Structure stability and electronic properties of the Zr-He system: First-principles calculations

Ab initio studies are carried out for the atomic and electronic structures of the Zr-He system. It is revealed that zirconium undergoes a phase transition induced by helium. The most preferred position of the impurity in the metal lattice is determined. The energy of dissolution of helium and the excess volume introduced by helium are calculated. It is shown that the presence of helium in the Zr lattice significantly perturbs the valence charge density of the metal and substantially changes its electronic structure.     

Electronic structure and elasticity of Z-phases in the Cr-Nb-V-N system

Structural properties and energetics of Cr-based Z-phases (CrNbN, Cr(Nb,V)N and CrVN) were investigated using the Vienna ab?initio simulation package (VASP) code employing the projector augmented wave (PAW) pseudopotentials by means of both local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange and correlation term. The geometry of all studied phases including NbN, VN and elemental constituents (nonmagnetic bcc Nb and V and antiferromagnetic bcc Cr) was fully relaxed, providing the equilibrium structure parameters and total energies. The calculated lattice parameters of Z-phases correspond very well to the experimental data and decrease with increasing molar fraction of vanadium. Enthalpies of formation show that all three Z-phases are stable at T?=?0?K. The electronic structures of Z-phases including densities of states and charge densities were analysed. The calculated bulk moduli and elastic constants were used to evaluate stability conditions and elastic anisotropy ratios. It was confirmed that Z-phases are mechanically stable. Additional information on ductility was obtained from Cauchy pressures, Pugh ratios, Young moduli, and Poisson ratios. The ductility evaluated using the Pugh ratio decreases with number of vanadium atoms.     

Electronic structure of the ferricenium cation

E.S.R. spectra of cations of substituted ferrocenes have been measured at 20° and 77°k. The g values are highly anisotropic, with g _¶ = 4·35 and g⊥=1·26 for Fe(C₅H₅)₂ ⁺. Owing to a fast spin-lattice relaxation the E.S.R. signals are much broader and less intense at 77° than at 20°k. A theoretical analysis shows that Fe(C₅H₅)₂ ⁺ has the orbitally degenerate ² E _2g [(e _2g )³(a _{1g ²} ] ground-state configuration. The combined action of the spin-orbit interaction H _so and a low-symmetry perturbation H ₁ splits the fourfold degenerate level into two Kramers doublets. Since H _so and H ₁ are of the same order of magnitude, the orbital momentum contribution to the g values is only partly quenched and the g values deviate strongly from the free-electron value. The predicted splitting of the Kramers doublets is small and this explains the fast relaxation.

The e _2g molecular orbitals are essentially localized on the iron atom. Substitution influences primarily the symmetry of the complex; it has no great influence on the mixing of the iron 3d ₂ and ligand e _2g π orbitals. g values and line widths of the E.S.R. signals indicate that in Fe(C₅H₅)₂ ⁺ the perturbation of the axial symmetry has an external origin. In the substituted complexes internal perturbations are also present.

By a combination of E.S.R., visible absorption and photoelectron spectra the order and character of the highest filled and lowest empty orbitals in ferrocene have been found to be:

     

Electronic structure and XPS spectra for the stoichiometric and substoichiometric Hf monocarbide-nitride system

We present results of electronic structure calculations for stoichiometric and substoichiometric HfC_xN_1-x (for x=0, 0.42, 0.6 and 1) obtained with the relativistic Korringa-Kohn-Rostocker Greens function (RKKR-GF) method. The mixed crystal and the finite concentrations of vacancies are treated within the coherent potential approximation (CPA), of which athree component version is needed to treat both chemical disorder and vacancies on the anion sublattice. We calculate X-ray photoemission intensities for all cases and find that, provided vacancies are taken into account, they agree rather well with experiment. We find that the densities of states at the Fermi level are but little affected by the presence of vacancies and briefly discuss the implications for the superconductivity in this series of compounds.     

Electronic structure of NiAl

Using the local approximation for exchange-correlation effects we employ the linearized augmented-plane-wave method (LAPW) to compute self-consistently the band structure, equilibrium lattice constant, and x-ray spectra of the B2-phase of the NiAl intermetallic alloy. The results are compared to other theoretical and experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fiziki, No. 7, pp. 41–45, July, 1987.     

Electronic structure of CsAu

Self-consistent non-relativistic and relativistic LMTO band structure calculations have been carried out for CsAu. In the non-relativistic model CsAu is a metal, whereas — in agreement with experiments — the relativistic calculations predict CsAu to be a semiconductor. The gap is not caused by the spin-orbit coupling. The importance of the core-like Cs-5s and Cs-5p states for the alloy formation is discussed, and charge distribution calculations are used to illustrate the ionic nature of the bonding.     

