Ab initio study of electronic and magnetic properties of the Fe3Zn intermetallic

First-principles calculations have been carried out to study the electronic structure and magnetic properties of the Fe₃Zn compound with the full-potential linearized augmented-plane wave (FLAPW) method. The results indicate a lower magnetostriction for Fe₃Zn as compared to Galfenol (Fe-Ga), as a result of a weaker spin-orbit coupling, which is due to a smaller magnetic moment induced on the Zn atom. With the Zn addition to Fe the bulk modulus and the cohesive energy (per atom) decrease, whereas the electronic specific heat coefficient γ has a substantial increase.     

Calculations of physical properties of pure and doped crystals: Ab initio and semi-empirical methods in application to YAlO3:Ce and TiO2

In this paper we demonstrate that two independent methods of calculations (DFT based ab initio and semi-empirical crystal field theory) can be used to form a complementary picture of the optical and electronic properties of the doped host and impurity ion. The crystals considered in the present paper are: (i) YAlO₃:Ce³⁺ and (ii) two dominant phases of TiO₂—rutile and anatase. As an example, detailed calculations of the band structure and crystal field energy level scheme of YAlO₃:Ce³⁺ are reported. From the analysis of the band structure and density of states, the character of the YAlO₃ energetic bands and positions of the Ce impurity energy levels were established. It was also shown how the ab initio methods can be used for calculations of the structural properties of solids under elevated pressure. Taking the two dominant phases of TiO₂ as an example, it was demonstrated how the elastic properties can be extracted from the calculated unit cell’s volume at different pressures. Particular attention was paid to the microscopic effects of crystal field, which were evidenced by the pressure-induced changes of the structure and shape of distribution of the Ti 3d electrons density of states. It was demonstrated how the difference in crystal structure of the anatase and rutile phases leads to remarkable difference in microscopic crystal field effects, which was explained by different Ti-O distances in both phases. In addition, the pressure dependence of the band gaps for anatase and rutile was investigated. It was shown that the hydrostatic pressure leads to the band gap narrowing in anatase and band gap widening in rutile, with pressure coefficients +0.00681 eV/GPa for rutile and −0.0088 eV/GPa for anatase.     

Ab initio calculation of structural, electronic and phonon properties of ZrRu and ZrZn in B2 phase

The structural, electronic and dynamic properties of cesium chloride, ZrRu and ZrZn were studied by employing an ab initio pseudopotential method and a linear response scheme, within the generalized gradient approximation. The calculated lattice constant, bulk modulus and first-order pressure derivative of the bulk modulus were reported in B2 structure and compared with available experimental and other theoretical results. The electronic band structure, partial and total density of states were determined by using the Quantum-Espresso ab initio simulation package based on pseudopotential method. Phonon dispersion curves and density of states were calculated by employing a density functional perturbation theory.     

Ab initio simulations on the atomic and electronic structure of single-walled BN nanotubes and nanoarches

To simulate the perfect single-walled boron nitride nanotubes and nanoarches with armchair- and zigzag-type chiralities and uniform diameter of ∼5 nm, we have constructed their one-dimensional (1D) periodic models. In this study, we have compared the calculated properties of nanotubes with those for both hexagonal and cubic phases of bulk: bond lengths, binding energies per B-N bond, effective atomic charges as well as parameters of total and projected one-electron densities of states. For both phases of BN bulk, we have additionally verified their lattice constants. In the density functional theory (DFT), calculations performed using formalism of the localized Gaussian-type atomic functions as implemented in the CRYSTAL-06 code we have applied Hamiltonians containing either PWGGA or hybrid (DFT+HF) B3PW exchange-correlation functionals. After calculation of Hessian matrix for the optimized structures of BN bulk (both phases) and nanotubes (both chiralities) using the CRYSTAL code we have estimated their normal phonon modes within the harmonic approximation. Applying both atomistic and continuum models we have calculated the elastic energies and moduli for SW BN nanoarches. Our calculations clearly show a reproducibility of the atomic structure, effective charges and total energy, as well as phonon and elastic properties when using either PWGGA or hybrid B3PW Hamiltonians. On other hand, there is a high sensitivity of the discrete energy spectra parameters (including band gap) to the choice of the first principles approach (the hybrid method reproduce them noticeably better).     

