Narrow Results

Ab initio calculations of the atomic and electronic structures of Cu_N and Na_N (N = 2 - 22) clusters show similar growth behaviors. These can be characterized into 3 regimes: planar, layered and 3D structures. Atomic structures lead to anomalies in the even-odd alternation behavior of the second order energy difference for N = 16 and 17 in both Cu_N and Na_N. The effects of sp-d hybridization on the atomic and electronic structures of Cu_N clusters are discussed.

We perform ab initio electronic structure calculations of the intermetallic compound CeFeGe₃ by means of the Tight Binding Linear Muffin-Tin Orbitals-Atomic Sphere Approximation (TB-LMTO-ASA) within the Local Spin Density Approximation containing the so-called Hubbard correction term (LSDA+U^SIC), using the Stuttgart's TB (Tight Binding)-LMTO-ASA code in the framework of the Density Functional Theory (DFT). At the LSDA-vBH level, our electronic structure calculations indicate an intermediate valence character compound whereas a DOS (density of states) analysis favors its metallic nature. We also study electronic correlation effects via the introduction of the U parameter on Ce and Fe, where a metallic and magnetic characters for this compound are found, along with certain features that may possibly be associated with a heavy fermion system.

A study of polyaromatic hydrocarbons by semiempirical PM3 and ab initio methods in MINI and STO 6G-31 bases has been performed for compounds with different numbers of rings. The optimized space and electronic structures have been derived. The multiplicity states effect on the energetic stability of the polyaromatic hydrocarbons is examined. It is shown that the high multiplicity states become more energetically preferable with the growth of the PAH size.

Interaction of the previously described [V. D. Khavryuchenko, Y. A. Tarasenko, V. V. Strelko, O. V. Khavryuchenko and V. V. Lisnyak, Quantum chemical study of polyaromatic hydrocarbons in high multiplicity states, Int. J. Modern. Phys. B21, 4507 (2007), in press] polyaromatic hydrocarbon (PAH) C₉₆H₂₄ with dioxygen molecule and KO₂ have been quantum chemically examined. The probability of existence of the oxygen superoxide ion-radical O₂ adsorbed on the surface of the PAH is critically discussed.

First principles total energy calculations have been performed using full potential linear augmented plane wave method (FP-LAPW) within density functional theory to study the structural, electronic and optical properties of MgS_xSe_1-x, MgS_xTe_1-x and MgSe_xTe_1-x alloys in the rock salt crystallographic phase. The generalized gradient approximation parameterization scheme has been used for calculating the ground state structural parameters and their deviation from the Vegard's law has been discussed. Full relativistic electronic band structures and density of states have been calculated to study the electronic properties of the end binary compounds and ternary alloys MgS_xSe_1-x, MgS_xTe_1-x and MgSe_xTe_1-x (0.25 < x < 0.75). Optical bowing for these semiconductor alloys has been discussed in term of volume deformation, electronegativity and structural relaxation. Optical properties of the binary and ternary magnesium chalcogenides have been calculated in terms of the complex dielectric function and the results are compared with available theoretical and experimental data.

High pressure effect on the structural evolution properties of intermetallic compounds PtX (X = Si, Ge, Sn AND Pb) was studied based on the first principle density functional theory. The compressibility of PtSi and PtGe under high pressure presents anisotropic behavior. The crystal stacking characteristic of the PtX (X = Si and Ge) along the three axes may be responsible for their anisotropic axial compressibility under high pressure. The sequence of axial compressibility for PtSi is c < a < b, whereas PtGe exhibits the sequence of a < c < b. The pressure derivative of c/a was qualified to be 8.92×10^-4GPa^-1 and 1.38×10^-3GPa^-1 for PtSn and PtPb, respectively. The applied pressure stabilized the crystal structures of PtSn and PtPb.

Using the first-principles plane wave pseudopotential method, the structural and electronic properties of intermetallic compound SrLiSb have been studied within generalized gradient approximation in the frame of density functional theory. The calculations of lattice parameters are in well agreement with the available experimental data. The geometry optimization results indicated the compressibility of SrLiSb is anisotropic under high pressure. The energy band structure and density of states of SrLiSb were also calculated, indicating that SrLiSb has an electronic phase transition from direct-gap semiconductor to indirect-gap semiconductor at approximate 8 GPa.

