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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Correlation consistent basis sets for actinides. I. The Th and U atoms

New correlation consistent basis sets based on both pseudopotential (PP) and all-electron Douglas-Kroll-Hess (DKH) Hamiltonians have been developed from double- to quadruple-zeta quality for the actinide atoms thorium and uranium. Sets for valence electron correlation (5f6s6p6d), cc − pV nZ − PP and cc − pV nZ − DK3, as well as outer-core correlation (valence + 5s5p5d), cc − pwCV nZ − PP and cc − pwCV nZ − DK3, are reported (n = D, T, Q). The -PP sets are constructed in conjunction with small-core, 60-electron PPs, while the -DK3 sets utilized the 3rd-order Douglas-Kroll-Hess scalar relativistic Hamiltonian. Both series of basis sets show systematic convergence towards the complete basis set limit, both at the Hartree-Fock and correlated levels of theory, making them amenable to standard basis set extrapolation techniques. To assess the utility of the new basis sets, extensive coupled cluster composite thermochemistry calculations of ThF{sub n} (n = 2 − 4), ThO{sub 2}, and UF{sub n} (n = 4 − 6) have been carried out. After accurately accounting for valence and outer-core correlation, spin-orbit coupling, and even Lamb shift effects, the final 298 K atomization enthalpies of ThF{sub 4}, ThF{sub 3}, ThF{sub 2}, and ThO{sub 2} are all within their experimental uncertainties. Bond dissociation energies of ThF{sub 4} and ThF{sub 3}, as well as UF{sub 6} and UF{sub 5}, were similarly accurate. The derived enthalpies of formation for these species also showed a very satisfactory agreement with experiment, demonstrating that the new basis sets allow for the use of accurate composite schemes just as in molecular systems composed only of lighter atoms. The differences between the PP and DK3 approaches were found to increase with the change in formal oxidation state on the actinide atom, approaching 5-6 kcal/mol for the atomization enthalpies of ThF{sub 4} and ThO{sub 2}. The DKH3 atomization energy of ThO{sub 2} was calculated to be smaller than the DKH2 value by ∼1 kcal/mol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

On the validity of the basis set superposition error and complete basis set limit extrapolations for the binding energy of the formic acid dimer

We report the variation of the binding energy of the Formic Acid Dimer with the size of the basis set at the Coupled Cluster with iterative Singles, Doubles and perturbatively connected Triple replacements [CCSD(T)] level of theory, estimate the Complete Basis Set (CBS) limit, and examine the validity of the Basis Set Superposition Error (BSSE)-correction for this quantity that was previously challenged by Kalescky, Kraka, and Cremer (KKC) [J. Chem. Phys. 140, 084315 (2014)]. Our results indicate that the BSSE correction, including terms that account for the substantial geometry change of the monomers due to the formation of two strong hydrogen bonds in the dimer, is indeed valid for obtaining accurate estimates for the binding energy of this system as it exhibits the expected decrease with increasing basis set size. We attribute the discrepancy between our current results and those of KKC to their use of a valence basis set in conjunction with the correlation of all electrons (i.e., including the 1s of C and O). We further show that the use of a core-valence set in conjunction with all electron correlation converges faster to the CBS limit as the BSSE correction is less than half than the valence electron/valence basis set case. The uncorrected and BSSE-corrected binding energies were found to produce the same (within 0.1 kcal/mol) CBS limits. We obtain CCSD(T)/CBS best estimates for D{sub e} = − 16.1 ± 0.1 kcal/mol and for D{sub 0} = − 14.3 ± 0.1 kcal/mol, the later in excellent agreement with the experimental value of −14.22 ± 0.12 kcal/mol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Non-resonant elastic scattering of low-energy photons by atomic sodium confined in quantum plasmas