Electron-density distribution and physical properties of plutonium-gallium alloys: Ab initio cluster calculations

A cluster model based on ab initio density-functional theory was used to model gallium-stabilized δ-plutonium alloys, and to calculate the electron-density distribution, its pressure dependence, bond lengths, elastic properties (second order and third order), and inelastic properties for Pu₁₂Ga (7.7 at% Ga) and Pu₁₈Ga (5.3 at% Ga). The electron distribution was found to contain localized, semi localized, and delocalized contributions, with the second possessing covalent character. Two of plutonium’s 8 valence electrons were found to be itinerant, consistent with a recent prediction based on an electrostatic model, with the electron configuration for plutonium being 7s^0.57p^0.55f¹ (itinerant) and 6d¹5f⁵ (localized), and that for gallium being 4s¹4p². Applied hydrostatic pressure shifts the charge density toward a more localized Pu(d)-based distribution. The onset of the pressure-induced δ-Pu to α-Pu phase change is accompanied by a ∼0.2 electron increase in the localized population that may serve as a driving force for the phase change. Interior bonding within the Pu₁₂Ga subunits is stronger than that of the surrounding plutonium lattice, and the Pu-Ga bonds therein relax in a direction opposite to lattice strain. This study predicts covalency in metallic plutonium, both in the Pu-Ga bonding and in the Pu-Pu bonding.     

Ab initio studies of superconductivity in monatomic metallic hydrogen under high pressure

A detailed study of the structural, electronic, dynamical, and superconducting properties in the monatomic metallic hydrogen is presented. At least up to 802 GPa, this phase is stable. Moreover, we find that a strong electron-phonon interaction drives a very high superconducting transition temperature TC≈291.40 K at 539 GPa.     

Ab initio studies on the electronic structure of CeSi5

We investigated the electronic properties of CeSi₅ by band structure calculation based on the density functional theory within LDA, LDA+U, and fully relativistic schemes. The calculated band structure scheme shows that the spin-orbit coupling splits the Ce 4f states into three manifolds. When the on-site Coulomb potential is added to the Ce-derived 4f orbitals, the degeneracy between the f orbitals would be lifted and they are split into lower Hubbard bands and upper Hubbard bands. It was found that quasiparticle mass enhancement inferred by comparing γ to the density of states (DOS) at the Fermi level indicates the effective mass of CeSi₅ is enhanced with the fully relativistic results.     

Optical properties of anatase and rutile titanium dioxide: Ab initio calculations for pure and anion-doped material

The optical properties of rutile and anatase titanium dioxide (TiO₂) are calculated from the imaginary part of the dielectric function using pseudopotential density functional method within its generalized gradient approximation (GGA) and a scissors approximation. The fundamental absorption edges calculated for the unit cell of both rutile and anatase are consistent with experimentally reported results of single crystal rutile and anatase TiO₂ and with previous theoretical calculations. A significant optical anisotropy is observed in the anatase structure which holds promise for investigating the band gap modification with better visible-light response and provides a reliable foundation for addressing the effect of impurities on the fundamental absorption edge/band gap of anatase TiO₂. Further calculations on the electronic structure and the optical properties of C-, N-, and S-doped anatase TiO₂ are performed. The results are analyzed and discussed in terms of optical anisotropy and scissors approximations.     

Adsorption site preference of CO2 on the Pt(1 0 0) surface by ab initio calculations

This paper investigates the adsorption sites and electronic structure of the adsorption of CO₂ on the Pt(1 0 0) surface by ab initio periodic density functional theory (DFT) methods. Several parallel and vertical adsorption sites are examined in detail. The results show that the adsorption energy for the atop hollow horizontal (thh) site is 0.34 eV. However, other sites only have small binding energies and these values are very close. For an atop hollow horizontal site, the calculated elecronic interaction is contributed to not only the Pt-O atoms, but also Pt-C atoms. So the results indicate that the thh site is the most favorable and stable.     

Structural, elastic, and electronic properties of orthorhombic NH3BH3: An ab initio study

At the generalized gradient approximation (GGA), the elastic constants of the orthorhombic phase of NH₃BH₃ were calculated with plane-wave pseudo-potential method. Our calculation showed that the orthorhombic phase NH₃BH₃ is a loose and brittle material, as well as hard to be deformed, also we calculated the elastic anisotropies and the Debye temperatures from the elastic constants. And from the band structure and density of state (DOS), we concluded that NH₃BH₃ is a wide-gap semiconductor and the band gap is almost 6.0 eV. The bonds between N atoms and H atoms show a strong covalent characteristic, B atoms and H atoms form ironic bonds, and so as to the B-N bonds. Electrons from the B atoms are absorbed by the H atoms around the B atoms, and the H atoms display electronegativity.     