This study describes structural, electronic and optical properties of Mg_xCd_1-xX (X = S, Se, Te) alloys in the complete range 0≤x ≤1 of composition x in the zinc-blende (ZB) phase with the help of full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within density functional theory (DFT). In order to calculate total energy, generalized gradient approximation (Wu–Cohen GGA) has been applied, which is based on optimization energy. For electronic structure calculations, the corresponding potential is being optimized by Engel–Vosko GGA formalism. Our calculations reveal the nonlinear variation of lattice constant and bulk modulus with different concentration for the end binary and their ternary alloys, which slightly deviates from Vegard's law. The calculated band structures show a direct band gap for all three alloys with increasing order in the complete range of the compositional parameter x. In addition, we have discussed the disorder parameter (gap bowing) and concluded that the total band gap bowing is substantially influenced by the chemical (electronegativity) contribution. The calculated density of states (DOS) of these alloys is discussed in terms of contribution from various s-, p- and d-states of the constituent atoms and charge density distributions plots are analyzed. Optical properties have been presented in the form of the complex dielectric function ε(ω), refractive index n(ω) and extinction coefficient k(ω) as function of the incident photon energy, and the results have been compared with existing experimental data and other theoretical calculations.

First-principles total energy calculations have been performed using full potential linear-augmented-plane-wave method within the framework of density functional theory to study the structural, electronic, magnetic and optical properties of the Pb_1-xEu_x Se and Pb_1-xEu_xTe(0 ≤ x ≤1) alloys in the ferromagnetic (FM) ordering. The calculations have been extended to treat the strongly localized f electrons of Eu atom by the self-interaction correction (SIC) approach. For structural optimization, the Wu and Cohen generalized gradient approximation (GGA) functional has been used, whereas for calculating electronic properties, the GGA parameterization scheme formulated by Engel and Vosko (EV) has also been utilized. It has been observed that the use of experimental value of Coulomb parameter (U_f-expt.) within the SIC does not yield an accurate EuSe and EuTe energy band structure. The improvement in the electronic band structures of nonmagnetic PbSe/PbTe and ferromagnetic EuSe/EuTe have been achieved by considering the effects of spin–orbit coupling for Pb atoms, by a suitable choice of U and by treating the U values for Eu atom's f and d electrons as parameters. The electronic and optical properties of FM Pb_1-xEu_xSe in agreement with experiments can be achieved by combining EV GGA with a Hubbard U < U_f-expt., however, a stronger and stable AFM coupling in EuTe leaves the above scheme unable to provide good electronic structure of FM Pb_1-xEu_xTe. In case of Pb_1-xEu_xSe the nonlinear behaviour of electronic structure is reflected in the optical properties of Eu-doped PbSe that have been studied in terms of incident photons' energy dependent complex dielectric function.

Electronic structure and electronic charge density in the interatomic bonds are investigated with ab initio calculations based on the density-functional theory. The full potential linearized augmented plane-wave method was used with the generalized gradient approximation. Considering the partial density of states the electron charge density distribution in the Bi, S, Se and Br atomic bonds is caused by Bi-6p, S-3p, Se-4p, Br-4p orbital hybridization. Electronic charge distribution of one BiSBr and BiSeBr molecule range suggest that the Bi–S, Bi–Se and Bi–Br bonds are covalent–ionic type. Bi–S and Bi–Se bonds are strong covalent with a not great ionicity factor (, Bi–S; , Bi–Se). Bi–Br bonds are covalent type with a larger ionicity factor (, Bi–Br).

First-principles calculations are performed to study the structural, electronic, thermodynamic and thermal properties of the InP and InAs bulk materials and InAs_xP_1-x ternary alloys using the full potential-linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT). The dependence of the lattice constant, bulk modulus, band gap, Debye temperature, heat capacity and mixing entropy on the composition x was analyzed. The lattice constant for InAs_xP_1-x alloys exhibits a marginal deviation from the Vegard's law. A large deviation of the bulk modulus from linear concentration dependence (LCD) was observed for our alloys. We found that the composition dependence of the energy band gap is almost linear by using the mBJ and EV-GGA approximations. The microscopic origins of the gap bowing were explained and detailed by using the approach of Zunger and co-workers. Furthermore, the calculated phase diagram shows a miscibility gap for these alloys with a high critical temperature. Thermal effects on some macroscopic properties of InAs_xP_1-x alloys are predicted using the quasi-harmonic Debye model, in which the phononic effects are considered. This is the first quantitative theoretical prediction of the thermal properties of the InAs_xP_1-x alloys, and we still expect the confirmation of experimental studies.