The non-resonant elastic scattering of low-energy photons by the bound valence electron in the ground state 3s of atomic sodium confined in quantum plasmas is investigated theoretically. The incident photon energy is assumed to be much smaller than the 3s-3p excitation energy. The alkali atom sodium is first formulated as an effective one-electron problem in which the attractive interaction between the valence electron and the atomic ion core is simulated by a spherically symmetric model potential. The Shukla-Eliasson oscillatory exponential cosine screened-Coulomb potential model is then used to mimic the effective two-body (valence-core) interaction within quantum plasmas. Non-relativistic calculations performed within the electric dipole approximation indicate that the non-resonant elastic photon scattering cross-section undergoes a dramatic growth by several orders of magnitude as the quantum wave number increases. A qualitative explanation of this phenomenon is presented. In the absence of the oscillatory cosine screening term, a similar growth is observed at larger values of the quantum wave number. Our computed relevant atomic data are in very good agreement with the experimental as well as the previous theoretical data for the zero-screening (free atom) case, and with the very limited, accurate theoretical results available for the case of exponential screened-Coulomb two-body interaction, without the cosine screening term.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Effects of polarization of polar semiconductor on electrical properties of poly(vinylidene fluoride-trifluoroethylene)/ZnO heterostructures

The electrical properties of heterostructures composed of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and ZnO with different crystallographic polarities, i.e., O- and Zn-polar ZnO, were investigated. Distinct differences in the capacitance-voltage and polarization-voltage characteristics between the P(VDF-TrFE)/O- and Zn-polar ZnO were obtained in the depletion regions of ZnO. The band configurations were determined by X-ray photoelectron spectroscopy (XPS) using a synchrotron radiation beam to analyze the differences in the electrical properties of the P(VDF-TrFE)/O- and Zn-polar ZnO. The XPS spectra indicated that the valence band maximum of P(VDF-TrFE) is 2.9 and 2.7 eV higher than Zn- and O-polar ZnO, respectively. Thus, both structures have staggered band configurations with large valence band offsets, and the spontaneous polarization of ZnO is less effective on the band lineup. The electrical properties of the P(VDF-TrFE)/ZnO heterostructures are modulated through carrier generation because of the polarization-mediated interface charges and the staggered band alignments of the P(VDF-TrFE)/ZnO with a large valence band offset.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Band alignment of atomic layer deposited (HfZrO{sub 4}){sub 1−x}(SiO{sub 2}){sub x} gate dielectrics on Si (100)

The band alignment of atomic layer deposited (HfZrO{sub 4}){sub 1−x}(SiO{sub 2}){sub x} (x = 0, 0.10, 0.15, and 0.20) gate dielectric thin films grown on Si (100) was obtained by using X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap, valence band offset, and conduction band offset values for HfZrO{sub 4} silicate increased from 5.4 eV to 5.8 eV, from 2.5 eV to 2.75 eV, and from 1.78 eV to 1.93 eV, respectively, as the mole fraction (x) of SiO{sub 2} increased from 0.1 to 0.2. This increase in the conduction band and valence band offsets, as a function of increasing SiO{sub 2} mole fraction, decreased the gate leakage current density. As a result, HfZrO{sub 4} silicate thin films were found to be better for advanced gate stack applications because they had adequate band gaps to ensure sufficient conduction band offsets and valence band offsets to Si.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic structure study of wide band gap magnetic semiconductor (La{sub 0.6}Pr{sub 0.4}){sub 0.65}Ca{sub 0.35}MnO{sub 3} nanocrystals in paramagnetic and ferromagnetic phases

X-ray circular magnetic dichroism (XMCD), X-ray photoemission spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS) techniques were used to study the electronic structure of nanocrystalline (La{sub 0.6}Pr{sub 0.4}){sub 0.65}Ca{sub 0.35}MnO{sub 3} near Fermi-level. XMCD results indicate that Mn{sup 3+} and Mn{sup 4+} spins are aligned parallel to each other at 20 K. The low M-H hysteresis curve measured at 5 K confirms ferromagnetic ordering in the (La{sub 0.6}Pr{sub 0.4}){sub 0.65}Ca{sub 0.35}MnO{sub 3} system. The low temperature valence band XPS indicates that coupling between Mn3d and O2p is enhanced and the electronic states near Fermi-level have been suppressed below T{sub C}. The valence band UPS also confirms the suppression of electronic states near Fermi-level below Curie temperature. UPS near Fermi-edge shows that the electronic states are almost absent below 0.5 eV (at 300 K) and 1 eV (at 115 K). This absence clearly demonstrates the existence of a wide band-gap in the system since, for hole-doped semiconductors, the Fermi-level resides just above the valence band maximum.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ferromagnetic bond of Li{sub 10} cluster: An alternative approach in terms of effective ferromagnetic sites