A theoretical-spectroscopy, ab initio-based study of the electronic ground state of SbH3

For the stibine isotopologue , we report improved theoretical calculations of the vibrational energies below 8000 cm⁻¹ and simulations of the rovibrational spectrum in the 0-8000 cm⁻¹ region. The calculations are based on a refined ab initio potential energy surface and on a new dipole moment surface obtained at the coupled cluster CCSD(T) level. The theoretical results are compared with the available experimental data in order to validate the ab initio surfaces and the TROVE computational method [Yurchenko SN, Thiel W, Jensen P. J Mol Spectrosc 2007;245:126-40] for calculating rovibrational energies and simulating rovibrational spectra of arbitrary molecules in isolated electronic states. A number of predicted vibrational energies of are provided in order to stimulate new experimental investigations of stibine. The local-mode character of the vibrations in stibine is demonstrated through an analysis of the results in terms of local-mode theory.     

Cu adsorption on pyrite (1 0 0): Ab initio and spectroscopic studies

Surface optimised S 2p photoelectron spectra show that both surface S²⁻ monomers and (S-S)²⁻ dimers are present at pyrite (1 0 0) fracture surfaces. In order to determine which sulfur species are involved in Cu adsorption, fresh pyrite surfaces were exposed to Cu²⁺ in solution. The S 2p spectra suggest that both types of S surface species are involved in the mechanism of Cu adsorption (activation). Ab initio density functional theory was used to model Cu adsorbed onto pyrite (1 0 0) to support the interpretation of the spectroscopy. Mulliken population analysis confirms the charge distribution suggested by the core line shifts as observed in the photoelectron spectra. The ab initio calculations were consistent with a two-coordinate bond between Cu(I), a surface S monomer and a surface S dimer.     

Ab initio dipole moment and theoretical rovibrational intensities in the electronic ground state of PH3

We report a six-dimensional CCSD(T)/aug-cc-pVTZ dipole moment surface for the electronic ground state of PH₃ computed ab initio on a large grid of 10 080 molecular geometries. Parameterized, analytical functions are fitted through the ab initio data, and the resulting dipole moment functions are used, together with a potential energy function determined by refining an existing ab initio surface in fittings to experimental wavenumber data, for simulating absorption spectra of the first three polyads of PH₃, i.e., (ν₂, ν₄), (ν₁, ν₃, 2ν₂, 2ν₄, ν₂ + ν₄), and (ν₁ + ν₂, ν₃ + ν₂, ν₁ + ν₄, ν₃ + ν₄, 2ν₂ + ν₄, ν₂ + 2ν₄, 3ν₂, 3ν₄). The resulting theoretical transition moments show excellent agreement with experiment. A line-by-line comparison of the simulated intensities of the ν₂/ν₄ band system with 955 experimental intensity values reported by Brown et al. [L.R. Brown, R.L. Sams, I. Kleiner, C. Cottaz, L. Sagui, J. Mol. Spectrosc. 215 (2002) 178-203] gives an average absolute percentage deviation of 8.7% (and a root-mean-square deviation of 0.94 cm⁻¹ for the transition wavenumbers). This is very remarkable since the calculations rely entirely on ab initio dipole moment surfaces and do not involve any adjustment of these surfaces to reproduce the experimental intensities. Finally, we predict the line strengths for transitions between so-called cluster levels (near-degenerate levels formed at high rotational excitation) for J up to 60.     

Quantum-chemical ab initio study of the crystal-field and charge transfer energies of nanocrystalline Y2O3: Eu

The 4f energy levels and crystal-field parameters for several clusters representing the local coordination surroundings of Eu³⁺ in the bulk and nanocrystalline cubic Y₂O₃: Eu³⁺ crystals are obtained by using a method based on the combination of the DV-X_α calculation and the effective Hamiltonian method initialized by M.F. Reid et al. (J. Phys.: Condens. Matter, 2011, 23: 045501). The results are in reasonable agreement with the measured energy levels and the crystal-field parameters obtained from the least-square fitting. The charge transfer energies are also obtained for all the clusters from the DV-X_α calculation. The results indicate that, compared with the bulk Y₂O₃: Eu³⁺ crystal, the charge transfer band in the excitation spectra is red-shifted in the nanocrystal.     

