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At least 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Syntheses, and crystal and electronic structures of the new Zintl phases Na{sub 2}ACdSb{sub 2} and K{sub 2}ACdSb{sub 2} (A=Ca, Sr, Ba, Eu, Yb): Structural relationship with Yb{sub 2}CdSb{sub 2} and the solid solutions Sr{sub 2-x}A{sub x}CdSb{sub 2}, Ba{sub 2-x}A{sub x}CdSb{sub 2} and Eu{sub 2-x}Yb{sub x}CdSb{sub 2}

Presented are the details of the syntheses, crystal and electronic structures of a new family of Zintl phases Na{sub 2}ACdSb{sub 2} and K{sub 2}ACdSb{sub 2} (A=Ca, Sr, Ba, Eu, Yb), as well as the solid solutions Sr{sub 2-x}A{sub x}CdSb{sub 2}, Ba{sub 2-x}A{sub x}CdSb{sub 2} and Eu{sub 2-x}Yb{sub x}CdSb{sub 2}. The structures of Na{sub 2}ACdSb{sub 2} and K{sub 2}ACdSb{sub 2} (A=Ca, Sr, Ba, Eu, Yb) were determined to be of a new type with the non-centrosymmetric space group Pmc2{sub 1} (no. 26), Pearson symbol oP12, with lattice parameters a=4.684(1)-4.788(1) A; b=9.099(3)-9.117(2) A; c=7.837(1)-8.057(2) A for the Na{sub 2}ACdSb{sub 2} series, and a=4.6637(9)-5.0368(8) A; b=9.100(2)-9.8183(15) A; and c=7.7954(15)-8.4924(13) A for K{sub 2}ACdSb{sub 2}, respectively. The solid solutions Sr{sub 2-x}A{sub x}CdSb{sub 2}, Ba{sub 2-x}A{sub x}CdSb{sub 2} and Eu{sub 2-x}Yb{sub x}CdSb{sub 2} (x{approx}1) are isostructural and isoelectronic to the recently reported Yb{sub 2}CdSb{sub 2} (space group Cmc2{sub 1} (no. 36), Pearson symbol cP20). All discussed structures are based upon CdSb{sub 2}{sup 4-} polyanionic layers, similar to the ones observed in Yb{sub 2}CdSb{sub 2}, with various alkali- and/or alkaline-earth cations coordinated to them. Magnetic susceptibility and Seebeck coefficient measurements on selected Eu{sub 2-x}Yb{sub x}CdSb{sub 2} samples, taken at low temperatures up to 300 K, are also reported. -- Graphical abstract: The quaternary Zintl phases Na{sub 2}ACdSb{sub 2} and K{sub 2}ACdSb{sub 2} (A=Ca, Sr, Ba, Eu, Yb) with novel layered structures have been synthesized for the first time and structurally characterized by single-crystal X-ray diffraction. Reported as well are the results from crystallographic and property studies of the closely related solid solutions Sr{sub 2-x}A{sub x}CdSb{sub 2}, Ba{sub 2-x}A{sub x}CdSb{sub 2} (x{approx}1), and Eu{sub 2-x}Yb{sub x}CdSb{sub 2} (1<x{<=}2). Display Omitted Research highlights: {yields} Na{sub 2}ACdSb{sub 2} and K{sub 2}ACdSb{sub 2} (A=Ca, Sr, Ba, Eu, Yb) are new quaternary Zintl phases. {yields} Sr{sub 2-x}A{sub x}CdSb{sub 2}, Ba{sub 2-x}A{sub x}CdSb{sub 2} (x{approx}1), and Eu{sub 2-x}Yb{sub x}CdSb{sub 2} (1<x<2) are solid solutions. {yields} Both structures are layered, based on corner-shared CdSb{sub 4} tetrahedra. {yields} Eu{sub 2-x}Yb{sub x}CdSb{sub 2} (1<x<2) exhibit high Seebeck coefficient (217 {mu}V/K at RT).