In this work, a model to explain the unusual stability of atomic lithium clusters in their highest spin multiplicity is presented and used to describe the ferromagnetic bonding of high-spin Li{sub 10} and Li{sub 8} clusters. The model associates the (lack of-)fitness of Heisenberg Hamiltonian with the degree of (de-)localization of the valence electrons in the cluster. It is shown that a regular Heisenberg Hamiltonian with four coupling constants cannot fully explain the energy of the different spin states. However, a more simple model in which electrons are located not at the position of the nuclei but at the position of the attractors of the electron localization function succeeds in explaining the energy spectrum and, at the same time, explains the ferromagnetic bond found by Shaik using arguments of valence bond theory. In this way, two different points of view, one more often used in physics, the Heisenberg model, and the other in chemistry, valence bond, come to the same answer to explain those atypical bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Holes localized in nanostructures in an external magnetic field: g-factor and mixing of states

The energy spectrum and wave functions of holes in the valence band in semiconductor nanosystems, including quantum wells, quantum wires, and quantum dots, in an external magnetic field are theoretically investigated. The dependence of Zeeman splitting of the hole ground state upon variation in the size-quantization parameters with regard to the complex structure of the valence band and magnetic field-induced mixing of hole states is traced. Analytical formulas for describing the Zeeman effect in the valence band in the limiting cases of a quantum disk, spherically symmetric quantum dot, and quantum wire are presented. It is demonstrated that the g-factor of a hole is extremely sensitive to the hole-state composition (heavy or light hole) and, consequently, to the geometry of the size-quantization potential.

36 MATERIALS SCIENCE↗
DOE OSTI2020

On the New Oxyarsenides Eu 5 Zn 2 As 5 O and Eu 5 Cd 2 As 5 O

The new quaternary phases Eu5Zn2As5O and Eu5Cd2As5O have been synthesized by metal flux reactions and their structures have been established through single-crystal X-ray diffraction. Both compounds crystallize in the centrosymmetric space group Cmcm (No. 63, Z = 4; Pearson symbol oC52), with unit cell parameters a = 4.3457(11) Å, b = 20.897(5) Å, c = 13.571(3) Å; and a = 4.4597(9) Å, b = 21.112(4) Å, c = 13.848(3) Å, for Eu5Zn2As5O and Eu5Cd2As5O, respectively. The crystal structures include one-dimensional double-strands of corner-shared MAs4 tetrahedra (M = Zn, Cd) and As–As bonds that connect the tetrahedra to form pentagonal channels. Four of the five Eu atoms fill the space between the pentagonal channels and one Eu atom is contained within the channels. An isolated oxide anion O2– is located in a tetrahedral hole formed by four Eu cations. Applying the valence rules and the Zintl concept to rationalize the chemical bonding in Eu5M2As5O (M = Zn, Cd) reveals that the valence electrons can be counted as follows: 5 × [Eu2+] + 2 × [M2+] + 3 × [As3–] + 2 × [As2–] + O2–, which suggests an electron-deficient configuration. The presumed h+ hole is confirmed by electronic band structure calculations, where a fully optimized bonding will be attained if an additional valence electron is added to move the Fermi level up to a narrow band gap (Eu5Zn2As5O) or pseudo-gap (Eu5Cd2As5O). In order to achieve such a formal charge balance, and hence, narrow-gap semiconducting behavior in Eu5M2As5O (M = Zn, Cd), europium is theorized to be in a mixed-valent Eu2+/ Eu3+ state.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Topotactic Reduction‐Driven Crystal Field Excitations in Brownmillerite Manganite Thin Films