Magnetic and electronic properties of Cr- and Mn-doped SnO2: ab initio calculations

The ab initio calculations, based on the Korringa–Kohn–Rostoker (KKR) approximation method combined with the coherent potential approximation (CPA), indicated as KKR–CPA, have been used to study the stability of ferromagnetic and ferrimagnetic states, for systems that are SnO₂ doped and co-doped with two transition metals, that is, chromium and manganese. Our results indicate that the ferromagnetic state is more stable than the spin-glass state for the (Sn_1−xCr_xO₂; x = 0.07, 0.09, 0.12 and 0.15)-doped system, while the spin-glass state is more stable than the ferromagnetic state for the (Sn_1−xMn_xO₂; x = 0.02 and 0.05)-doped system. However, the ferromagnetic and/or the ferrimagnetic states are stable for the (Sn_0.98−xMn_0.02Cr_xO₂; x = 0.05, 0.09 and 0.13)-doped system depending on the Cr concentration. Moreover, we estimated the Curie temperature (T_c) for the Cr-doped tin dioxide (SnO₂), and we explained the origin of magnetic behaviour through the total density of states for different doped and co-doped SnO₂ systems.     

Ab initio calculation of electronic transition moments for singlet excited states of the H2 molecule

Ab initio CI electronic dipole transition moments have been calculated for the transitions between singlet states of the hydrogen molecule correlating asymptotically with H(nl)+H(1s) (n=1,2,3). The investigated singlet-singlet transitions include the 30 (n=3) inter-Rydberg transitions and the 32 transitions which may contribute to absorption in the far wings of the Balmer α line of atomic hydrogen perturbed by another hydrogen atom in its ground state. Results are presented for internuclear distances 1.0a₀?R?12a₀. The present results compare well with the previous theoretical calculations available for about half of the transitions treated in the present work. Thirty eight new transitions are presented. Adiabatic potential energies for the and and improved energies for the and states are reported as well.     

Structural, electronic, magnetic and elastic properties of tetragonal layered diselenide KCo2Se2 from first principles calculations

The structural, elastic, magnetic and electronic properties of the layered tetragonal phase KCo₂Se₂ have been examined in details by means of the first-principles calculations and analyzed in comparison with the isostructural KFe₂Se₂ as the parent phase for the newest group of ternary superconducting iron-chalcogenide materials. Our data show that KCo₂Se₂ should be characterized as a quasi-two-dimensional ferromagnetic metal with highly anisotropic inter-atomic bonding owing to mixed ionic, covalent, and metallic contributions inside [Co₂Se₂] blocks, and with ionic bonding between the adjacent [Co₂Se₂] blocks and K sheets. This material should behave in a brittle manner, adopt enhanced elastic anisotropy rather in compressibility than in shear, and should show very low hardness.     

Structural, elastic, electronic and magnetic properties of ThCr2Si2 from first-principles calculations

The tetragonal (s.g. I4/mmm; #139) ThCr₂Si₂ is widely known as a structural type of the broad family of the so-called 122-like ternary phases which includes now more than 800 members. Among them the superconducting iron-pnictides (discovered in 2008, -earth metals) and the newest superconducting iron-chalcogenides (discovered in 2010, metals) have attracted recently enormous interest in this class of materials. Meanwhile, the data about the electronic, magnetic, and elastic properties of the ThCr₂Si₂ phase itself are still practically absent. Here, by means of first-principles calculations, the optimized structural parameters, spin ordering of the magnetic ground state, independent elastic constants, bulk, shear, and Young’s moduli, elastic anisotropy indexes, total and partial densities of states, and inter-atomic bonding picture for ThCr₂Si₂ were obtained for the first time and analyzed in comparison with the aforementioned most popular 122-like systems and .     

Phase stability and electronic structure of Si2Sb2Te5 phase-change material

On the basis of an ab initio computational study, the present work provide a full understanding on the atomic arrangements, phase stability as well as electronic structure of Si₂Sb₂Te₅, a newly synthesized phase-change material. The results show that Si₂Sb₂Te₅ tends to decompose into Si₁Sb₂Te₄ or Si₁Sb₄Te₇ or Sb₂Te₃, therefore, a nano-composite containing Si₁Sb₂Te₄, Si₁Sb₄Te₇ and Sb₂Te₃ may be self-generated from Si₂Sb₂Te₅. Hence Si₂Sb₂Te₅ based nano-composite is the real structure when Si₂Sb₂Te₅ is used in electronic memory applications. The present results agree well with the recent experimental work.