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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 649 records · Page 36
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DOE OSTI2022

Flux Growth of Cerium Nickel Gallides Studied by In Situ Neutron Diffraction

In this work, reactions of cerium and nickel in excess molten gallium were monitored by neutron diffraction during heating and cooling. The formation of binary intermediates CeGa 2 and Ni 2 Ga 3 was observed during heating. During cooling of the molten mixture from 900 °C, precipitation of BaAl 4 -type CeNi 0.74 Ga 3.26 occurred at 850 °C. Upon cooling to 650 °C, this compound reacted in the flux to form Ce 2 NiGa 10 and then Ce 2 NiGa 12 , the latter of which persisted to room temperature. Making use of this information, subsequent reactions were quenched at 750 °C to isolate crystals of CeNi 0.74 Ga 3.26 for further study. Similar reactions replacing Ce with La and quenching above 750 °C yielded LaNi 0.35 Ga 3.65 crystals. Magnetic susceptibility studies on CeNi 0.74 Ga 3.26 indicate that the cerium is trivalent; the Ce 3+ moments undergo a strongly anisotropic ferromagnetic ordering with moment perpendicular to the c axis below 7 K. Heat capacity data show little evidence of heavy fermion behavior. Resistivity measurements show that both LaNi0.35Ga3.65 and CeNi 0.74 Ga 3.26 exhibit metallic behavior. Density of states calculations support this and indicate that Ni/Ga mixing in the compound stabilizes the structure.

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

Ternary Complexes of BiI 3 /CuI and SbI 3 /CuI with Tetrahydrothiophene

Reactions of BiI 3 /CuI mixtures with tetrahydrothiophene (THT) in toluene produce 2-D sheet networks BiCu 3 I 6 (THT) n (n = 2, 3, or 4), depending on reaction conditions. All three structures are based on BiI 6 octahedra, which share pairs of (μ 2 -I) 2 with Cu 3 (THT) n units. BiCu 3 I 6 (THT) 2 features Cu 2 (μ 2 -I) 2 rhombs with close Cu···Cu interactions and is accompanied by formation of the very complex HBi 3 Cu 12 I 22 (THT) 8 . Reactions of SbI 3 /CuI with THT in toluene produced a SbCu 3 I 6 (THT) 2 network shows Cu 3 (μ 2 -THT) 2 units, like its Bi congener, but Cu 6 (μ 2 -I) 6 barrels rather than rhombs. Isolated SbI 3 units are stacked above the Cu 6 I 6 barrels. A molecular compound, Sb 3 Cu 3 I 12 (THT) 6 consists of a face-sharing Sb 3 I 12 stack, in which the Cu-THT units are bonded in asymmetric fashion about the central SbI 6 . Metal-halide bonds were investigated via QTAIM and NLMO analyses, demonstrating that these bonds are largely ionic and occur between the Bi/Sb and I p orbitals. Hirshfeld analysis shows significant H···H and H···I interactions. Diffuse reflectance spectroscopy (DRS) reveals band edges for the Bi species of 1.71–1.82 eV, while those for the neutral Sb complexes are in the range of 1.94–2.06 eV. Mapping of the electronic structure via density of state calculations indicates population of antibonding Bi/Sb–I orbitals in the excited state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

BaCu 4/3 Si 2/3 P 2 and BaCu 2–( x + y ) Zn x Si y P 2 : Expanding the Semiconducting Landscape in the ThCr 2 Si 2 -Type Family