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electronic structure and magnetic properties of disordered Co{sub 2}FeAl Heusler alloy

The effects of disorder on the magnetic properties of Co{sub 2}FeAl alloy are reported. X-ray diffraction exhibit A2-type disordered structure. Room temperature Mössbauer studies show the presence of two sextets with hyperfine field values of 31T and 30T along with a nonmagnetic singlet. The electronic structure of ordered and disordered Co{sub 2}FeAl alloys, investigated by means of the KKR Green's-function method shows that the magnetic moment of the ordered structure is 5.08μ{sub B} and is 5.10μ{sub B} when disordered. However, a much higher magnetic moment of 5.74μ{sub B} is observed experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Density Functional Theory Study on Electronic Structures and Magnetism for Nitrogen-Doped ZnS

Using the full-potential linearized augmented plane wave method, together with the generalized gradient approximation and modified Becke-Johnson as correlation potential, the electronic structures and magnetism for nitrogen-doped ZnS are investigated. Based on the generalized gradient approximation, calculations show nitrogen-substituting sulfur (N{sub S}) would induce the ZnS to be paramagnetic metal. As the band gap increasing to the experimental results (obtained by the modified Becke-Johnson potential), the N{sub S} defects would induce the ZnS to be a ferromagnetic metal. The total magnetic moment for ZnS supercell with single N{sub S} defect is 0.85 μ{sub B}. Positive chemical pair interactions imply that N{sub S} defects would form homogeneous distribution in ZnS host. Sulfur vacancies would give rise to the ZnS with N{sub S} defects system losing the magnetism. Moderate formation energy (0.71 eV) indicates ZnS with N{sub S} defects could be fabricated experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Communication: Recovering the flat-plane condition in electronic structure theory at semi-local DFT cost

The flat-plane condition is the union of two exact constraints in electronic structure theory: (i) energetic piecewise linearity with fractional electron removal or addition and (ii) invariant energetics with change in electron spin in a half filled orbital. Semi-local density functional theory (DFT) fails to recover the flat plane, exhibiting convex fractional charge errors (FCE) and concave fractional spin errors (FSE) that are related to delocalization and static correlation errors. We previously showed that DFT+U eliminates FCE but now demonstrate that, like other widely employed corrections (i.e., Hartree-Fock exchange), it worsens FSE. To find an alternative strategy, we examine the shape of semi-local DFT deviations from the exact flat plane and we find this shape to be remarkably consistent across ions and molecules. We introduce the judiciously modified DFT (jmDFT) approach, wherein corrections are constructed from few-parameter, low-order functional forms that fit the shape of semi-local DFT errors. Here we select one such physically intuitive form and incorporate it self-consistently to correct semi-local DFT. We demonstrate on model systems that jmDFT represents the first easy-to-implement, no-overhead approach to recovering the flat plane from semi-local DFT.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Structural, electronic and elastic properties of KCaF{sub 3} and RbCaF{sub 3} for vacuum-ultraviolet-transparent lens materials

The first principles calculation within the full potential linearized augmented plane wave (FP-LAPW) method is applied to study the structural, electronic and elastic properties of cubic perovskite-type compounds KCaF{sub 3} and RbCaF{sub 3}. The exchange correlation effects are included through the LDA, GGA and modified Becke-Johnson (mBJ) exchange potential. The calculated structural properties such as equilibrium lattice constant, the bulk modulus and its pressure derivative are in good agreement with the available data. KCaF{sub 3} and RbCaF{sub 3} have wide and indirect band gaps and they agree with experimental values. The elastic properties such as elastic constants, anisotropy factor, shear modulus, Young’s modulus and Poisson’s ratio are obtained for the first time. KCaF{sub 3} and RbCaF{sub 3} are elastically anisotropic and the B/G ratio indicate that these are ductile materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

First-Principles Calculations to Investigate the Structural, Electronic, and Half-Metallic Properties of Ti{sub 2}RhSn{sub 1-x}Si{sub x}, Ti{sub 2}RhSn{sub 1-x}Ge{sub x}, and Ti{sub 2}RhGe{sub 1-x}Si{sub x} (x = 0, 0.25, 0.5, 0.75, and 1) Quaternary Heusler Alloys