In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO₃ perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO₂-terminations of the nonpolar (001) surface and A, BO, and O terminations of the polar (011) surface, as well as B and AO₃-terminations of the polar (111) surface were considered. On the AO-terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO₂-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all ABO₃ perovskites, the surface rumpling is much larger for the AO-terminated than for the BO₂-terminated (001) surface, but their surface energies are always quite similar. In contrast, different terminations of the (011) ABO₃ surface lead to very different surface energies for the O-terminated, A-terminated, and BO-terminated (011) surface, respectively. A considerable increase in the Ti–O or Zr–O, respectively, chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface in ABO₃ perovskites were predicted. According to the results of ab initio calculations for Nb doped SrTiO₃, Nb is a shallow donor; six nearest O ions are slightly displaced outwards from the Nb ion. The F center in ABO₃ perovskites resembles electron defects in the partially-covalent SiO₂ crystal rather than usual F centers in ionic crystals like MgO and alkali halides. The results of calculations for several perovskite KNb_xTa_1-xO₃ (KTN) solid solutions, as well as hole and electron polarons in ABO₃ perovskites are analyzed.

Structural, electronic, elastic and thermodynamic properties of Rh₃X(X = Zr, Nb, Ta) intermetallic compounds are investigated in the framework of density functional theory (DFT). The exchange-correlation (XC) potential is treated with the generalized gradient approximation (GGA) and local density approximation (LDA). The computed ground state properties agree well with the available theoretical and experimental values. The elastic constants are obtained by calculating the total energy versus volume conserving strains using Mehl model. The electronic and bonding properties are discussed from the calculations of band structures (BSs), densities of states and electron charge densities. The volume and bulk modulus at high pressure and temperature are investigated. Additionally, thermodynamic properties such as the heat capacity, thermal expansion and Debye temperature at high pressures and temperatures are also analyzed.

The structural, stability and electronic properties of C15-AB₂ (A = Ti, Zr; B = Cr) isomeric intermetallic compounds were systematically investigated by using density functional theory (DFT) and plane-wave pseudo-potential (PW-PP) method. The macroscopic properties including the lattice constant, bulk modulus and stability for these compounds were studied before and after hydrogenation. For parent compounds, the enthalpy of formation was evaluated with regard to their bulk modules and electronic structures. After hydrogenation of compounds at different interstitial tetrahedral sites (A₂B₂, A₁B₃, B₄), a volume expansion was found for hydrides. The stability properties of hydrides characterized the A₂B₂ sites as the site preference of hydrogen atoms for both compounds. The Miedema's "reverse stability" rule is also satisfied in these compounds as lower the enthalpy of formation for the host compound, the more stable the hydride. Analysis of microscopic properties (electronic structures) after hydrogenation at more stable interstitial site (A₂B₂) shows that the H atoms interact stronger with the weaker (or non) hydride forming element B (Cr) than the hydride forming element A (Ti/Zr). A correlation was also found between the stability of the hydrides and their electronic structure: the deeper the hydrogen band, the less stable the hydride.

The band structure, density of states, elastic properties and thermal properties of semiconductor GaX (X = N, P, As, Sb) with zinc blende were calculated by using the first principle plane-wave pseudo-potential methods based on density functional theory (DFT). The band structure and density of states for GaN, GaP, GaAs and GaSb show that GaX compounds are semiconductors with a direct band gap of 1.542, 1.445, 0.34 and 0.257 eV, respectively. The elastic constants and modulus are calculated showing that GaX are mechanically stable and GaN has the largest modulus. The anisotropy factor, internal-strain parameter, shear to bulk modulus and Poisson's ratio are also calculated indicating that GaX exhibit a brittle, anisotropic and plastic character. The dependencies of the Debye temperature, heat capacity, enthalpy, the entropy and free energy on temperature are also investigated. Comparisons with the available experiment and other theoretical calculation show reasonable agreement.

Adsorption and dissociation of O₂ molecule on the MoSe₂ and MoTe₂ monolayers are studied by using density functional theory (DFT) within the generalized gradient approximation (GGA) and a supercell approach. The physisorbed O₂ molecule on MoSe₂ and MoTe₂ with a magnetic moment (MM) close to that for an isolated O₂ molecule has small adsorption energy and long distance from the surface. The dissociative adsorption of configuration R5(R6) is the most stable adsorption site, whereas the chemisorption of O₂ is unfavorable at all adsorption sites. The dissociative adsorption of configuration R4 induces dramatic changes of electronic structures and localized spin polarization both for monolayer MoSe₂ and MoTe₂. The analysis of electronic density of states (DOSs) shows that the contribution of spin polarization is mainly from the hybridization between O–p, Se(Te)–p and Mo–d orbitals.

