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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Much more to explore with an oxidation state of nearly four: Pr valence instability in intermetallic m -Pr 2 Co 3 Ge 5

For some intermetallic compounds containing lanthanides, structural transitions can result in intermediate electronic states between trivalency and tetravalency; however, this is rarely observed for praseodymium compounds. The dominant trivalency of praseodymium limits potential discoveries of emergent quantum states in itinerant 4$f$ 1 systems accessible using Pr 4+ -based compounds. Here, we use in situ powder x-ray diffraction and in situ electron energy-loss spectroscopy (EELS) to identify an intermetallic example of a dominantly Pr 4+ state in the polymorphic system Pr 2 Co 3 Ge 5 . The structure-valence transition from a nearly full Pr 4+ electronic state to a typical Pr 3+ state shows the potential of Pr-based intermetallic compounds to host valence-unstable states and provides an opportunity to discover previously unknown quantum phenomena. In addition, this work emphasizes the need for complementary techniques like EELS when evaluating the magnetic and electronic properties of Pr intermetallic systems to reveal details easily overlooked when relying on bulk magnetic measurements alone.

36 MATERIALS SCIENCE↗
DOE OSTI2018

“Extremum Loop” Model for the Valence-Band Spectrum of a HgTe/HgCdTe Quantum Well with an Inverted Band Structure in the Semimetallic Phase

The effective mass and spectrum of Landau levels are calculated for the valence band of a HgTe/HgCdTe quantum well with an inverted band structure in the quasi-classical “extremum loop” model. The Landau-level fan of the valence band in the semimetallic phase starts at B = 0 from the energy corresponding to that of the lateral maxima of this band and is overlapped with the Landau-level fan of the conduction band.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Valence structures of light and strange mesons from the basis light-front quantization framework

In this work, we apply the basis light-front quantization (BLFQ) framework to solve for the structures of mesons with light and strange valence quarks. Our approach treats mesons as relativistic bound states with quarks confined in both the transverse direction and the light-front longitudinal direction. The spin-orbit interactions of these confined quarks are further specified by the Nambu–Jona-Lasinio model. We address the U(1)A axial anomaly by including the Kobayashi-Maskawa-’t Hooft interaction regularized by our basis. We present the structures of the pion, the kaon, the eta meson, and the eta-prime meson in terms of their valence light-front wave functions obtained from the eigenvalue problem of our light-front Hamiltonian.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Spectroscopic evidence for temperature-dependent convergence of light- and heavy-hole valence bands of PbQ (Q = Te, Se, S)

Here, we have conducted a temperature- dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light -hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

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

Eu speciation in apatite at 1 bar: An experimental study of valence-state partitioning by XANES, lattice strain, and Eu/Eu* in basaltic systems

Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts.

Geochemistry & Geophysics↗
NASA NTRS1973

Valence resonance states of N2/-/

Resonant scattering observed in the transmission of electrons between 9 and 11 eV in collision with N2 has been attributed to the attachment of the scattered electron to an excited valence state. Resonance valence excited states of N2(-) are calculated that could support this hypothesis. Although calculated and observed resonance energies are in close agreement, the calculations do not reflect the observed complex angular behavior of the scattered electron.

Krauss, M.↗
NASA NTRS1989

Correlation of molecular valence- and K-shell photoionization resonances with bond lengths

The relationship between the interatomic distance and the positions of valence-shell and K-shell sigma(asterisk) photoionization resonances is investigated theoretically for the molecules C2, F2, N2, O2, CO, NO, C2H2, C2H4, C2H6, HCN, H2CO, N20, CO2, and C2N2. The results of molecular-orbital computations are presented in three-dimensional diagrams, which are shown to be similar to the wave functions of a particle in a cylindrical well, confirming the validity of free-electron molecular-orbital (FEMO) approximations for modeling the potential along the symmetry axis. FEMO orbital energies and resonance positions are found to be in good agreement with previous theoretical and experimental results. Also included is a Feshbach-Fano analysis of the relevance of virtual-valence orbitals to the appearance of single-channel resonances in molecular photoionization cross sections.

