Ab-initio study of the electronic structure and optical properties of KNO3 in the ferroelectric phase

The electronic energy band structure, partial (PDOS) and total densityof states (DOS), and linear optical properties of the paraelectricKIO3 single crystal are calculated using density functional theory (DFT) in its localdensity approximation (LDA). The calculated band structure for paraelectricKIO3 indicates that the crystal has a direct bandgap. Using LDA and generalized gradient approximation(GGA), structural optimization has been performed. The optical spectra of the paraelectricKIO3 in the photon energy range up to 30 eV are investigated under the scissor approximation. Thereal and imaginary parts of the frequency-dependent linear dielectric function, as well asrelated quantities such as energy-loss function, refractive index and effective number of valenceelectrons, are calculated. The calculated structural optimization and bandgap of the paraelectricKIO3 have been compared with experimental data and have been found to be in good agreementwith the experimental results.

The first-principle investigation of SiFe2O4 (SFO) spinel was performed with the help of a plane-wave pseudopotential technique within the generalized gradient approximation (GGA) and local density approximation (LDA) as implemented in Quantum Espresso Simulation package. The Electronic band structure and optical properties of SFO spinel-type material have been investigated and discussed in this paper. The calculated band structure reveals that SFO spinel-type material is a direct bandgap semiconductor. Using GGA + U and LDA + U the band gap value so obtained is 3.52 eV and 2.96 eV respectively. The contribution to valence and conduction bands due to different bands was analyzed on the basis of the total and partial density of state. The Optical properties of SFO spinel-type material have been calculated and discussed in detail. The real, and the imaginary, part of the complex dielectric constants is found to be 6.52 and 5.42 at energies of 3.44 eV and 6.21 eV respectively. The refractive index and the reflectivity index at zero energy value were found to be 1.88 and 10% respectively. We found that SFO spinel-type material has good properties for optical devices.

The electronic energy band structure and linear optical properties of the ferroelectricsemiconductor SbSI in the paraelectric phase are calculated by an ab initio pseudopotentialmethod using density functional theory in the local density approximation. Thecalculated electronic band structure shows that SbSI has an indirect band gap of1.45 eV and that the smallest direct gap is at the S point of the Brillouin zone(1.56 eV). The total density of states has been analysed. The linear energy dependentdielectric functions and some optical constants such as the absorption coefficient,extinction coefficient, refractive index, energy-loss function, reflectivity and opticalconductivity, including self-energy effects, are calculated. The effective number ofvalence electrons and the effective optical dielectric constant are also calculated.

The electronic-energy band structure, site and angular momentum decomposed density of states (DOS) and charge-density contours of perovskite CaTiO3 are calculated by the first principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows an indirect (R-Γ) band gap of 1.5 eV. The total DOS as well as the partial density of states (PDOS) are compared with the experimental photoemission spectra. The calculated DOS are in reasonable agreement with the experimental energy spectra and the features in the spectra are interpreted by a comparison of the spectra with the PDOS. The origin of the various experimentally observed bands have been explained. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ca and TiO3 is mainly ionic and that the TiO3 entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of CaTiO3. The real and imaginary parts of the dielectric function and hence the optical constants such as refractive index and extinction coefficient are calculated. The calculated spectra are compared with the experimental results for CaTiO3 and are found to be in good agreement with the experimental results. The effective number of electrons per unit cell participating in the interband transitions are calculated. The role of band structure calculation as regards the optical properties of CaTiO3 is discussed.

Electronic band structure, optical and thermodynamic properties of ternary hydrides MBeH3 (M = Li, Na, and K) were studied using ab initio density functional theory (DFT). The effect of the adopted approximation to the exchange-correlation functional of the DFT is explicitly investigated by considering four different expressions of two different classes (local-density approximation and generalized-gradient approximation). The calculated magnitude of B classifies MBeH3 (M = Li, Na, and K) as easily compressible materials. The bonding interaction in these compounds is quite complicated. The interaction between M and BeH6 is ionic and that between Be and H comprises both ionic and covalent characters. The electronic structure of the complex hydride was investigated by calculating the partial and total densities of states, and electron charge density distribution. Large gaps in the density of states appear at the Fermi energy of LiBeH3, NaBeH3, and KBeH3 indicating that these classes of hydrides are insulators. Optical properties, including the dielectric function, reflectivity, and absorption coefficient, each as a function of photon energy, are calculated and show an optical anisotropy for LiBeH3 and KBeH3. Through the quasi-harmonic Debye model, in which the phononic effects are considered, temperature dependence of volume V(T), bulk modulus B(T), and thermal expansion coefficient α(T), constant-volume and constant-pressure specific heat (Cv and Cp) and Debye temperature ΘD, the entropy S, and the Grüneisen parameter γ were calculated at wide pressure and temperature ranges. The principal aspect of the obtained results is the close similarity of MBeH3 (M = Li, Na, and K) compounds.

