Self-consistent Band Calculations Research Articles

Interface coupling induced built-in electric fields accelerate electro-assisted uranium extraction over Co3O4@FeOx nanosheet arrays

Revealing the nature of enhanced electro-assisted uranium extraction activity by hetero-interface and M-O-H coordination bonds is an important and challenging task. Hence, Co3O4@FeOx nanosheet arrays with abundant M-O-H coordination bonds is fabricated as a binder-free and self-supporting electrocatalyst for electro-assisted uranium extraction from fluorine-containing uranium wastewater. As revealed by self-consistent energy band calculations and in-situ Kelvin Probe Force Microscopy (KPFM) analysis, the p-n heterojunction formed by Co3O4@FeOx provides a well-designed built-in electric field (BIEF), triggering the interfacial accumulation of uranium and accelerating the electro reduction kinetics of uranium. Consequently, Co3O4@FeOx shows remarkable U(VI) extraction ability (>95 %) in fluorine-containing uranium wastewater. By virtue of the XAFS spectra, free uranyl ions are captured by M-O-H on Co3O4@FeOx to form a sturdy 2 Oax-1 U-3 Oeq configuration, confirming the electrically driven separation of uranium and fluorine. Furthermore, DFT calculation indicates that the existent of M-O-H enhances the adsorption energy of uranium species, thus promoting uranium extraction efficiency.

In Situ Metal-Oxygen-Hydrogen Modified B-Tio2 @Co2 P-X S-Scheme Heterojunction Effectively Enhanced Charge Separation for Photo-assisted Uranium Reduction.

Photo-assisted uranium reduction from uranium mine wastewater is expected to overcome the competition between impurity ions and U(VI) in the traditional process. Here, B-TiO2 @Co2 P-X S-scheme heterojunction with metal-oxygen-hydrogen (M-O-H) is developed insitu modification for photo-assisted U(VI) (hexavalent uranium) reduction. Relying on the DFT calculation and Hard-Soft-Acid-Base (HSAB) theory, the introduction of metal-oxygen-hydrogen (M-O-H, hard base) metallic bonds in the B-TiO2 @Co2 P-X is found to enhance the hydrophilicity and the capture capability for uranyl ion (hard acid). Accordingly, B-TiO2 @Co2 P-500 hybrid nanosheets exhibit excellent U(VI) reduction ability (>98%) in the presence of competing ions. By self-consistent energy band calculations and in-situ KPFM spectral analysis, the formation of the internal electric field between B-TiO2 and Co2 P at the heterojunction is proven, offering a strong driving force and atomic transportation highway for accelerating the S-scheme charge carriers directed migration and promoting the photocatalytic reduction of uranium. This work provides a valuable route to explore the functionally modified photocatalyst with high-efficiency photoelectron separation for U(VI) reduction.

DensityTool: A post-processing tool for space- and spin-resolved density of states from VASP

The knowledge of the local electronic structure of heterogeneous solid materials is crucial for understanding their electronic, magnetic, transport, optical, and other properties. VASP, one of the mostly used packages for density-functional calculations, provides local electronic structure either by projecting the electronic wave functions on atomic spheres, or as a band-decomposed partial charge density. Here, we present a simple tool which takes the partial charge density and the energy eigenvalues calculated by VASP as input and constructs local charge and spin densities. The new data provides a much better spatial understanding than the projection on the atomic spheres. It can be visualized directly in the real space e.g. with Vesta, or averaged along planes spanned by two of the lattice vectors of the periodic unit cell. The plane-averaged local (spin) density of states can be easily plotted e.g. as color-coded data using almost any plotting program. DensityTool can be applied to visualize and understand the local electronic structure of any system calculated with VASP. We expect it to be useful especially for researchers concerned with inhomogeneous systems, such as interfaces, defects, surfaces, adsorbed molecules, or hybrid inorganic-organic composites. Program summaryProgram Title: DensityToolCPC Library link to program files:https://doi.org/10.17632/26mcg4jwvd.1Developer's repository link:https://github.com/llodeiro/DensityToolLicensing provisions: MITProgramming language: FORTRANSupplementary material: A complementary User Manual and examples can be found in the developer's repository link.Nature of problem: Total and local density of states (LDOS) are widely-used quantities for understanding the electronic structure of condensed matter. In VASP, one of the mostly used packages for numerically solving the Kohn-Sham equations of density-functional theory, one does not directly obtain the LDOS. Usually, one works with partial density of states (PDOS), where the wave function is projected onto atomic orbitals. Alternatively, it is possible to calculate the band- and wavevector-dependent charge densities. Together with the band structure, LDOS can be obtained using post-processing tools.Solution method: DensityTool combines the band structure with band- and wavevector-dependent charge densities calculated by VASP to construct LDOS. In addition, spin-resolved LDOS (LSDOS) can be calculated for magnetic systems. Thus obtained LDOS and LSDOS can be further processed with DensityTool. In particular, it can be averaged along chosen spatial directions, which is convenient for example to demonstrate the spatially-resolved electronic structure of inhomogeneous systems.DensityTool can handle all three types of approaches implemented in VASP (non-magnetic, collinear magnetic, non-collinear magnetic including spin-orbit coupling), as well as all types of self-consistent band calculations as DFT (e.g., LDA, GGA, mGGA, BJ, and hybrid exchange-correlation functionals) and HF.For convenience, the output of DensityTool follows the format of VASP (the CHGCAR file), which can be easily used in other programs for visualization. We provide a sample python script to plot averaged L(S)DOS from the calculated data.

