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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 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Reduction of π–Expanded Cyclooctatetraene with Lithium: Stabilization of the Tetra–Anion through Internal Li + Coordination

The chemical reduction of a π-expanded polycyclic framework comprising a cyclooctatetraene moiety, octaphenyltetrabenzocyclooctatetraene, with lithium metal readily affords the corresponding tetra-anion instead of the expected aromatic dianion. As revealed by X-ray crystallography, the highly contorted tetra-anion is stabilized by coordination of two internally bound Li + , while two external cations remain solvent separated. The variable-temperature 7 Li NMR spectra in THF confirm the presence of three types of Li + ions and clearly differentiate internal binding, consistent with the crystal structure. Density-functional theory calculations suggest that the formation of the highly charged tetra-reduced carbanion is stabilized through Li + coordination under the applied experimental conditions.

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

Pressure-Induced Structural Phase Transition in CeNi: X-ray and Neutron Scattering Studies and First-Principles Calculations

The pressure-induced structural phase transition in the intermediate-valence compound CeNi has been investigated by x-ray and neutron powder diffraction techniques. It is shown that the structure of the pressure-induced CeNi phase (phases) can be described in terms of the Pnma space group. Equations of state for CeNi on both sides of the phase transition are derived and an approximate P-T phase diagram is suggested for P<8 GPa and T<300 K. The observed Cmcm→Pnma structural transition is then analyzed using density functional theory calculations, which successfully reproduce the ground state volume, the phase transition pressure, and the volume collapse associated with the phase transition.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Noble gas encapsulation into carbon nanotubes: Predictions from analytical model and DFT studies

The energetics for the interaction of the noble gas atoms with the carbon nanotubes (CNTs) are investigated using an analytical model and density functional theory calculations. Encapsulation of the noble gas atoms, He, Ne, Ar, Kr, and Xe into CNTs of various chiralities is studied in detail using an analytical model, developed earlier by Hill and co-workers. The constrained motion of the noble gas atoms along the axes of the CNTs as well as the off-axis motion are discussed. Analyses of the forces, interaction energies, acceptance and suction energies for the encapsulation enable us to predict the optimal CNTs that can encapsulate each of the noble gas atoms. We find that CNTs of radii 2.98 − 4.20 Å (chiral indices, (5,4), (6,4), (9,1), (6,6), and (9,3)) can efficiently encapsulate the He, Ne, Ar, Kr, and Xe atoms, respectively. Endohedral adsorption of all the noble gas atoms is preferred over exohedral adsorption on various CNTs. The results obtained using the analytical model are subsequently compared with the calculations performed with the dispersion-including density functional theory at the M06 − 2X level using a triple-zeta basis set and good qualitative agreement is found. The analytical model is however found to be computationally cheap as the equations can be numerically programmed and the results obtained in comparatively very less time.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electron Counting and High-Pressure Phase Transformations in Metal Hexaborides

Pressure-induced structural transitions of the alkaline earth hexaborides, CaB 6 , SrB 6 , and BaB 6 , are studied theoretically using electron counting rules and density functional theory calculations. Here, we demonstrate the applicability of gas-phase borane electron counting methods to solid-state metal borides under pressure and validate the assumptions of the rules by density functional theory (DFT) calculations. All three compounds share ambient-pressure and high-pressure structures, but BaB 6 differs from CaB 6 and SrB 6 at intermediate pressures. The unique BaB 6 phase is shown to break electron counting rules, while all other phases obey them. This anomaly is resolved by DFT, which reveals B–Ba covalency and unusual B–B π bonding under pressure. The relationships between structure and bonding can help us to understand the exotic behavior of lanthanide hexaborides and design new borides with desirable properties. Developing electron counting procedures for solids will enhance materials discovery efforts with chemical intuition.