The structural, electronic, and magnetic properties of Ti{sub 2}RhSn{sub 1-x}Si{sub x}, Ti{sub 2}RhSn{sub 1-x}Ge{sub x}, and Ti{sub 2}RhGe{sub 1-x}Si{sub x} (x = 0, 0.25, 0.5, 0.75, and 1) quaternary Heusler alloys have been investigated using first-principles density functional theory within generalized gradient approximation (GGA) and local spin-density approximation with Hubbard-U corrections (LSDA+U). The calculations have been performed using the full-potential linearized augmented Plane wave plus local orbitals method. The crystal structures of Ti{sub 2}RhSn{sub 1-x}Si{sub x}, Ti{sub 2}RhSn{sub 1-x}Ge{sub x}, and Ti{sub 2}RhGe{sub 1-x}Si{sub x} were found to be cubic in (F-43m) symmetry for x = 0, 1 and cubic in (P-43m) symmetry for x = 0.25, 0.75, while tetragonal in (P-4m2) symmetry for x = 0.5. The results show that all Ti{sub 2}RhSn1-xSix, Ti{sub 2}RhGe{sub 0.75}Si{sub 0.25}, and Ti{sub 2}RhGe alloys are half-metallic ferrimagnetic. These new materials are good candidates for potential applications in spintronic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1981

Study of certain features of the electronic structure of the ternary alloys Ni2(Mn, Fe) and Ni3(Mn, Co)

The changes in electronic structure related to transport processes occurring during the alloying of he alloy Ni3Mn with iron and cobalt, and the ordering of the ternary alloys thus formed are presented. The Hall effect, the absolute thermal emf, the internal saturation induction, the Nernst-Ettingshausen constant, and the electrical resistivity were measured. Results show a decrease in the contribution of hole sections of the Fermi surface to the transport process occurs together with a considerable increase in the contribution of electron sections. In this case, the mobility of 3 dimensional holes decreases and the mobility of 4s electrons increases considerably.

Zhukova, V. M.↗
DOE OSTI2015

Electronic structure and magnetism in g-C{sub 4}N{sub 3} controlled by strain engineering

Regulation of magnetism and half-metallicity has attracted much attention because of its potential in spintronics. The magnetic properties and electronic structure of graphitic carbon nitride (g-C{sub 4}N{sub 3}) with external strain are determined theoretically based on the density function theory and many-body perturbation theory (G{sub 0}W{sub 0}). Asymmetric deformation induced by uniaxial strain not only regulates the magnetic characteristics but also leads to a transformation from half-metallicity to metallicity. However, this transition cannot occur in the structure with symmetric deformation induced by biaxial strain. Our results suggest the use of strain engineering in metal-free spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Ab initio studies of structural, electronic, optical, elastic and thermal properties of CuGaTe{sub 2}

We have performed ab initio calculations for the structural, electronic, optical, elastic and thermal properties of CuGaTe{sub 2}. In this study, we used an accurate full potential linearized augmented plane wave (FP-LAPW) method to find the equilibrium structural parameters and to compute the full elastic tensors. We report electronic and optical properties with the recently developed density functional of Tran and Blaha. Furthermore, optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients, optical conductivities, were calculated for photon energies up to 30 eV. The thermodynamical properties such as Debye temperature, entropy and Gruneisen parameter, bulk modulus and hardness were calculated employing the quasi-harmonic Debye model at different temperatures (0–1000 K) and pressures (0–8 GPa) and the silent results were interpreted. Most of the investigated parameters are reported for the first time.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electronic structure and electrical transport in ternary Al-Mg-B films prepared by magnetron sputtering

Nanostructured ternary Al-Mg-B films possess high hardness and corrosion resistance. In the present work, we study their electronic structure and electrical transport. The films exhibit semiconducting characteristics with an indirect optical-bandgap of 0.50 eV, as deduced from the Tauc plots, and a semiconductor behavior with a Fermi level of {approx}0.24 eV below the conduction band. Four-probe and Hall measurements indicated a high electrical conductivity and p-type carrier mobility, suggesting that the electrical transport is mainly due to hole conduction. Their electrical properties are explained in terms of the film nanocomposite microstructure consisting of an amorphous B-rich matrix containing AlMgB{sub 14} nanoparticles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Pushing the limits of the periodic table — A review on atomic relativistic electronic structure theory and calculations for the superheavy elements