ThCr 2 Si 2 -type layered materials are a large family of compounds with applications ranging from thermoelectricity to magnetism, with the vast majority of the members exhibiting metallic behavior. Here, in this study, we synthesized a new group of materials with Cu-Si and Cu-Zn-Si square nets with the general formula BaCu 1.33 Si 0.67 P 2 and BaCu 2–(x+y) Zn x Si y P 2 (0 ≤ x ≤ 0.9; 0.3 ≤ y ≤ 0.7). Several synthesized compounds are charge-balanced semiconductors, which are rare in the ThCr 2 Si 2 family. All the reported compounds crystallize in the ThCr 2 Si 2 -type tetragonal I4/mmm space group, with Cu/Zn/Si jointly occupying the same 4d crystallographic site. In the Zn-free composition, BaCu 1.33 Si 0.67 P 2 , Ba, and P each occupy a single crystallographic site. The introduction of Zn results in the expansion of the unit cell and splitting the Ba atomic sites along the [001] direction. Such structural displacement of the Ba atoms was confirmed by the heat capacity measurements. Band structure and density-of-states calculations on ordered hypothetical structural models reveal either a small bandgap (∼0.2 eV) or semimetallic band structures. The compounds reported here exhibit high Seebeck coefficients and ultralow thermal conductivity, making them promising candidates for the development of thermoelectric materials.

crystal structure↗
DOE OSTI2026

CoSn-type NiIn1–xSbx (0 ≤ x ≤ 0.17): Site-Selective Substitution, Electronic Structure, Chemical Bonding, and Structural Transformation

CoSn-type intermetallic compounds have emerged as a model platform for Kagome-derived flat-band physics, where subtle chemical perturbations can strongly influence electronic structure and phase stability. Here, we present a combined experimental and theoretical study of Sb-substitution in CoSn-type NiIn1–xSbx (0 ≤ x ≤ 0.17) to elucidate the interplay between site selectivity, solubility limit, chemical bonding, and electronic structure. Rietveld refinements on Neutron powder diffraction data confirmed the selective Sb-substitution at the electron-rich In2 (2d) site forming the honeycomb substructure, while the In1 (1a) site within the Kagome layer remains exclusively occupied by In. Density functional theory (DFT) calculations revealed that pristine CoSn-type NiIn hosts Ni 3d-dominated flat bands near the Fermi level (EF), originating from the Kagome-like Ni substructure. Partial replacement of In by Sb within the honeycomb layer alters these flat-band features below EF, reducing the density of states and suppressing the flat-band topology near the Fermi level. Orbital-resolved electronic structure and chemical-bonding analyses show that Sb-substitution enhances Ni-p-block (In/Sb) covalency and optimizes charge compensation, stabilizing the CoSn-type structure up to the solubility limit x ≈ 0.17. Beyond the limit, the higher-Sb compositions show satellite reflections consistent with an incommensurately modulated phase. These results establish a link between site-selective chemical substitution, bonding optimization, and flat-band electronic structure evolution, providing fundamental insights into how chemical substitution influences the electronic properties of Kagome-based intermetallic compounds.

Roy, Nilanjan [National Institute of Technology Si↗
DOE OSTI2020

Persistent Protein Motions in a Rugged Energy Landscape Revealed by Normal Mode Ensemble Analysis

Proteins are allosteric machines that couple motions at distinct, often distant, sites to control biological function. Low-frequency structural vibrations are a mechanism of this long-distance connection and are often used computationally to predict correlations, but experimentally identifying the vibrations associated with specific motions has proved challenging. Spectroscopy is an ideal tool to explore these excitations, but measurements have been largely unable to identify important frequency bands. The result is at odds with some previous calculations and raises the question what methods could successfully characterize protein structural vibrations. Here we show the lack of spectral structure arises in part from the variations in protein structure as the protein samples the energy landscape. However, by averaging over the energy landscape as sampled using an aggregate 18.5 μs of all-atom molecular dynamics simulation of hen egg white lysozyme and normal-mode analyses, we find vibrations with large overlap with functional displacements are surprisingly concentrated in narrow frequency bands. These bands are not apparent in either the ensemble averaged vibrational density of states or isotropic absorption. However, in the case of the ensemble averaged anisotropic absorption, there is persistent spectral structure and overlap between this structure and the functional displacement frequency bands. We systematically lay out heuristics for calculating the spectra robustly, including the need for statistical sampling of the protein and inclusion of adequate water in the spectral calculation. The results show the congested spectrum of these complex molecules obscures important frequency bands associated with function and reveal a method to overcome this congestion by combining structurally sensitive spectroscopy with robust normal mode ensemble analysis.