Topotactic reduction of perovskite oxides offers a powerful approach for discovering novel phenomena, such as superconducting infinite-layer nickelates and polar metallicity, and is commonly accompanied by the emergence of multiple valence states and/or complex crystal fields of transition metals. However, understanding the complex interplay between crystal chemistry, electronic structure, and physical properties at the spin- and orbital-resolved levels in these reduced systems remains elusive. Here, in this study, x-ray absorption spectroscopy, resonant inelastic x-ray scattering (RIXS), and density functional theory calculations are used to uncover topotactic metal-insulator transition and orbital-specific crystal field excitations in brownmillerite La 0.67 Ca 0.33 MnO 2.5 thin films. The Mn valence states are found to be Mn 2+ /Mn 3+ , along with their corresponding populations at octahedral and tetrahedral sites, which effectively weaken the Mn-O hybridization compared to the parent perovskite phase. As a result, La 0.67 Ca 0.33 MnO 2.5 films exhibit an antiferromagnetic insulating ground state. Moreover, by combining the RIXS measurements on selected single-valence manganites, specifically MnO, LaMnO 3 , and CaMnO 3 , with orbital- and spin-resolved density-of-states calculations, the study identifies the dd excitations of octahedrally and tetrahedrally coordinated Mn 2+ /Mn 3+ ions, directly linking the microscopic electronic structure to the macroscopic magnetic/electrical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. In this work, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m -1 K -1 ), a high power factor (11.6 μ W cm -1 K -2 ), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe 4 tetrahedra and long Tl-Se bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl + lone-pair electrons, boosts phonon-phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl + as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Gamma-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

An Anomalous Electron Configuration Among 3d Transition Metal Atoms

Physical properties of materials are mainly determined by valence electron configurations, where different valence shells would induce divergent phenomena. In compounds containing Sc 2+ , 3d electron occupancy is expected, same as other transition metal atoms like Ti 3+ . But this situation still awaits experimental verification. In this work, we selected ScS to measure the valence electron density and orbital population of Sc2+ through delicate quantitative convergent-beam electron diffraction. Though the [Ar]3d 1 configuration is confirmed in ionic Sc 2+ , the unintuitive occupation of 4s orbital is concluded with the absence of 3d orbital features around Sc-atom sites and nearly bare population of t2g orbital. It should be the first time to report this special electron configuration in a transition metal compound, in which 4s rather than 3d orbital is preferred. Our findings reveal the distinct behavior of Sc, and probable ways to modulate material properties by controlling electron orbitals.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Reversible Iron Oxyfluoride (FeOF)–Graphene Composites as Sustainable Cathodes for High Energy Density Lithium Batteries

Two large barriers are impeding the wide implementation of electric vehicles, namely driving-range and cost, primarily due to the low specific energy and high cost of mono-valence cathodes used in lithium-ion batteries. Iron is the ideal element for cathode materials considering its abundance, low cost and toxicity. However, the poor reversibility of (de)lithiation and low electronic conductivity prevent iron-based high specific energy multi-valence conversion cathodes from practical applications. In this work, a sustainable FeOF nanocomposite is developed with extraordinary performance. The specific capacity and energy reach 621 mAh g –1 and 1124 Wh kg –1 with more than 100 cycles, which triples the specific capacity, and doubles the specific energy of current mono-valence intercalation LiCoO 2 . This is the result of an effective approach, combing the nanostructured FeOF with graphene, realized by making the (de)lithiation reversible by immobilizing FeOF nanoparticles and the discharge products over the graphene surface and providing the interparticle electric conduction. Importantly, it demonstrates that introducing small amount of graphene can create new materials with desired properties, opening a new avenue for altering the (de)lithiation process. Finally, such extraordinary performance represents a significant breakthrough in developing sustainable conversion materials, eventually overcoming the driving range and cost barriers.

25 ENERGY STORAGE↗
DOE OSTI2025

Three-dimensional imaging of pion using lattice QCD: generalized parton distributions

In this work, we report a lattice calculation of x-dependent valence pion generalized parton distributions (GPDs) at zero skewness with multiple values of the momentum transfer −t. The calculations are based on an N f = 2 + 1 gauge ensemble of highly improved staggered quarks with Wilson-Clover valence fermion. The lattice spacing is 0.04 fm, and the pion valence mass is tuned to be 300 MeV. We determine the Lorentz-invariant amplitudes of the quasi-GPD matrix elements for both symmetric and asymmetric momenta transfers with similar values and show the equivalence of both frames. Then, focusing on the asymmetric frame, we utilize a hybrid scheme to renormalize the quasi-GPD matrix elements obtained from the lattice calculations. After the Fourier transforms, the quasi-GPDs are then matched to the light-cone GPDs within the framework of large momentum effective theory with improved matching, including the next-to-next-to-leading order perturbative corrections, and leading renormalon and renormalization group resummations. We also present the 3-dimensional image of the pion in impact-parameter space through the Fourier transform of the momentum transfer −t.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Transition probabilities of the low-lying levels of Kr XXIV