Self-consistent band calculation of the slab phase in the neutron-star crust

Fully self-consistent band calculation has been performed for slab phase in neutron-star inner crust, using the BCPM energy density functional. Optimized slab structure is calculated at given baryon density either with the fixed proton ratio or with the beta-equilibrium condition. Numerical results indicate the band gap of in order of keV to tens of keV, and the mobility of dripped neutrons are enhanced by the Bragg scattering, which leads to the macroscopic effective mass, $\bar{m}^*_z/m_n=0.65\sim 0.75$ near the bottom of the inner crust in neutron stars. We also compare the results of the band calculation with those of the Thomas-Fermi approximation. The Thomas-Fermi approximation becomes invalid at low density with high proton ratio.

Electronic Structure for Europium Chalcogenides with Modified Becke–Johnson Potential Plus an On-Site Coulomb U (mBJ+U)

The full potential linearized-augmented-plane-wave (FP-LAPW) method is generalized to a case of an all-electron fully-relativistic spin-polarized self-consistent band calculation based on the relativistic spin-density functional theory and the modified Becke–Johnson potential (TB-mBJ) plus an on-site coulomb U employed for greater generation of the band gap. The results show that these materials are semiconducting materials. The indirect energy gap obtained in this calculation is 1.63[Formula: see text]eV, 1.79[Formula: see text]eV and 1.96[Formula: see text]eV for EuS, EuSe and EuTe, respectively. It is clear from the plots that LSDA[Formula: see text][Formula: see text][Formula: see text]U is a poor technique for the calculation of the band gaps of chalcogenides (EuX). The calculated results for EuX (S, Se and Te) by mBJ[Formula: see text][Formula: see text][Formula: see text]U are in good agreement with the experimental values as compared to the other calculated results.

Structural, elastic, electronic and phonon properties of scandium-based compounds ScX3 (X = Ir, Pd, Pt and Rh): An ab initio study

Self-consistent band calculations on four compounds of the L12 structure are presented. The structural, elastic, electronic and phonon properties of ScX3 (X=Ir, Pd, Pt and Rh) compounds within density functional theory have been investigated. The calculated lattice constants for those compounds are in good agreement with the available theoretical and experimental values. The elastic constants (C11, C12 and C44) in L12 phase for ScX3 (X=Ir, Pd, Pt and Pd) compounds are calculated using the energy-strain method. These calculated elastic constants satisfy the mechanical stability criterion and the ductility of ScX3 (X=Ir, Pd, Pt and Rh) is predicted by Pugh’s criterion. The band structure and density of states (DOS), and phonon dispersion curves have been obtained and compared with the available results as well as with existing theoretical calculations. The present band structure calculations predict that the L12–ScX3 (X=Ir, Pd, Pt and Pd) compounds are metals. Finally specific heat at constant volume versus temperature is calculated and discussed using the quasi harmonic approximation.

