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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Titanium, vanadium and chromium valences in silicates of ungrouped achondrite NWA 7325 and ureilite Y-791538 record highly-reduced origins

Titanium, Cr, and V valences were determined by applying micro-X-ray Absorption Near Edge Structure (micro-XANES) spectroscopy methods to individual grains of olivine and pyroxene in the ungrouped achondrite NWA 7325 and ureilite Y-791538, as well as to plagioclase in NWA 7325. The advantages of applying multiple, multivalent-element-based oxybarometers to individual grains are (1) the ability to cover the entire oxygen fugacity (fO2) range encountered in nature, and (2) the increased reliability from consistent results for semi-independent fO2 proxies. fO2 values were inferred from each mineral valence determination after correcting with available laboratory-experiment-derived, valence-specific partition coefficients to obtain melt valences and then calibrating with the fO2 values of the relevant equal species proportions points suggested for igneous (primarily basaltic) systems. The resulting olivine and pyroxene valences are highly reduced and similar in the two meteorites with substantial fractions of Cr2+, Ti3+ and V2+. The exception is Cr in NWA 7325 pyroxene which is much more oxidized than the Cr in its olivine. Chromium and Ti in plagioclase in NWA 7325 is relatively oxidized (V valence not determined). The anomalously oxidized Cr in NWA 7325 pyroxene may be due to a secondary reheating event that oxidized Cr in the pyroxene without similarly oxidizing Ti and V. Such a separation of the redox couples may be an effect of re-equilibration kinetics, where the valence of Cr would be more rapidly modified. These valences yielded similar mean fO2s for the two meteorites; IW-3.1 ± 0.2 for NWA 7325 and IW-2.8 ± 0.2 for Y-791538, consistent with an origin of NWA 7325 in either Mercury or an asteroid that experienced redox conditions similar to those on the ureilite parent body.

58 GEOSCIENCES↗
DOE OSTI2016

Effect of Pressure on Valence and Structural Properties of YbFe 2 Ge 2 Heavy Fermion Compound A Combined Inelastic X-ray Spectroscopy, X-ray Diffraction, and Theoretical Investigation

The crystal structure and the Yb valence of the YbFe 2 Ge 2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. Furthermore, the measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr 2 Si 2 -type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ~9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

Kumar, Ravhi S.↗
DOE OSTI2018

A valence balanced rule for discovery of 18-electron half-Heuslers with defects

Using first-principles DFT we systematically investigate the thermodynamic stability and off-stoichiometry in 108 nominal 19-electron half-Heusler (hH) compounds. We demonstrate unambiguously that considering a cation deficiency towards the off-stoichiometric valence balanced, VEC = 18 composition is necessary for explaining the stability of all previously reported nominal VEC = 19 compounds. This is understandable in terms of an energy benefit from valence balance considering the valence of each atom using Zintl chemistry that offsets the energy penalty of forming defects in nearly all cases. Thus, we propose a valence balanced rule to understand the ground state stability of half-Heuslers irrespective of stoichiometry and nominal electron count (8, 18 or 19). Using this generalized rule we (a) predict 16 previously unreported nominal 19-electron XYZ half-Heuslers and (b) rationalize the reports of giant off-stoichiometries in compounds such as Ti (1–x) NiSb which has been known for over 50 years. Of the 16 new compounds predicted here, Ti (1–x) PtSb was synthesized and the half-Heusler phase confirmed through X-ray studies. Here, the flexibility in stoichiometry of the half-Heusler systems to attain a stable valence balanced composition by accommodating large defect concentrations opens up multiple dimensions for discovery of multi-component defective half-Heuslers based on intrinsic and extrinsic defects which compensate for the nominally non-18-electron count of the structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Accurate determination of the valence band edge in hard x-ray photoemission spectra using GW theory

We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal–Organic Framework Fe(1,2,3-triazolate) 2 (BF 4 ) x

Metal–organic frameworks are of interest for use in a variety of electrochemical and electronic applications, although a detailed understanding of their charge transport behavior, which is of critical importance for enhancing electronic conductivities, remains limited. Herein, we report isolation of the mixed-valence framework materials, Fe(tri) 2 (BF 4 ) x (tri – = 1,2,3-triazolate; x = 0.09, 0.22, and 0.33), obtained from the stoichiometric chemical oxidation of the poorly-conductive iron(II) framework Fe(tri) 2 , and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe(tri) 2 (BF 4 ) 0.33 , displays a room-temperature conductivity of 0.3(1) S/cm, which represents an increase of eight orders of magnitude from that of the parent material and is one of the highest conductivity values reported among three-dimensional metal–organic frameworks. Detailed characterization of Fe(tri) 2 and the Fe(tri) 2 (BF 4 ) x materials via powder X-ray diffraction, Mössbauer spectroscopy, and IR and UV-vis-NIR diffuse reflectance spectroscopies reveals that the high conductivity arises from intervalence charge transfer between mixed-valence low-spin Fe II/III centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized Fe II/III species in Fe(tri) 2 (BF 4 ) x at 290 K, one of the first such observations for a metal–organic framework. The electronic structure of valence-pure Fe(tri) 2 and the charge transport mechanism and electronic structure of mixed-valence Fe(tri) 2 (BF 4 ) x frameworks are discussed in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Simulating Ru L 3 -Edge X-ray Absorption Spectroscopy with Time-Dependent Density Functional Theory: Model Complexes and Electron Localization in Mixed-Valence Metal Dimers