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

Reconciling Work Functions and Adsorption Enthalpies for Implicit Solvent Models: A Pt (111)/Water Interface Case Study

Implicit solvent models are a computationally efficient method of representing solid/liquid interfaces prevalent in electrocatalysis, energy storage, and materials science. However, electronic structure changes induced at the metallic surface by the dielectric continuum are not fully understood. To address this, we perform DFT calculations for the Pt(111)/water interface, in order to compare Poisson–Boltzmann continuum solvation methods with ab initio molecular dynamics (AIMD) simulations of explicit solvent. We show that the implicit solvent cavity can be parametrized in terms of the electric dipole moment change at the equilibrated explicit Pt/water interface to obtain the potential of zero charge (PZC). We also compare the accuracy of aqueous enthalpies of adsorption of phenol on Pt(111) using geometry and charge density based dielectric cavitation methods. The ability to parametrize the cavity according to individual atoms, as afforded in the geometry based approach, is key to obtaining accurate enthalpy changes of adsorption under aqueous conditions. Additionally, we show that the electronic structure changes induced by explicit solvent and our proposed implicit solvent parametrization scheme yield comparable density difference profiles and d-band projected density of states. We therefore demonstrate the capability of implicit solvent approaches to capture both the energetics of adsorption processes and the main electronic effects of aqueous solvent on the metallic surface. Therefore, this work provides a scheme for computationally efficient simulations of interfacial processes for applications in areas such as heterogeneous catalysis and electrochemistry.

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

Molecular Dynamics Driven by the Many-Body Expansion (MBE-MD)

Here, we present a protocol for classical and nuclear quantum dynamics, in which the energies and forces are generated by the many-body expansion (MBE), and apply it to water clusters using the TTM2.1-F and MB-Pol interaction potentials at various temperatures. We carry out MBE-MD classical and nuclear quantum dynamical simulations in which the energies and forces of the full system are approximated by the 2-, 3-, and 4-body terms of the MBE expansion and compare the average potential and the vibrational density of states (VDOS) with the full simulation, i.e., the one for which no MBE is used. Our results indicate that the thermally averaged potential energy from the MBE up to the 4-body term converges with near identical behavior to the one from the full simulation. The 3-body makes a substantial contribution (~20%) to the energy, whereas the 4- body is necessary for obtaining quantitatively accurate energetics and forces, albeit making a small contribution to each (~2%). We further show that the harmonic frequencies are reproduced to within a few wavenumbers (cm -1 ) at the 4-body level, and that the slowest modes to converge with the MBE rank are those involving the strongest hydrogen bonds. Anharmonicity exacerbates this effect, so that a 4-body description of the energies and forces is needed to achieve accurate anharmonic vibrational frequencies in the hydrogen-bonded OH-stretching region. We also discuss the asymptotic scaling of the MBE-MD protocol with respect to the cost of the underlying potential energy evaluation, suggesting that electronic structure methods that scale at least as N 4 ,N being the size of the system, are needed to result in savings over the traditional full MD simulation. We anticipate that MBE-MD protocol can evolve into a powerful and practical method, which will allow for highly accurate ab initio MD simulations on a much broader range of molecular systems than can be currently handled.

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

Stochastically Realized Observables for Excitonic Molecular Aggregates

In this work, we show that a stochastic approach enables calculations of the optical properties of large 2-dimensional and nanotubular excitonic molecular aggregates. Previous studies of such systems relied on numerically diagonalizing the dense and disordered Frenkel Hamiltonian, which scales approximately as $\mathcal{O}$($N$ 3 ) for $N$ dye molecules. Our approach scales much more efficiently as $\mathcal{O}$($N$ log($N$)), enabling quick study of systems with a million of coupled molecules on the micrometer size scale. We calculate several important experimental observables, including the optical absorption spectrum and density of states, and develop a stochastic formalism for the participation ratio. Quantitative agreement with traditional matrix diagonalization methods is demonstrated for both small- and intermediate-size systems. The stochastic methodology enables the study of the effects of spatial-correlation in site energies on the optical signatures of large 2D aggregates. Our results demonstrate that stochastic methods present a path forward for screening structural parameters and validating experiments and theoretical predictions in large excitonic aggregates.