Structural, electronic and vibrational properties of ordered intermetallic alloys CoZ (Z = Al, Be, Sc and Zr) from first-principles total-energy calculations

Self-consistent band calculations on four intermetallic compounds of the CsCl structure are presented. The calculations were performed employing the self-consistent ultrasoft pseudopotential method based on the density functional theory, within the local density approximation and the generalized gradient approximation. The calculations predicted that the equilibrium lattice constants are in excellent agreement with the experiment for CoAl and are 1% smaller than experimental values for CoBe, CoSc and CoZr, respectively. In the present study, ordered CoAl do not show any magnetic moment, whereas the other three compounds have moderate magnetic moments of about 0.2 and 0.7 Bohr magnetons (μB ) per atom. The elastic constants are calculated using two approaches, the energy-strain method and the use of phonon dispersion curves. The values obtained from the two methods are in reasonable agreement for the studied intermetallic compounds CoZ (Z = Al, Be, Sc and Zr). The brittleness and ductility properties of CoZ (Z = Al, Be, Sc and Zr) are determined by Poisson’s ratio σ criterion and Pugh’s criterion. The calculated elastic constants satisfy the mechanical stability criterion and the ductility of CoZr and CoSc is predicted by Pugh’s criterion. The band structure and density of states, and phonon dispersion curves have been obtained and compared with the available experimental results as well as with existing theoretical calculations. We studied and discussed the position of Fermi level for the selected four intermetallic compounds.

Magnetovolume Effects in Strongly and Intermediately Correlated Electron Systems

Analyses of magnetovolume effects such as the forced volume magnetostriction and the thermal expansion anomaly can give estimation of local amplitude of spin fluctuations. We have shown that bcc Fe has a rigid magnetic moment and that the magnetic moment of Invar type alloys is rather soft. It is pointed out that the total energy difference between the magnetized state and the nonmagnetic state obtained by self-consistent energy band calculations using a local-spin-density approximation is much higher than the Curie temperature for rigid spin systems and comparable for soft spin systems. The effects of zero point spin fluctuations observed in the YMn2 system are discussed.

Comparison of various exchange potentials in self-consistent OPW energy band calculations for cubic ZnS and ZnSe

Self-consistent energy band calculations using the OPW method have been performed on cubic ZnS and ZnSe using Slater's, Kohn-Sham's and Liberman's exchange potentials. Term values are given for the four symmetry points T, ≥X, L, and W. The variation of the energy bands with the exchange coefficient is shown to be linear. The imaginary part of the dielectric constant (ϵE2) is calculated by means of a pseudopotential interpolation technique for the three exchange potentials. It is found that both the band gap and the position of the peaks of the ϵE2 curves agree very closely with experiment for the self-consistent results derived from Slater's exchange, while the disagreement is substantial for the self-consistent results derived from Kohn-Sham's and Liberman's exchange potential.

Point defects in aluminum: Single vacancy

The results of a first-principles calculation for the formation energy of a single vacancy in aluminum are reported. Solid-state scattering theory is modified to use localized Gaussian orbitals (instead of Wannier functions) and applied to evaluate the one-electron contribution. Energy bands and wavefunctions, obtained from a self-consistent LCGO band calculation, are used to evaluate the Green's function matrix. The defect potential consists of that due to the absence of a neutral atom, including the lattice relaxation. It is further made partially self-consistent by including the electronic dielectric function for screening.

Temperature dependence of the built-in electric field strength of AlGaN/GaN heterostructures on Si(111) substrate

Photoreflectance spectroscopy was used to measure the barrier electric field strength F of as-grown AlGaN/GaN heterostructures on Si(111) substrate with two-dimensional electron gases in the temperature range from 80 up to 295 K. The Al-contents were in the range from 12 to 20%. Despite the difference of Al-contents and the large temperature variation we find only minute changes of F . This behaviour is explained by an almost constant strain state and thus a constant piezoelectric polarisation, which was concluded from the analysis of the GaN free excitonic transitions observed by photoluminescence excitation spectroscopy. Self-consistent conduction band calculations point to a pinning of the potential of the bare surface at 0.6 V, attributed to a large density of surface donor states.

The change in magnetic properties of Fe3Al compound due to substitution of Fe by Co

The magnetic moment using self-consistent spin-polarized energy band calculations of Fe 3 Al and Fe 2 CoAl Heusler phases are presented. These results are compared with the experimental values obtained from the magnetization curves of these materials.

Orbital magnetism in UFe2

Abstract The magnetization density of UFe 2 has been evaluated from self-consistent energy band calculations using screened exchange interactions. The calculated total magnetic moment is 1.4 μ B /f.u. and almost entirely due to an Fe moment of 0.72 μ B . The small uranium moment consists of spin and orbital moments of −0.19 and 0.17 μ B , respectively. The calculated magnetic amplitude is in excellent agreement with the results of neutron diffraction experiments. The calculated orbital contribution to the Fe moment of 0.1 μ B is one order of magnitude larger than measured.