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

Influence of Rotational Distortions on Li + - and Na + -Intercalation in Anti-NASICON Fe 2 (MoO 4 ) 3

Anti-NASICON Fe 2 (MoO 4 ) 3 (P2 1 /c) shows significant structural and electrochemical differences in the intercalation of Li + and Na + ions. To understand the origin of this behavior, we have used a combination of in situ X-ray and high-resolution neutron diffraction, total scattering, electrochemical measurements, density functional theory calculations, and symmetry-mode analysis. We find that for Li + -intercalation, which proceeds via a two-phase monoclinic-to-orthorhombic (Pbcn) phase transition, the host lattice undergoes a concerted rotation of rigid polyhedral subunits driven by strong interactions with the Li + ions, leading to an ordered lithium arrangement. Na + -intercalation, which proceeds via a two-stage solid solution insertion into the monoclinic structure, similarly produces rotations of the lattice polyhedral subunits. Furthermore, using a combination of total neutron scattering data and density functional theory calculations, we find that while these rotational distortions upon Na + -intercalation are fundamentally the same as for Li + -intercalation, they result in a far less coherent final structure, with this difference attributed to the substantial difference between the ionic radii of the two alkali metals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

A Combined Machine Learning and High-Energy X-ray Diffraction Approach to Understanding Liquid and Amorphous Metal Oxides

Determining the structure-property relations of liquid and amorphous metal oxides is challenging, due to their variable short-range order and polyhedral connectivity. To predict chemically realistic structures, we have developed a Machine Learned, Gaussian Approximation Potential (GAP) for HfO 2 , with a focus on enhanced sampling of the training database and accurate density functional theory calculations. By using training datasets for the GAP model at the level of Density Functional Theory-Strongly Constrained and Appropriately Normed (DFT-SCAN) level of theory, our results show that the topology of both the low viscosity liquid and the amorphous form are dominated by edge-shared chains and small corner-shared rings of polyhedra. This topology is shown to be consistent with the structure of other liquid and amorphous transition metal oxides of variable ion size, such as TiO 2 and ZrO 2 . Current limitations of the ML-GAP modeling method for obtaining glass structures and future perspectives are also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2022

High-pressure structural systematics of dysprosium metal compressed in a neon pressure medium to 182 GPa

Here we present an experimental and theoretical study of dysprosium metal compressed in the soft pressure transmitting medium Ne up to 182 and 300 GPa, respectively. Angle-dispersive x-ray powder diffraction data from each of the high-pressure polymorphs shows anisotropic compression behavior indicating changes to the electron density distribution throughout its polymorphic landscape. We compare the monoclinic (mC4) and orthorhombic (oF16) structures for the collapsed structure for Dy above 82 GPa and verify that the oF16 structure offers a better fit to our data than the previously reported mC4 structure. Further, we have found that the oF16 structure undergoes similar anisotropic compression of its lattice parameters, with a turning point above 160 GPa; suggesting a potential phase transition at pressures much higher than achieved in this study. Density functional theory calculations show the likely candidate for this new high-pressure phase is the isosymmetric oF8 structure, which is predicted to be lower in energy than the oF16 structure above 275 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ab-initio vibrational dynamics study of silver nanoclusters

The size dependent structural, electronic and vibrational properties of silver metal clusters along with its bulk counterparts have been studied using the first principles calculations within density functional theory. The size effect was studied by calculating a series of clusters discussed in terms of the increase in band gap when going from the bulk to cluster. The phonon calculations show that the clusters with smaller size are dynamically stable. Our findings will give some reference to the insight understanding of the electronic and vibrational properties of size orientation dependent Ag metal cluster.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Surface Termination Dependent Work Function and Electronic Properties of Ti 3 C 2 T x MXene

MXenes, an emerging family of 2D transition metal carbides and nitrides, have shown promise in various applications, such as energy storage, electromagnetic interference shielding, conductive thin films, photonics, and photothermal therapy. Their metallic nature, wide range of optical absorption, and tunable surface chemistry are the key to their success in those applications. The physical properties of MXenes are known to be strongly dependent on their surface terminations. In this study, we investigated the electronic properties of Ti 3 C 2 T x for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photoelectron spectroscopy, and density functional theory calculations. We find that fluorine occupies solely the face-centered cubic adsorption site, whereas oxygen initially occupies at least two different adsorption sites, followed by a rearrangement after fluorine desorption at high annealing temperatures. The measured electronic structure of Ti 3 C 2 T x showed strong dispersion of more than 1 eV, which we conclude to stem from Ti–O bonds by comparing it to calculated band structures. We further measured the work function of Ti 3 C 2 T x as a function of annealing temperature and found that it is in the range of 3.9–4.8 eV, depending on the surface composition. A comparison of the experimental work function to detailed density functional theory calculations shows that the measured value is not simply an average of the work function values of uniformly terminated Ti 3 C 2 surfaces but that the interplay between the different surface moieties and their local dipoles plays a crucial role.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic transition above room temperature in CaMn{sub 7}O{sub 12} films