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

Toward Quantum Confinement in Graphitic Carbon Nitride-Based Polymeric Monolayers

raphitic carbon nitride (g-C 3 N 4 ) has garnered much attention due to its potential as an efficient metal-free photocatalyst. This study examines the evolution of properties in zero-dimensional quantum dots up to sizable clusters that mimic extended g-C 3 N 4 monolayers. We employ density functional theory to investigate systematically the structural, electronic, and optical properties of the g-C 3 N 4 -based melamine and heptazine building blocks using a “bottom-up” construction of polymeric monolayers. The results from our computations indicate that the melamine- and heptazine-based polymeric g-C 3 N 4 systems must be reduced to at least 2.74 and 4.00 nm, respectively, to observe an increase of its optical gap with a size reduction. The present study also examines the nature of the electronic transitions exhibited by g-C 3 N 4 -based monolayers through full natural transition orbital and density of state analyses. Furthermore, the most promising sites for water splitting and subsequent chemical doping studies are identified, which generally correspond to the nitrogen and carbon atoms, respectively.

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

Cation, Anion, and Radical Isomers of C4H4N: Computational Characterization and Implications for Astrophysical and Planetary Environments

Nitrogen-containing ions and molecules in the gas phase have been detected in non-Earth environments such as dark molecular clouds and more recently in the atmosphere of Saturn's moon Titan. These molecules may serve as precursors to larger heterocyclic structures that provide the foundation of complex biological molecules. On Titan, molecules of m/z 66 have been detected by the Cassini mission, and species of the empirical formula C 4 H 4 N may contribute to this signature. We have characterized seven isomers of C 4 H 4 N in anionic, neutral radical, and cationic states using density functional theory. Structures were optimized using the range-separated hybrid ωB97X-V with the cc-pVTZ and aug-cc-pVTZ basis sets. Anionic and radical C4H4N favor cyclic structures with aromatic and quasi-aromatic electron arrangements, respectively. Interestingly, ionization from the radical surface to the cation induces significant changes in structural stability, and the global minimum for positively charged isomers is CH 2 CCHCNH + , a pseudo-linear species reminiscent of cyanoallene. Select formation pathways to these structures from Titan's existing or postulated gas-phase species, reactions that are also relevant for other astrophysical environments, are discussed. By characterizing C 4 H 4 N isomers, we have identified energetically stable anionic, radical, and cationic structures that may be present in Titan's atmosphere and dark molecular clouds.

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

Anharmonic phonon dispersion in polyethylene

The second-order Green's function method for anharmonic crystals has been applied to an infinite, periodic chain of polyethylene taking into account up to quartic force constants. The frequency-independent approximation to the Dyson self-energy gives rise to numerous divergent resonances, which are fortuitous. Instead, solving the Dyson equation self-consistently with a frequency-dependent self-energy resists divergences from resonances or zero-frequency acoustic vibrations. Here, the calculated anharmonic phonon dispersion, which nonetheless displays many true resonances, and anharmonic phonon density of states furnish hitherto unknown details that explain smaller features of observed vibrational spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Defect Energetics in Pseudo-Cubic Mixed Halide Lead Perovskites from First-Principles

Owing to the increasing popularity of lead-based hybrid perovskites for photovoltaic (PV) applications, it is crucial to understand their defect energetics and its influence on their optoelectronic properties. In this work, we simulate various point defects in pseudocubic structures of mixed iodide-bromide and bromide-chloride methylammonium lead perovskites with the general formula MAPbI 3-y Br y or MAPbBr 3-y Cl y (where y is between 0 and 3), and use first-principles based density functional theory computations to study their relative formation energies and charge transition levels. We identify vacancy defects and Pb on MA antisite defect as the lowest energy native defects in each perovskite. Here, we observe that while the low energy defects in all MAPbI 3-y Br y systems only create shallow transition levels, the Br or Cl vacancy defects in the Cl-containing pervoskites have low energy and form deep levels which become deeper for higher Cl content. We examine the structures and density of states of pure and defect-containing perovskite systems to obtain an understanding of the nature of defect levels. Further, we study extrinsic substitution by different elements at the Pb site in MAPbBr 3 , MAPbCl 3 , and the 50-50 mixed halide perovskite, MAPbBr 1.5 Cl 1.5 , and identify some transition metals that create lower energy defects than the dominant intrinsic defects and also create midgap charge transition levels.