The change in magnetic properties of Fe3Al compound due to substitution of Fe by Co

The magnetic moment using self-consistent spin-polarized energy band calculations of Fe3Al and Fe2CoAl Heusler phases are presented. These results are compared with the experimental values obtained from the magnetization curves of these materials.

Effect of Small Pt Doping on the Electronic Structure of NiMnSb Half-Heusler Alloys

The effect of small Pt doping on the electronic and magnetic properties of NiMnSb compound were studied using the spin-polarised tight-binding linear muffin-tin orbital method. The electronic structure of half-Heusler alloys depends on the relative arrangement of the atoms in the unit cell, changing from half-metal to metal. The local ordering drastically influences the value of the band gap. The self-consistent band calculations were performed for the experimental lattice constant and for the lattice parameter estimated from the minimum of the total energy.

Self-consistent Calculation of the Quasi-particle Energy Spectrum of Sodium using the Correlated Hartree–Fock Method

Self-consistent band calculation of sodium is performed in the correlated Hartree–Fock scheme proposed by Yasuhara and Takada [Phys. Rev. B 43 (1991) 7200], which contains information on the effective mass of the electron liquid in the form of a nonlocal spin-parallel potential, and the remaining information of the self-energy operator in the form of a local potential. The bandwidth of occupied states is somewhat increased under the influence of the non-local spin-parallel potential, compared with the free electron value. No significant difference can be found in the distortion of the Fermi surface between the present theory and the LDA.

Spin waves and large electron-phonon coupling near the metal-insulator transition in hole-doped high-Tcoxides

A theory for spin and lattice couplings to the electronic states in high-T$_C$ oxides is presented, with HgBa$_2$CuO$_4$ as an example. A simple analytical model suggests that the barrel band is sensitive to potential perturbations with long-range Fourier components, and that gaps can be opened at the Fermi energy. This is confirmed in self-consistent band calculations for modulations in elongated supercells, where spin waves and phonon coupling compete for equal q-vectors. It is argued that the wavelengths of spin-wave modulations and of the phonon modes with large coupling, depend on the doping. This mechanism supports the idea that pseudogaps are caused by stripe like spin-modulations in underdoped systems, while superconductivity is attributed to enhanced electron-phonon coupling for long wavelength phonons.

Orbital polarization effect on electronic structure and Fermi surface in ferromagnetic UGa 2

A self-consistent band calculation including an orbital polarization is performed for ferromagnetic UGa 2 on the basis of an all-electron fully-relativistic spin-polarized LAPW (RSPLAPW) method with exchange-correlation potentials in a local density approximation (LDA). The obtained magnetic moment is composed of the spin of about −2.0 μ B and the orbital moment of about 4.9 μ B. Accordingly, it can be interpreted as if the configuration were a localized 5f 2, though the f character contains about 2.5 as valence electrons. From the band structure, the Fermi surface is constructed in order to analyze frequencies observed by the de Haas–van Alphen (dHvA) effect.

Electronic Band Structure and Bonding Nature of Ternary Compound Ni1/3TiS2

The nature of bonding in intercalation compound Ni1/3TiS2 is studied by calculating the bond orders on the basis of self-consistent APW band calculations. It is found that the 3dγ states of Ni atoms make covalent-like bonds with the S 3p states while the 3de states of Ni and Ti atoms form bonding and antibonding bands. The results are discussed in connection with the recent photoemmission experiments.

Band Structure of SrLaGaO3+δ and SrLaAlO3+δ

The influence of oxygen on the electronic structure of strontium lanthanum gallate SrLaGaO 3 δ (SLG) and strontium lanthanum alluminate SrLaAlO 3+δ (SLA) crystals is presented. The band structures of SLA and SLG crystals were calculated by ab-initio tight-binding linear muffin tin orbital (TB LMTO) method within an atomic sphere approximation. The self-consistent band calculations indicate that SrLaGaO 4 , SrLaAlO 4 and SrLaAlO 3 are semiconductors, while SrLaGaO 3 , SrLaGaO 3+δ and SrLaAlO 3+δ for δ = 2 and 3 have the metallic character.