We report on the electronic phase transition in CaMn{sub 7}O{sub 12} quadruple perovskite films synthesized by oxide molecular beam epitaxy on SrLaAlO{sub 4} and La{sub 0.3}Sr{sub 0.7}Al{sub 0.65}Ta{sub 0.35}O{sub 3} substrates. We use x-ray diffraction and transmission electron microscopy to confirm that the CaMn{sub 7}O{sub 12} phase has been realized. Temperature dependent resistivity measurements reveal a signature of a charge ordering phase transition at ≈425 K, consistent with bulk CaMn{sub 7}O{sub 12}. The transition temperature is found to be relatively invariant to changes in the cation stoichiometry. Density functional theory calculations reveal the changes in atomic and electronic structure induced by the charge ordering transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Insights into the Electrochemical Oxygen Evolution Reaction with ab Initio Calculations and Microkinetic Modeling: Beyond the Limiting Potential Volcano

Density functional theory calculations are potentially useful for both understanding the activity of experimentally tested catalysts and screening for new catalyst materials. For electrochemical oxygen evolution reaction (OER) catalysts, these analyses are usually performed considering only the thermodynamics of the reaction path, which typically consists of adsorbed OH*, O*, and OOH*. Scaling relationships between the stability of these intermediates lead to a limiting potential volcano whose optimum is constrained by the roughly constant offset between the binding energies of OH* and OOH*. In this work, we evaluate OER kinetics at rutile IrO 2 , RuO 2 , RhO 2 , and PtO 2 surfaces by computing reaction barriers with an explicit model of the electrochemical interface. We conclude that the kinetics of proton transfer between oxygen atoms at the surface and in the electrolyte is facile and that O–O bond formation is most likely rate-determining in all cases. Combining these results with a microkinetic model and a scaling relationship for the OOH* formation barrier, we construct a new activity volcano whose optimum is similar to that of the limiting potential volcano for typical current densities. This kinetic volcano is also shown to agree reasonably well with experimental observations. Based on this analysis, we propose a more precise requirement for improving OER catalysts beyond the state of the art: the transition state for OOH* formation must be stabilized as opposed to the fully formed OOH* final state as has been previously presumed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Transition-metal and metalloid substitutions in L1{sub 0}-ordered FeNi

The effect of atomic substitutions on the magnetization, exchange, and magnetocrystalline anisotropy energy of L1{sub 0}-ordered FeNi (tetrataenite) is computationally investigated. The compound naturally occurs in meteorites but has attracted renewed attention as a potential material for permanent magnets, and elemental additives will likely be necessary to facilitate the phase formation. Our density functional theory calculations use the Vienna ab-initio simulation package, applied to 4-atom unit cells of Fe{sub 2}XNi and 32-atom supercells (X = Al, P, S, Ti, V, Cr, Mn, Fe, Co). While it is found that most additives deteriorate the magnetic properties, there are exceptions: excess substitutional Fe and Co additions improve the magnetization, whereas Cr, S, and interstitial B additions improve the magnetocrystalline anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Design and Synthesis of PtPdNiCoMn High‐Entropy Alloy Electrocatalyst for Enhanced Alkaline Hydrogen Evolution Reaction: A Theoretically Supported Predictive Design Approach

Electrocatalytic hydrogen generation requires a multifunctional electrocatalyst with abundant active sites to drive multielectron transfer reactions. High entropy alloys (HEA) are five or more-elements with high configurational entropy are considered unique materials for next-generation electrocatalysts. Here, in this work, based on new screening guidelines for catalyst selections that combine density-functional theory calculated Gibbs formation-enthalpy with bond length and electronegativity variance, a novel HEA electrocatalyst consisting of five elements, namely, Pt, Pd, Ni, Co, and Mn has been designed. By simple room temperature electrodeposition, the designed catalyst is prepared and its hydrogen evolution reaction (HER) is explored and validated through experimental and theoretical approaches. The HEA demonstrated a superior HER activity with an overpotential of 22.6 mV at -10 mA cm -2 which outperforms Pt/C commercial catalyst. No evident degradation of the material is detected even after 100 hours of continuous operation under high current density. Moreover, the HEA has shown exceptional performance in harsh electrolyte conditions such as in simulated seawater and actual seawater. Remarkably, the density-functional theory calculated Gibbs formation-enthalpy is small (≈0 eV) compared to Pt/C placing the new HEA near the apex of Trasatti's model of Volcano plot, which is also suggestive of superior HER activity.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Trigonal polymorph of Li 2 MnO 3