We review the progress in atomic structure theory with a focus on superheavy elements and their predicted ground state configurations important for an element’s placement in the periodic table. To understand the electronic structure and correlations in the regime of large atomic numbers, it is essential to correctly solve the Dirac equation in strong Coulomb fields, and to take into account quantum electrodynamic effects. We specifically focus on the fundamental difficulties encountered when dealing with the many-particle Dirac equation. We further discuss the possibility for future many-electron atomic structure calculations going beyond the critical nuclear charge $Z_{crit}≈170$, where levels such as the $1s$ shell dive into the negative energy continuum ($E_{nκ} < –m_{e}c^2$). The nature of the resulting Gamow states within a rigged Hilbert space formalism is highlighted.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Evolution of the Kondo lattice electronic structure above the transport coherence temperature

Significance The temperature ( T )-dependent evolution of the Kondo lattice electronic structure is a long-standing topic of theoretical and experimental investigation, still lacking a truly microscopic theory that agrees with a full experimental characterization. Here multiple characteristic T scales of the interaction of localized f moments with conduction electrons in the Kondo lattice CeCo I n 5 are identified and investigated using angle-resolved photoemission measurements that substantiate dynamical mean-field theory which newly includes the full realism of crystalline electric-field (CEF) f splittings. Thereby errors in the itinerant versus localized f -state classification from standard density functional theory are corrected, microscopic insight into the broad T -range crossover of f -hybridization effects is gained, and a prediction of CEF degeneracy crossover below the lattice coherence T is made.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Overlapping fragments method for electronic structure calculation of large systems

Here, we introduce a method for the calculation of the electronic structure of systems that contain tens of thousands of atoms. The method is based on the division of the system into mutually overlapping fragments and the representation of the single-particle Hamiltonian in the basis of eigenstates of these fragments. In practice, for the range of the system size that we studied (up to tens of thousands of atoms), the dominant part of the calculation scales linearly with the size of the system when all the states within a fixed energy interval are required. Our technique is highly suitable for making good use of parallel computing architectures. We illustrate the method by applying it to diagonalize the single-particle Hamiltonian obtained using the density functional theory based charge patching method in the case of amorphous alkane and polythiophene polymers.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Density functional theory study of the structural, electronic, lattice dynamical, and thermodynamic properties of Li4SiO4 and its capability for CO2 capture

The structural, electronic, lattice dynamical, optical, thermodynamic, and CO{sub 2} capture properties of monoclinic and triclinic phases of Li{sub 4}SiO{sub 4} are investigated by combining density functional theory with phonon lattice dynamics calculations. We found that these two phases have some similarities in their bulk and thermodynamic properties. The calculated bulk modulus and the cohesive energies of these two phases are close to each other. Although both of them are insulators, the monoclinic phase of Li{sub 4}SiO{sub 4} has a direct band gap of 5.24 eV while the triclinic Li{sub 4}SiO{sub 4} phase has an indirect band gap of 4.98 eV. In both phases of Li{sub 4}SiO{sub 4}, the s orbital of O mainly contributes to the lower-energy second valence band (VB{sub 2}) and the p orbitals contribute to the fist valence band (VB{sub 1}) and the conduction bands (CBs). The s orbital of Si mainly contributes to the lower portions of the VB1 and VB{sub 2}, and Si p orbitals mainly contribute to the higher portions of the VB{sub 1} and VB{sub 2}. The s and p orbitals of Li contribute to both VBs and to CBs, and Li p orbitals have a higher contribution than the Li s orbital. There is possibly a phonon soft mode existing in triclinic {gamma}-Li{sub 4}SiO{sub 4}; in the monoclinic Li{sub 4}SiO{sub 4}, there are three phonon soft modes, which correspond to the one type of Li disordered over a few sites. Their LO-TO splitting indicates that both phases of Li{sub 4}SiO{sub 4} are polar anisotropic materials. The calculated infrared absorption spectra for LO and TO modes are different for these two phases of Li{sub 4}SiO{sub 4}. The calculated relationships of the chemical potential versus temperature and CO{sub 2} pressure for reaction of Li{sub 4}SiO{sub 4} with CO{sub 2} shows that Li{sub 4}SiO{sub 4} could be a good candidate for a high-temperature CO{sub 2} sorbent while used for postcombustion capture technology.