Electronic structure of Ce-pnictides

High-resolution x-ray photoemission has been used to study the electronic structure of the Ce-pnictides (CeN, CeP, CeAs and CeSb). This series of isostructural compounds allows us to follow the evolution of the 4f level as the distance between Ce atoms varies. Core level spectra show clearly that the 4f state is localized in all the compounds. In the valence band region, the width of the 4f peak is strongly influenced by the energy overlap with the extended states originating from anion p states. The spectra of CeN reveal the existence of a mixed configuration.     

Electronic structure of polyperylene

The electronic structure of polyperylene is studied on the basis of the one-dimensional tight-binding SCF-CO (self consistent field-crystal orbital) method. The geometry of this polymer is optimized from the energetic point of view. The analysis of the electronic properties of polyperylene reveals that this polymer will be a promising member of the one-dimensional graphite as a new electrically conductive or semi-conductive material.     

Electronic structure of SmOFeAs

The electronic structure of SmOFeAs, a parent compound of iron arsenic superconductors, is measured by angle resolved photoemission spectroscopy. Due to the surface contribution, the measured electronic structure deviates strongly from the calculations. One of the bulk bands is identified by photon energy dependence measurements. Moreover, the appearance of sharp quasiparticle peaks at low temperatures implies the drastic reduction of the scattering rate. No energy gap is observed at Fermi level, indicating that the Fermi surface nesting is irrelevant in the spin density wave formation.     

Electronic structure of silicene

In this topical review, we discuss the electronic structure of free-standing silicene by comparing results obtained using different theoretical methods. Silicene is a single atomic layer of silicon similar to graphene. The interest in silicene is the same as for graphene, in being two-dimensional and possessing a Dirac cone. One advantage of silicene is due to its compatibility with current silicon electronics. Both empirical and first-principles techniques have been used to study the electronic properties of silicene. We will provide a brief overview of the parameter space for first-principles calculations.However, since the theory is standard, no extensive discussion will be included. Instead, we will emphasize what empirical methods can provide to such investigations and the current state of these theories. Finally, we will review the properties computed using both types of theories for free-standing silicene, with emphasis on areas where we have contributed.Comparisons to graphene is provided throughout.     

Electronic structure of MnSb

On the basis of a two center tight binding model the recursion method of Haydock et al. was applied to calculate spin polarized and non-polarized electronic densities of states and magnetic moments for the B8₁ crystal structure and a fictitious B32 structure of MnSb. The model implies the itinerant nature of the Mn d-states hybridizing with Sb p-states. For the non-polarized densities of states the self-consistent charge transfer amounts to 0.6 electrons from Mn to Sb with a band center difference of E_p ? E_d = - 0.75 eV. For the magnetic moments, which were also calculated self-consistently, 3.5 μ_B for the d-band and 3.34 μ_B in total were obtained for the B8₁ crystal structure. For the B31 structure bigger moments of 3.67 μ_B and 3.53 μ_B were obtained, correspondingly. The results for MnSb are also discussed with respect to very recent results for MnAs calculated within the same model. The local densities of states as obtained for MnSb are in good agreement with experimental XPS results.     

Electronic structure of CsAu

X-ray photoemission spectra of CsAu prepared in vacuum are in good agreement with the ionic character expected for this material. The Cs 5p doublet lies well below flat Au 5d bands. The Au 6s valence band is located somewhat closer to the 5d states than recent band structure calculations indicate. The charge transfer to gold is in the range 0.6 to 0.8 electrons.     

Electronic structure of PCBM

We have studied the electronic structure of [6,6]-phenyl-C₆₁-butyric-acid-methyl-ester (PCBM) using synchrotron radiation photoelectron spectroscopy (PES) measurements and first-principles calculations. The PES spectrum of the entire occupied valence band is reported, which exhibits abundant spectral features from the Fermi level to ～24 eV binding energy. All the spectral features are broadened as compared with the cases of C₆₀. The reasons for the broadening are analysed by comparing the experimental data with the calculated energy levels and density of states. Special attention is paid to the analysis of the C₆₀ highest occupied molecular orbital (HOMO)-1 derived states, which can play a crucial role in the bonding at the interfaces of PCBM/polymer blenders or PCBM/electrodes. Besides the well-known energy level splitting of the C₆₀ backbone caused by the lowered symmetry, C 2p states from the side chain mix or hybridize with the molecular orbitals of parent C₆₀. The contribution of the O 2p states can substantially modify the PES spectrum.