Ruthenium L3-edge X-ray absorption (XA) spectroscopy probes unoccupied 4d orbitals of the metal atom and is increasingly being used to investigate the local electronic structure in ground and excited electronic states of Ru complexes. The simultaneous development of computational tools for simulating Ru L3-edge spectra is crucial for interpreting the spectral features at a molecular level. This study demonstrates that time-dependent density functional theory (TDDFT) is a viable and predictive tool for simulating ruthenium L3-edge XA spectroscopy. We systematically investigate the effects of exchange correlation functional and implicit and explicit solvent interactions on a series of RuII and RuIII complexes in their ground and electronic excited states. The TDDFT simulations reproduce all of the experimentally observed features in Ru L3-edge XA spectra within the experimental resolution (0.4 eV). Our simulations identify ligand-specific charge transfer features in complicated Ru L3-edge spectra of [Ru(CN)6]4- and RuII polypyridyl complexes illustrating the advantage of using TDDFT in complex systems. We conclude that the B3LYP functional most accurately predicts the transition energies of charge transfer features in these systems. We use our TDDFT approach to simulate experimental Ru L3-edge XA spectra of transition metal mixed-valence dimers of the form [(NC)5MII-CN-RuIII(NH3)5] (where M = Fe or Ru) dissolved in water. Our study determines the spectral signatures of electron delocalization in Ru L3-edge XA spectra. We find that the inclusion of explicit solvent molecules is necessary for reproducing the spectral features and the experimentally determined valencies in these mixed-valence complexes. This study validates the use of TDDFT for simulating Ru 2p excitations using popular quantum chemistry codes and providing a powerful interpretive tool for equilibrium and ultrafast Ru L3-edge XA spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films

We use a high signal-to-noise X-ray photoelectron spectrum of bulk PbS, GW calculations, and a model assuming parabolic bands to unravel the various X-ray and ultraviolet photoelectron spectral features of bulk PbS as well as determine how to best analyze the valence band region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are commonly used to probe the difference between the Fermi level and valence band maximum (VBM) for crystalline and thin-film semiconductors. However, we find that when the standard XPS/UPS analysis is used for PbS, the results are often unrealistic due to the low density of states at the VBM. Instead, a parabolic band model is used to determine the VBM for the PbS QD films, which is based on the bulk PbS experimental spectrum and bulk GW calculations. Our analysis highlights the breakdown of the Brillioun zone representation of the band diagram for large band gap, highly quantum confined PbS QDs. We have also determined that in 1,2-ethanedithiol-treated PbS QD films the Fermi level position is dependent on the QD size; specifically, the smallest band gap QD films have the Fermi level near the conduction band minimum and the Fermi level moves away from the conduction band for larger band gap PbS QD films. This change in the Fermi level within the QD band gap could be due to changes in the Pb:S ratio. In addition, we use inverse photoelectron spectroscopy to measure the conduction band region, which has similar challenges in the analysis of PbS QD films due to a low density of states near the conduction band minimum.

14 SOLAR ENERGY↗
DOE OSTI2016

A low-temperature study of manganese-induced ferromagnetism and valence band convergence in tin telluride

SnTe is renowned for its promise in advancing energy-related technologies based on thermoelectricity and for its topological crystalline insulator character. Here, we demonstrate that each Mn atom introduces ∼4 μ{sub B} (Bohr magneton) of magnetic moment to Sn{sub 1−x}Mn{sub x}Te. The Curie temperature T{sub C} reaches ∼14 K for x = 0.12, as observed in the field dependent hysteresis of magnetization and the anomalous Hall effect. In accordance with a modified two-band electronic Kane model, the light L-valence-band and the heavy Σ-valence-band gradually converge in energy with increasing Mn concentration, leading to a decreasing ordinary Hall coefficient R{sub H} and a favorably enhanced Seebeck coefficient S at the same time. With the thermal conductivity κ lowered chiefly via point defects associated with the incorporation of Mn, the strategy of Mn doping also bodes well for efficient thermoelectric applications at elevated temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo2As2 (A = Eu and Ca)

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo(2)As(2) (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 eleetron) change in the population of the 3d orbitals. The mixed valence state of En observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.

Tan, Xiaoyan↗