Structural, electronic and nonlinear optical properties of B3 and B20 compounds: A first-principles investigation within the LDA, GGA and modified Becke–Johnson exchange potential plus LDA

The geometric structural optimization, electronic band structure, total density of states for valence electrons, density of states for phonons, optical, dynamical, and thermodynamical features of cesium chloride have been investigated by linearized augmented plane wave method using the density functional theory under the generalized gradient approximation. Ground state properties of cesium chloride are studied. The calculated ground state properties are consistent with experimental results. Calculated band structure indicates that the cesium chloride structure has an indirect band gap value of 5.46 eV and is an insulator. From the obtained phonon spectra, the cesium chloride structure is dynamically stable along the various directions in the Brillouin zone. Temperature dependent thermodynamic properties are studied using the harmonic approximation model.

Some physical properties of RbGeCl3 crystal are investigated with ABINIT computer program within the generalized gradient approximation (GGA) and the local density approximation (LDA), using density functional theory (DFT). We studied the geometry optimization, electronic band structure, electron density of states, optical properties such as the dielectric functions, reflectivity, refractive index, extinction coefficients, energy-loss functions for volume, the effective number of valence electrons per unit cell and elastic properties of RbGeCl3. The calculated electronic band structure shows that the RbGeCl3 has a direct band gap and this compound is a semiconducting material with a wide bandgap.

MgH2 and LiH metal hydrides crystals as novel hydrogen storage material: Electronic structure and optical properties

The structural, electronic band structure and optic properties of the Ni doped MgSiP2 chalcopyrite compound have been performed by using first-principles method in the density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke and Ernzerhof (PBE) is used for the exchange and correlation functional. The present lattice constant (a) follows generally the Vegard’s law. The electronic band structure, total and partial density of states (DOS and PDOS) are calculated. We present data for the frequency dependence of imaginary and real parts of dielectric functions of Ni doped MgSiP2. For further investigation of the optical properties the reflectivity, refractive index, extinction coefficient and electron energy loss function are also predicted. Our obtained results indicate that the lattice constants, electronic band structure and optical properties of this compound are dependent on the substitution concentration of Ni.

The structural, electronic and optical properties of In xGa 1 −xP alloys

Ground-state electronic structure and optical properties of distorted perovskite DyMnO3 are investigated in the frame of density functional theory within generalized gradient approximation (GGA). Optimized lattice constants are reasonable in agreement with experimental data. Electronic band structure, density of states and partial density of states of elements are obtained. It shows that DyMnO3 exhibit an indirect band gap of 0.91 eV. Dielectric function, optical reflectivity, refractive index, extinction coefficient, electron energy loss, and absorption coefficient are calculated and analyzed for radiation up to 35 eV.

Zinc blende (zb) and wurtzite (wz) structure of cadmium sulfide (CdS) are analyzed using density functional theory within local density approximation (LDA), generalized gradient approximation (GGA), Hubbard correction (GGA + U), and hybrid functional approximation (PBE0 or HSE06). To assure the accuracy of calculation, the convergence test of total energy with respect to energy cutoff and k-point sampling is performed. The relaxed atomic position for the CdS in zb and wz structure is obtained by using total energy and force minimization method following the Hellmann–Feynman approach. The structural optimization and electronic band structure properties of CdS are investigated. Analysis of the results shows that LDA and GGA underestimate the bandgap due to their poor approximation of exchange-correlation functional. However, the Hubbard correction to GGA and the hybrid functional approximation give a good bandgap value which is comparable to the experimental result. Moreover, the optical properties such as real and imaginary parts of the dielectric function, the absorption coefficient, and the energy loss function of CdS are determined.

Optoelectronic properties of Nd3+ doped CaTa2O6: Insights from the GGA + U calculations

The electronic structure, linear, and non-linear optical properties of ferroelectric-semiconductor SbSBr are investigated in the non-polar (paraelectric) and polar (ferroelectric) phase, using the density functional methods in the generalized gradient approximation. The electronic band structure obtained shows that SbSBr has an indirect forbidden gap of 2.16 and 2.21 eV in the paraelectric and ferroelectric phase, respectively. The linear photon-energy dependent dielectric functions and some optical functions, such as absorption and extinction coefficients, refractive index, energy-loss function, reflectivity, and optical conductivity in both phases and photon-energy dependent second-order susceptibilities in the ferroelectric phase are calculated. Moreover, some important optical parameters, such as the effective number of valence electrons and the effective optical dielectric constant, are calculated in both phases.