We report the discovery of a trigonal polymorph of the prospective Li ion battery material Li 2 MnO 3 and its synthesis in bulk, single crystal form. Crystal growth of trigonal Li 2 MnO 3 is strongly dependent upon the quality of a polycrystalline LiMnO 2 precursor consumed in the synthesis process. The crystal structure of the new trigonal phase is composed of ordered honeycomb layers of LiO 6 and MnO 6 octahedra segregated by layers of LiO 6 octahedra and represents an ordered stacking variant of the known monoclinic polymorph. Diffuse reflectance spectroscopy reveals a direct optical gap of 2.47 ± 0.11 eV and a series of charge excitations that are well explained by the expected Mn 4+ 3d 3 valence. Density functional theory calculations are in excellent agreement with the spectroscopic measurements and find a near degeneracy in the formation energies of the two polymorphs. Furthermore, our results suggest that the trigonal structure resolves the compositional and structural disorder often manifested in the monoclinic phase.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Synthesis Strategies for Ultrastable Zeolite GIS Polymorphs as Sorbents for Selective Separations

Abstract Designing zeolites with tunable physicochemical properties can substantially impact their performance in commercial applications, such as adsorption, separations, catalysis, and drug delivery. Zeolite synthesis typically requires an organic structure‐directing agent to produce crystals with specific pore topology. Attempts to remove organics from syntheses to achieve commercially viable methods of preparing zeolites often lead to the formation of impurities. Herein, we present organic‐free syntheses of two polymorphs of the small‐pore zeolite P (GIS), P1 and P2. Using a combination of adsorption measurements and density functional theory calculations, we show that GIS polymorphs are selective adsorbents for H 2 O relative to other light gases (e.g., H 2 , N 2 , CO 2 ). Our findings refute prior theoretical studies postulating that GIS‐type zeolites are excellent materials for CO 2 separation/sequestration. We also show that P2 is significantly more thermally stable than P1, which broadens the operating conditions for GIS‐type zeolites in commercial applications and opens new avenues for exploring their potential use in processes such as catalysis.

Oleksiak, Matthew D.↗
DOE OSTI2016

Perspective: Fundamental aspects of time-dependent density functional theory

In the thirty-two years since the birth of the foundational theorems, time-dependent density functional theory has had a tremendous impact on calculations of electronic spectra and dynamics in chemistry, biology, solid-state physics, and materials science. Alongside the wide-ranging applications, there has been much progress in understanding fundamental aspects of the functionals and the theory itself. This Perspective looks back to some of these developments, reports on some recent progress and current challenges for functionals, and speculates on future directions to improve the accuracy of approximations used in this relatively young theory.

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

Eshelby-twisted three-dimensional moiré superlattices

Twisted bilayers of van der Waals materials have recently attracted great attention due to their tunable, strongly correlated phenomena. In this work, we investigate the chirality-specific physics in three-dimensional moiré superlattices induced by Eshelby twist. Our direct density functional theory calculations reveal helical rotation leads to optical circular dichroism and chirality-specific nonlinear Hall effect, even though there is no magnetization or magnetic field. Both these phenomena can be reversed by changing the structural chirality. This provides a path to constructing chirality-specific materials.

2-dimensional systems↗
DOE OSTI2016

Tailoring graphene magnetism by zigzag triangular holes: A first-principles thermodynamics study

In this study, we discuss the thermodynamic stability and magnetic property of zigzag triangular holes (ZTHs) in graphene based on the results of first-principles density functional theory calculations. We find that ZTHs with hydrogen-passivated edges in mixed sp 2 /sp 3 configurations (z 211 ) could be readily available at experimental thermodynamic conditions, but ZTHs with 100% sp 2 hydrogen-passivation (z 1 ) could be limitedly available at high temperature and ultra-high vacuum conditions. Graphene magnetization near the ZTHs strongly depends on the type and the size of the triangles. While metallic z 1 ZTHs exhibit characteristic edge magnetism due to the same-sublattice engineering, semiconducting z 211 ZTHs do show characteristic corner magnetism when the size is small < 2 nm. Our findings could be useful for experimentally tailoring metal-free carbon magnetism by simply fabricating triangular holes in graphene.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