We present the electronic, magnetic and structural properties of the magnetic transition metal oxides PbMO₃ (M = Fe, Co, Ni) in cubic perovskite structure. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and local spin density approximation (LSDA) of the exchange-correlation functional. On-site Coulomb interaction is also included in calculations (LSDA+U). The systems are considered in ferromagnetic (FM) and G-type antiferromagnetic (G-AFM) order. FM structures are energetically more favored than G-AFM and than non-magnetic states for all the systems studied. The spin-polarized electronic band structures show that all the structures have metallic property in FM order without Hubbard-U interaction (U_eff = 0). However, the inclusion of on-site Coulomb interaction (U_eff = 7 eV) opens a semiconducting gap for majority spin channel of PbFeO₃ and of PbNiO₃ resulting in a half-metallic character. PbCoO₃ system remains as metallic with LSDA+U scheme. Bonding features of all structures are largely determined by the hybridizations between O–p and d-states of transition metal atoms. The partial magnetic moment of Fe atom in PbFeO₃ is enhanced by inclusion of Hubbard-U interaction (2.55 μ_B ⇒ 3.78 μ_B). Total magnetic moments of half-metallic PbFeO₃ and of PbNiO₃ compounds are very close to integer values.

Nitinol as a superelastic shape memory alloy (SMA) has been the focus of physical-chemical studies in recent decades in respect to functionality of biocompatibility in the body. Superelastic properties of nitinol are the direct results of the electronic structure of this material while dealing with the ab initio behavior of microstructure. In the present work, the elastic properties and electronic structure of B2-phase binary TiNi_(1-x)Cu_x (x = 0, 0.25 and 0.75) shape memory alloys are discussed aiming at understanding of the physical properties underlying superelastic behavior. The calculations have been performed with the program package WIEN2K, in the framework of first-principle, all-electron density functional theory (DFT) within the scheme of the generalized gradient approximation (GGA). The optimized lattice parameters and independent elastic constants are obtained for use in the calculation of the bulk and shear moduli, Young modulus, Poisson ratio and Zener anisotropy parameter. For different alloying fractions x, the tetragonal (C′) and trigonal (C₄₄) shear constants are calculated and brittle/ductile behavior of these compounds is discussed. Finally, a qualitative discussion of dependence of elastic behavior of these compounds upon the electronic density of states (DOS) is presented.

First principle calculations are performed to investigate the electronic structure, structural phase stability, optical and vibrational properties of double perovskite oxide semiconductors namely Ba₂ScMO₆ (M = Nb, Ta) in the cubic symmetry using WIEN2k. In order to study the ground state properties of these compounds, the total energies are calculated as a function of reduced volumes and fitted with Brich Murnaghan equation. The estimated ground state parameters are comparable with the available experimental data. Calculations of electronic band structure on these compounds reveal that both Ba₂ScNbO₆ and Ba₂ScTaO₆ exhibit a semiconducting behavior with a direct energy gap of 2.78 and 3.15 eV, respectively. To explore the optical transitions in these compounds, the real and imaginary parts of the dielectric function, refractive index, extinction coefficient, reflectivity, optical absorption coefficient, real part of optical conductivity and the energy-loss function are calculated at ambient pressure and analyzed. The collective Raman active modes of the atoms of these materials are also calculated in order to understand the structural stability of these compounds.

In a comprehensive study, structural properties, electronic structure and optical response of crystalline o-phenanthroline were investigated. Our results show that in generalized gradient approximation (GGA) approximation, o-phenanthroline is a direct bandgap semiconductor of 2.60 eV. In the framework of many-body approach, by solving the Bethe–Salpeter equation (BSE), dielectric properties of crystalline o-phenanthroline were studied and compared with phenanthrene. Highly anisotropic components of the imaginary part of the macroscopic dielectric function in o-phenanthroline show four main excitonic features in the bandgap region. In comparison to phenanthrene, these excitons occur at lower energies. Due to smaller bond lengths originated from the polarity nature of bonds in presence of nitrogen atoms, denser packing, and therefore, a weaker screening effect, exciton binding energies in o-phenanthroline were found to be larger than those in phenanthrene. Our results showed that in comparison to the independent-particle picture, excitonic effects highly redistribute the oscillator strength.