Sheehy, J. A.↗
NASA NTRS1990

Inelastic collisions of positrons with one-valence-electron targets

The total elastic and positronium formation cross sections of the inelastic collisions between positrons and various one-valence-electron atoms, (namely hydrogen, lithium, sodium, potassium and rubidium), and one-valence-electron ions, (namely hydrogen-like, lithium-like and alkaline-earth positive ions) are determined using an elaborate modified coupled-static approximation. Special attention is devoted to the behavior of the Ps cross sections at the energy regions lying above the Ps formation thresholds.

Abdel-Raouf, Mohamed Assad↗
NASA NTRS1991

The role of valence-band excitation in laser ablation of KCl

We present recent measurements of excited-atom and ion emission from KCl surfaces illuminated by vacuum-ultraviolet synchrotron radiation (h-nu = 8-28 eV) and ultraviolet laser light (h-nu = 4 eV). At low intensities characteristic of the synchrotron experiments, excited atoms are desorbed by simple valence-band excitation process involving the metallization of the KCl surface. At the higher intensities typical of laser desorption and ablation, we observe a strong decrease in K emission as a function of the number of laser shots, but an essentially constant yield of Cl. K(+) and Cl(-) emission at high intensities show similar behavior. The energetics of these desorption phenomena can be treated in a bond-orbital model which shows that creation of a single valence hole is sufficient to excite an ion to an anti-bonding state.

Haglund, Richard F., Jr.↗
NASA NTRS2005

Comparative Planetary Mineralogy: Valence State Partitioning of Cr, Fe, Ti, and V Among Crystallographic Sites in Olivine, Pyroxene, and Spinel from Planetary Basalts

We have considered the valence-state partitioning of Cr, Fe, Ti, and V over crystallographic sites in olivine, pyroxene, and spinel from planetary basalts. The sites that accommodate these cations are the M2 site (6-8 coordinated) and M1 site (6 coordinated) in pyroxene, the M2 site (6-8 coordinated) and M1 site (6 coordinated) in olivine, and the tetrahedral and octahedral sites in spinel. The samples we studied are basalts from Earth, Moon, and Mars, which have fO2 conditions that range from IW-2 (Moon) to IW+6 (Earth) with Mars somewhere between at IW to IW+2. In this range of fO2 the significant elemental valences are (from low to high fO2) Ti4+, V3+, Fe2+, Cr2+, Cr3+, V3+, V4+, and Fe3+. V2+ and Ti3+ play a minor role in the phases considered for the Moon, and are probably in very low concentrations. V5+ plays a minor role in these phases in terrestrial basalts because it is probably in lower abundance than V4+ and it has an ionic radii that is so small (0.054 nm, 6- coordinated,[1]) that it is almost at the lower limit for octahedral coordination. The role of Cr2+ in the Moon is significant, as Sutton et al. [2] found that lunar olivine contains mostly Cr2+ while coexisting pyroxene contains mostly Cr3+. Hanson one vacancy only accommodates one V4+. Thus more vacancies are required in V4+ substitutions into olivine. In the Moon V3+ is much more abundant than V4+ [7, 8]. Thus in lunar chromite V3+ follows Cr3+, whereas in Earth V4+ (which is much greater in abundance than V3+) follows Ti4+ (eg in ulv spinel). We could go on in this vein for some time but space limitations do not permit us to do so.