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

Exploring Responses of Contact Kelvin Probe Force Microscopy in Triple-Cation Double-Halide Perovskites

Hybrid organic–inorganic perovskite (HOIP) solar cells have undergone unprecedentedly rapid improvement in power conversion efficiencies during the last decade, but controversy remains regarding the role of local phenomena and heterogeneity in grain by grain. These challenges in turn necessitate understanding the fundamental mechanisms operating at individual grains and exploring how these mechanisms give rise to the collective behaviors of the films. Although numerous efforts have targeted local phenomena in these materials using classical scanning probe microscopy modalities, the measurements remain difficult to interpret due to intrinsic coupling between possible ferroelectric, ion migration, bulk and surface electrochemical phenomena, and the effects of light on these behaviors. Here, we explore the electromechanical responses in (Cs 0.05 FA 0.85 MA 0.15 )Pb(I 0.85 Br 0.15 ) 3 perovskite thin film in the dark and under illumination using band excitation-piezoresponse force microscopy (BE-PFM). To identify and separate ferroic, electrostatic, and electrochemical responses, we utilize contact-Kelvin probe force microscopy (cKPFM). The cKPFM responses show weak variability with morphological features in the dark, while such variability significantly increases under illumination. Deconvolution of the cKPFM responses via principal component analysis (PCA) shows heterogeneity in grain to grain possibly due to variation in migration and segregation of halide ions in mixed perovskites at specific grains. Further, we propose an electrostatic mechanism based on the physics of semiconductors, which can directly link the observed cKPFM behaviors with the ionic and electronic density of states (DOS) and their time dynamics. Here, these findings suggest that cKPFM is a powerful probe of local ionic behaviors on the nanoscale, both in HOIPs and other materials.

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

Adsorbate Free Energies from DFT-Derived Translational Energy Landscapes

Adsorption free energies are fundamental to surface chemistry and catalysis. Standard models combine some assumed analytical form of the translational potential energy surface, often parameterized against density functional theory (DFT) calculations, with an analytical expression for the resultant translational densities of states (DOS), free energy, and entropy. Here we compare the performance of such models against numerical evaluations of the DOS and thermodynamic functions derived from solutions to the translational Schrodinger equation. We compare results for a translational potential energy surface (PES) derived from nudged eleastic band calculations with those obtained from adsorbate rastering across a series of monatomic (O, S, C, N, H) and polyatomic (NH x ) adsorbates on (100) Pt and Au facets. We find that analytical models as commonly parameterized have mixed performance for describing the translational PES and that the consequences for computed free energies are modest but potentially significant in microkinetic models. Here, numerical solutions are possible for modest to no additional computational cost over analytical models and thus should be considered when reliable free energy estimates are needed or translational potential energy surfaces are available.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

X-ray Transient Absorption Studies of Exciton Self-Trapping

Here, we present optical pump/X-ray absorption probe studies to probe changes in electronic distribution and local structure upon formation of a self-trapped exciton. The experiments were carried out on the mixed-valence halide-bridged transition metal linear chain charge density wave material [Pt(en 2 )][Pt(en 2 )Cl 2 ]·(ClO 4 ) 4 . Photoinduced changes in the Pt L III XANES spectrum were measured on a picosecond time scale following generation of excitons via optical excitation of the intervalence charge transfer transition. Ab initio FEFF9 modeling of the spectra and the associated angular momentum projected density of states was used to interpret the response. We find photoinduced changes reflecting electronic redistribution and local lattice distortions that correspond to strong localization of the self-trapped state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