Energy levels, radiative rates, and wavelengths are calculated for all levels of 3s{sup 2}3p, 3s3p{sup 2}, 3s{sup 2}3d, 3p{sup 3}, 3s3p3d, 3p{sup 2}3d and 3s3d{sup 2} configurations of Al-like krypton ion (Kr XXIV). Multiconfigurational Dirac–Hartree–Fock (MCDHF) method is adopted for calculating energy levels and radiative rates. Transition probabilities are reported for E1 and M2 transitions from the ground level. The valence–valence and core–valence correlation effects are accounted for in a systematic way. Breit interactions and quantum electrodynamics effects are estimated in subsequent relativistic configuration interaction calculations. Comparisons are made with the available data in the literature and good agreement has been found which confirms the reliability of our results.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Effects of Co doping on the structure and physicochemical properties of hausmannite (Mn 3 O 4 ) and its transformation during aging

Hausmannite is a common low-valence manganese oxide mineral with a distorted spinel structure in surficial sediments. Although natural Mn oxides often contain various transition metal (TM) impurities, few studies have addressed the substitution mechanisms of TMs, such as cobalt (Co), with hausmannite and induced changes in the mineral physicochemical properties. In this study, Co-doped hausmannites with final Co/Mn molar ratios of 0.05 and 0.11 were synthesized and characterized by wet chemical analysis, powder X-ray diffraction (XRD), Fourier transformed infrared spectra (FTIR), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (XAFS). Co-doped samples have larger crystallite sizes and smaller specific surface areas than those of the hausmannite without Co. The Co cations exist in a mixed-valence of +3 and + 2. Powder XRD and acid dissolution experiments indicate that Co incorporates into the mineral structures, and increases the mineral structural stability. Co K-edge extended X-ray adsorption fine structure (EXAFS) analysis demonstrates that Co likely substitutes for lattice Mn(III). After aging at room temperature for three years, 14.1 ± 1.9% of pure hausmannite is transformed to manganite while only 2.6 ± 0.7% of manganite appears in the aged Co-doped sample with an initial Co/Mn molar ratio of 0.10. Finally, these results provide new insights into the incorporation mechanisms of TMs into low valence Mn oxides, and the stability of these minerals in natural environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Photoionization of C 60 at high energies

Calculations of the photoionization cross section of C60 including all valence and core subshells have been carried out for photon energies up to 1 keV using density functional theory (DFT) and time-dependent DFT (TDDFT) within the framework of a fully molecular model. The high-energy valence subshell cross sections behave rather differently than the results from model potentials where the cross section falls far too rapidly with energy. This unphysical behavior of the model calculations is traced to smearing out of the carbon nuclei in the model potentials which makes it difficult for momentum to be conserved at the higher energies. Comparison with 60 times carbon atomic cross section sheds light on similarities and differences. The high energy behavior of individual valence cross sections reflects the amount of C 2s/2p mixing in the molecular orbitals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Unraveling transition-metal-mediated stability of spinel oxide via in situ neutron scattering

The energy materials performance is intrinsically determined by structures from the average lattice structure to the atom arrangement, valence, and distribution of the containing transition metal (TM) elements. Understanding the mechanism of the structure transition and atom rearrangement via synthesis or processing is key to expediting the exploration of excellent energy materials. In this work, in situ neutron scattering is employed to reveal the real-time structure evolution, including the TM-O bonds, lattice, TM valence and the migration of the high-voltage spinel cathode LiNi 0.5 Mn 1.5 O 4 . The transition-metal-mediated spinel destabilization under the annealing at the oxygen-deficient atmosphere is pinpointed. Additionally, the formation of Mn 3+ is correlated to the TM migration activation, TM disordered rearrangement in the spinel, and the transition to a layered-rocksalt phase. The further TM interdiffusion and Mn 2+ reduction are also revealed with multi-stage thermodynamics and kinetics. The mechanisms of phase transition and atom migrations as functions of temperature, time and atmosphere present important guidance on the synthesis in various-valence element containing oxides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