Papike, J. J.↗
NASA NTRS2012

Relationship Between Iron Valence States of Serpentine in CM Chondrites and Their Aqueous Alteration Degrees

The 0.6-0.7 micron absorption band observed for C-type asteroids is caused by the presence of Fe(3+) in phyllosilicates . Because Fe-bearing phyllosilicates, especially serpentine, are the most dominant product of aqueous alteration in the most abundant carbonaceous chondrites, CM chondrites, it is important to understand the crystal chemistry of serpentine in CM chondrites to better understand spectral features of C-type asteroids. CM chondrites show variable degrees of aqueous alteration, which should be related to iron valences in serpentine. It is predicted that the Fe(3+)/Sum of (Fe) ratios of serpentine in CM chondrites decrease as alteration proceeds by Si and Fe(3+) substitutions from end-member cronstedtite to serpentine, which should be apparent in the absorption intensity of the 0.6-0.7 micron band from C-type asteroids. In fact, the JAXA Hayabusa 2 target (C-type asteroid: 1993 JU3) exhibits heterogeneous spectral features (0.7 micron absorption band disappears by rotation). From these points of view, we have analyzed iron valences of matrix serpentine in several CM chondrites which span the entire observed range of aqueous alteration using Synchrotron Radiation X-ray Absorption Near-Edge Structure (SR-XANES). In this abstract we discuss the relationship between obtained Fe(3+)/Sum of (Fe) ratios and alteration degrees by adding new data to our previous studies

Mikouchi, T.↗
NASA NTRSDate not supplied

High-Throughput Screening of Li Solid-State Electrolytes With Bond Valence Methods and Graph Neural Networks

Li-based solid-state electrolyte (Li-SSE) materials enable safer, all-solid-state batteries but the computational search for candidates with favorable stability and Li-ion conductivity is challenging due to the size of the search space and the cost of evaluating transport properties with ab initio methods. We present a high-throughput screening approach for Li-SSE materials using a combination of bond-valence methods and graph neural networks. We demonstrate the screening approach with a dataset containing tens of thousands of Li-containing compounds. Furthermore, we combine the machine-learning screening procedure with an isovalent substitution scheme to generate and screen additional Li SSE candidates beyond existing databases. Finally, we discuss relative importances of geometric and bond-valence quantities in the training of graph neural networks, providing insight for future modeling of ionic conductivity in Li-SSE materials.

Materials discovery↗
NASA NTRSDate not supplied

Screening of Li-Based Solid Electrolytes Using Bond-Valence Methods and Graph Neural Networks

Li-based solid-state electrolyte (Li-SSE) materials enable safer, all-solid-state batteries but the computational search for candidates with favorable stability and Li-ion conductivity is challenging due to the size of the search space and the cost of evaluating transport properties with ab initio methods. We present a high-throughput screening approach for Li-SSE materials using a combination of bond-valence methods and graph neural networks. We demonstrate the screening approach with a dataset containing tens of thousands of Li-containing compounds. Furthermore, we combine the machine-learning screening procedure with an isovalent substitution scheme to generate and screen additional Li SSE candidates beyond existing databases. Finally, we discuss relative importances of geometric and bond-valence quantities in the training of graph neural networks, providing insight for future modeling of ionic conductivity in Li-SSE materials.

Materials discovery↗
DOE OSTI2018

Comparative Study on the Charge-Ordered and Mixed-Valence Phases of LuFe{sub 2}O{sub 4} via First-Principles Calculations

The first-principles density functional theory calculations have been used to investigate the relation between magnetic and structural properties of LuFe{sub 2}O{sub 4} for both the charge-ordered (CO) and the mixed-valence (MV) phases and also to compare the electronic properties for both phases. The computed structural parameters of rhombohedral and monoclinic systems are found to be in good agreement with experimental findings. An analysis of the magnetic properties is provided considering nonmagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic model orderings. In agreement with experimental results, CO modification of LuFe{sub 2}O{sub 4} is predicted to be ferrimagnetic. The GGA + U electronic structures show a semiconducting behavior of CO phase while a metallic character for MV LuFe{sub 2}O{sub 4} phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Study of Electronic, Magnetic, and Thermoelectric Properties of 24 Valence-Electron Fe{sub 2}TiSn Heusler Compound Using Modified Becke-Johnson Scheme

