotube. Furthermore, the calculated vertical singlet-singlet electronic transitions of the f-(n,0)-SWCNTs significantly altered in the optical structure of the nanotube and exhibited charge transfer states for both the functionalized (8,0)- and (12,0)-SWCNTs. The small distances among the calculated dipole-allowed and forbidden electronic energy levels also suggested an internal crossing (IC) processes.     

Structural relaxations and energy effects of intramolecular motions inN,N-dimethylnitramine: A theoretical study

The equilibrium geometry of theN,N-dimethylnitramine molecule and changes in the energy and structural parameters due to the internal rotation of the nitro group and the inversion of the N atom in the amino fragment were calculated by the restricted Hartree-Fock (RHF) method and at the second-order Møller-Plesset (MP2) level of perturbation theory with inclusion of electron correlation using the 6–31 G^* and 6–31 G^** basis sets. The one-dimensional potential functions of these motions calculated at the RHF/6–31 G^* level were approximated by a truncated Fourier and power series, respectively. The frequencies of torsional and inversion transitions were determined by solving direct vibrational problems for a non-rigid model,i.e., taking into account the molecular geometry relaxation. The equilibrium conformation of the molecular skeleton ofN,N-dimethylnitramine is nonplanar. Transition states of the internal rotation of the nitro group and inversion of the amine N atom are characterized by pronounced concerted changes in its bond angles and the length of the N?N bond. In the MP2/6–31 G^* approximation, the height of the barrier to internal rotation calculated taking into account the difference in the zero-point vibrational energies is equal to 9.7 kcal mol^?1. Inversion in the amino fragment is accompanied by a relatively small energy change at the barrier height of ?1.0 kcal mol^?1 calculated in the same approximation.     

Comparative first-principles study of structural and optical properties of alkali metal azides

A comparative first-principles study of the structural and optical properties of the alkali metal azides has been performed with density functional theory within the generalized gradient approximation. The crystal structures of the alkali azides compare well with experimental data. Their ionic character is manifested by the closeness of their internitrogen distances to the calculated N-N bond length for the free azide ion. An analysis of electronic structure, charge transfer, and bond order shows that the alkali azides are all wide-gap insulators and ionic compounds. The energy band and density of states for lithium azide and alpha-sodium azide are very similar, while these for potassium azide, alpha-rubidium azide, and alpha-cesium azide are alike, but some modifications are observed with the increment of alkali metals' electropositivity. These changes are closely related to the differences of the crystal structures. The general shapes of the real and imaginary parts of the dielectric function, adsorption coefficient, and electron energy-loss spectra are quite similar. The peaks originate from the electron transitions from the alkali metal s and p states to the conduction band. Our calculated optical properties for the alkali azides are found to be in good agreement with available experimental data. The absorption spectra of the alkali azides show a number of absorption peaks, which are believed to be associated with different exciton states, in the fundamental absorption region. In general, the electron energy-loss spectra have two plasma frequencies.     

Long-range interaction and the dynamics of nonadiabatic transitions in collisions of the I2(E) molecule with inert gas atoms

A special form of perturbation theory based on intermolecular interaction was used to analyze the contributions to the interaction energy between a homonuclear diatomic molecule with dipole-coupled electronic states n and n′ and a neutral particle A. At large distances, the energy of the system in both states n and n′ is determined by the induction contribution similar to the induction interaction of particle A and a polar molecule. The presence of a constant electric moment of particle A gives an electrostatic contribution to the matrix element of the nonadiabatic coupling of the states n and n′. The equations obtained were specialized to describe the interaction of the iodine molecule excited to ion-pair states with an inert gas atom and used as corrections to the potential energy surfaces of this system constructed within the diatomics-in-molecule approximation. The dynamics of transitions between the ion-pair states of the I₂ molecule induced by collisions with the Ar and He atoms was calculated. The results demonstrate the importance of correctly including long-range interaction.