36 MATERIALS SCIENCE↗
NASA NTRS1996

Electronic Structure of Tl2Ba2CuO(6+Delta) Epitaxial Films Measured by X-Ray Photoemission

The valence electronic structure and core levels of Tl2Ba2CuO(6 + delta) (Tl-2201) epitaxial films have been measured with X-ray photoelectron spectroscopy and are compared to those of Tl2Ba2CaCu2O(8 + delta) (Tl-2212). Changes in the Tl-2201 core-level binding energies with oxygen doping are consistent with a change in the chemical potential. Differences between the Tl-2201 and Tl-2212 measured densities of states are consistent with the calculated Cu 3d and Tl 6s partial densities of states.

Vasquez, R. P.↗
DOE OSTI2015

Photoelectron Spectroscopy at the Graphene–Liquid Interface Reveals the Electronic Structure of an Electrodeposited Cobalt/Graphene Electrocatalyst

Abstract Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X‐ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl‐like species, namely Co(CO) x , promoting the reduction of Co 3+ to Co 2+ , which is believed to be the active site of the catalyst.

Velasco‐Velez, Juan J.↗
DOE OSTI2018

Magnetic Phase Transition in MnSi on the Base of the LSDA + U + SO Calculations of the Electronic Structure and the Spin-Fluctuation Theory

Spin states appearing near the magnetic phase transition in helicoidal ferromagnet MnSi are studied on the basis of the spin-fluctuation theory and the LSDA + U + SO calculations of the electronic structure. The temperature dependence of the uniform magnetic susceptibility is calculated near the magnetic phase transition temperature, and the result agrees well with the experiment. Spin correlators corresponding to various solutions of the equation of magnetic state are determined in the region of the magnetic phase transition expanded in temperature. It is shown that, in this region, a helicoidal short-range order appears in the form of the superposition of left and right spin spirals with stochastic weight coefficients. It is shown that the magnetic susceptibility divergences on the helicoid wave vector at the temperature of disappearance of local magnetization and during the transition to the paramagnetic state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetic order effects on the electronic structure of K M MnS 2 ( M = Cu , Li ) with the ThCr 2 Si 2 -type structure

We study the relationship between antiferromagnetic order and the electronic properties of KCuMnS 2 with the ThCr 2 Si 2 -type structure. We propose two magnetic structures for KCuMnS 2 with the ThCr 2 Si 2 -type structure. Powder samples of KCuMnS 2 and KLiMnS 2 were prepared for structural studies and magnetization measurements. In both compounds, the Mn 2+ site is alloyed by either Cu + or Li + . We also prepared single crystals of KCuMnS 2 for x-ray and neutron-diffraction studies and resistivity measurements. We relate these properties to the electronic structure calculated with density functional theory. Neutron-diffraction studies reveal that KCuMnS 2 exhibits long-range magnetic ordering with a Néel temperature near 160 K and a moment of 0.92(2) μ B /Mn 2+ at 6 K. In contrast, KLiMnS 2 never exhibits long-range magnetic ordering down to 3.5 K. Here, both sulfides never display a crystallographic phase transition from our temperature-dependent x-ray and neutron-diffraction studies. We discuss the magnetic phases in detail and how they relate to isostructural phases such as iron-based superconductors and related chalcogenides. Electrical resistance measurements indicate that while KCuMnS 2 is semiconducting, there is an anomaly around the Néel temperature, which indicates that long-range magnetism influences its electronic structure.

36 MATERIALS SCIENCE↗