The first-principle investigation of electronic and transport properties of 24 valence-electron Fe{sub 2}TiSn Heusler compound have been carried out using modified Becke-Johnson scheme. The compound possesses Fm-3m structure configuration in the ground state. Spin-polarized electronic calculations reveal Fe{sub 2}TiSn is a semiconductor in both the spin configurations with a total magnetic moment as 0μ{sub B}, which is consistent with the Slater-Pauling rule. The thermoelectric properties were calculated by means of the semiclassical Boltzmann transport theory. The average values of electrical conductivity, thermal conductivity, and Seebeck coefficient at 300, 600, and 900 K temperatures versus chemical potential are reported and discussed in detail. Further, the results show the possibility to create n-type and p-type thermoelectrics within one type of Heusler compound. The large Seebeck coefficient advocates the compounds to be good for thermoelectric applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

High pressure synthesis of a new phase of YbAg 2 : Structure, valence of Yb and properties

The new phase of YbAg 2 was obtained using high-pressure and high-temperature reaction. YbAg 2 crystallizes in the MgZn 2 structure (the space group P6 3 /mmc space group, No 194) with a = 5.68153(3) Å and c = 9.31995(7) Å and the unit cell volume V = 260.54(3) Å 3 . The XANES analysis showed that the valence state of Yb is +2.8. The low-temperature dependences of the electrical resistivity and magnetic susceptibility can be adequately described by a T 2 term that supports the Fermi-liquid picture. Furthermore, the Kadowaki–Woods relation gives a low value of the degeneracy (N = 2).

36 MATERIALS SCIENCE↗
DOE OSTI2021

Hydrogen bond network and bond valence analysis on uranyl sulfate compounds with organic-based interstitial cations

Seven new uranyl sulfate compounds with organic charge-balancing cations have been synthesized and structurally characterized. The structural unit topologies of the two chains and five sheets have been previously reported in uranyl sulfate crystal chemistry, although they were synthesized using different organic molecules. With the inclusion of six of these structures to the 48 previously published uranyl sulfate compounds, a total of 54 known uranyl sulfate compounds with organic charge-balancing cations were compiled and analyzed. A graphical approach was used to compare the structural unit topologies, and a bond valence approach was used to quantify the hydrogen bond networks that exist between the interstitial cationic and solvent species to the uranyl sulfate anionic structural units. Finally, this analysis helped elucidate which oxygen atoms in the structural unit receive hydrogen bonds and how the organic cations stabilize the overall crystal structures in this subclass of U(VI) materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Impurity-enhanced core valence luminescence via Zn-doping in cesium magnesium chlorides

Scintillators with faster timing capabilities are currently in high demand for use in radiation detection systems in the fields of nuclear and medical physics. The limited number of suitable materials that meet the performance criteria of next generation detection systems presents an opportunity for discovery of new fast scintillator materials. In this work, the effects of doping several ultrafast core-valence luminescent (CVL) scintillators with divalent Zn is explored. Three compounds are investigated – CsMgCl 3 , Cs 2 MgCl 4 , and Cs 3 MgCl 5 – and single crystals of each doped with 5 mol% Zn are grown via the Bridgman method. Additionally, mixing across the full range of concentrations (from 0 % to 100 % Zn) is explored in the Cs 2 Mg 1-x Zn x Cl 4 and Cs 3 Mg 1-x Zn x Cl 5 systems. For low concentrations of Zn, light yields of all three compounds are enhanced (by up to ~60 %) compared to the pure crystals, achieving what we believe to be the brightest known CVL, CsMgCl 3 :Zn 5 % (3400 ± 170 ph/MeV light yield). More importantly, Zn doping does not affect the ultrafast timing properties, with each composition maintaining a single-component decay time around 1–3 ns. A sub-100 ps coincidence time resolution (CTR) is also achieved with CsMgCl 3 :Zn 5 %. The results of this work reveal a new avenue towards obtaining brighter CVL materials, which could open up possibilities for more advanced ultrafast scintillators to be discovered moving forward.

36 MATERIALS SCIENCE↗