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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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13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Exomoons in Systems with a Strong Perturber: Applications to α Cen AB

The presence of a stellar companion can place constraints on occurrence and orbital evolution of satellites orbiting exoplanets, i.e., exomoons. In this work we revise earlier orbital stability limits for retrograde orbits in the case of a three-body system consisting of a star, planet, and satellite. The revised limit reads a{sub sat}{sup crit}≈0.668(1-1.236e{sub p}) for e {sub p} ≤ 0.8 in units of the Hill Radius and represents the lower critical orbit as a function of the planetary eccentricity e {sub p}. A similar formula is determined for exomoons hosted by planets in binary star systems, where e {sub p} is replaced with the components of free and forced eccentricity from secular orbit evolution theory. By exploring the dynamics of putative exomoons in α Centauri AB we find that the outer stability limit can be much less than half the Hill Radius due to oscillations in the planetary orbital eccentricity caused by the gravitational interaction with the binary star. We show, furthermore, how the resulting truncation of the outer stability limit can affect the outward tidal migration and potential observability of exomoons through transit-timing variations (TTVs). Typical TTV (rms) amplitudes induced by exomoons in binary systems are ≲10 minutes and appear more likely for planets orbiting the less massive stellar component.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Expanded Analogs of Three‐Dimensional Lead‐Halide Hybrid Perovskites

Abstract Replacing the Pb−X octahedral building unit of A I PbX 3 perovskites (X=halide) with a pair of edge‐sharing Pb−X octahedra affords the expanded perovskite analogs: A II Pb 2 X 6 . We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium‐based cations with low‐lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with A II =pyridinium and ammonium, we can isolate the contributions of the pyrazinium‐based orbitals in the band gap transition of A II Pb 2 X 6 . The organic‐molecule‐based conduction band and the inorganic‐ion‐based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

Umeyama, Daiki↗
DOE OSTI2020

Expanded Analogs of Three-Dimensional Lead-Halide Hybrid Perovskites

Replacing the Pb-X octahedral building unit of A I PbX 3 perovskites (X=halide) with a pair of edge-sharing Pb-X octahedra affords the expanded perovskite analogs: A II Pb 2 X 6 . We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium-based cations with low-lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with AII=pyridinium and ammonium, we can isolate the contributions of the pyrazinium-based orbitals in the band gap transition of A II Pb 2 X 6 . The organic-molecule-based conduction band and the inorganic-ion-based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

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

Error-Free and Current-Driven Synthetic Antiferromagnetic Domain Wall Memory Enabled by Channel Meandering

We propose a new type of energy-efficient multi-bit magnetic memory based on current-driven, field-free, controlled domain wall motion. A meandering domain wall channel with precisely interspersed pinning regions provides the multi-bit capability of a magnetic tunnel junction memory. The magnetic free layer of the memory device has perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii-Moriya interaction (DMI) so that spin-orbit torques (SOTs) induce efficient domain wall motion. Using micromagnetic simulations, we find two different cell designs: two-way switching and four-way switching. The memory cell design choices and the physics of pinning mechanisms are discussed in detail. Furthermore, we show that switching reliability and speed may be significantly improved by replacing the ferromagnetic free layer with a synthetic antiferromagnetic (SAF) layer. Switching behavior and material choices will be discussed for the two memory implementations.

magnetic domain wall↗
DOE OSTI2026

Mo 4 FeGa 17.25– x Ge x : Complementary Point Substitutions, Buffering Frameworks, and Merging of the 18-n and Octet Bonding Schemes

We present the discovery of Mo 4 FeGa 17.25–x Ge x (x ∼ 2.1(4), based on determination of Ge content), a complex gallide featuring a variety of point substitution phenomena. Its crystal structure is derived from the Ti 2 Ni type, in which a diamond network of face-sharing octahedra is interpenetrated by a second diamond network of vertex-sharing stella quadrangula. However, while in the ideal Ti 2 Ni type these two frameworks run uninterrupted through the crystal, Mo 4 FeGa 17.25–x Ge x shows three variations. First, the inner tetrahedron of every other stella quadrangula is replaced with a main group atom, creating tetrahedra reminiscent of the Zintl phase NaTl. Next, a selection of Ga 6 octahedra are filled with Fe atoms, in a manner analogous to the stuffed AuCu 3 -type phases. Finally, the refined crystal structure shows that in each unit cell a single Ga2 atom in the octahedral network is substituted with a dumbbell of Ga/Ge atoms. Electronic structure calculations on ordered models of Mo 4 FeGa 17.25–x Ge x reveal a narrow band near the Fermi energy, which is explained in terms of the 18-n and octet bonding schemes using reversed approximation Molecular Orbital analysis. A DFT-chemical pressure analysis connects the tetrahedron/atom and atom/dumbbell substitutions to the relief of atomic packing tensions and highlights soft atomic motions within the octahedral framework and driving forces for atom/dumbbell substitution. The combination of soft vibrational modes, disorder, and a narrow band gap could make this phase of interest for potential thermoelectric properties.

Chemical structure↗
DOE OSTI2025

Centrifugal-mirror confinement with strong azimuthal magnetic field

One practical challenge for the centrifugal-mirror confinement concept is the large radial voltage necessary to drive supersonic azimuthal rotation. In principle, the addition of a strong azimuthal field could reduce the required voltage, since the simple azimuthal E x B drift would be replaced by more rapid azimuthal trapped-particle precession. Also, if the mirror ratio is large enough, newly ionized ions are accelerated to the necessary parallel velocities in their first bounce orbit, both confining and significantly heating them. Unfortunately, MHD analysis shows that the centrifugal-force-confining plasma current is purely azimuthal. This implies that only the axial magnetic field contributes to the confining magnetic pressure, severely limiting the usefulness of the azimuthal magnetic field in a beta-limited plasma scenario.

Grad-Shafranov↗
DOE OSTI2026

First-principles insights into Si substitution effects in Sm 2 (Fe,Si) 17 C x magnet

The partial substitution of Fe by Si enhances the phase stability of Sm 2 Fe 17 C x magnets with x > 1.0. Here, we elucidate the Si-substitution scheme and its impact on phase stability and magnetic properties in Sm 2 (Fe,Si) 17 C 3 from first-principles calculations and chemical bonding analysis. The calculated substitution energies for Si at various Fe sites are negative, indicating improved phase stability. Si preferentially substitutes Fe atoms at the 9d site in Sm 2 (Fe,Si) 17 C 3 while it tends to enter the Fe 18h site in Sm 2 (Fe,Si) 17 . This difference in site preference is attributed to the distinct chemical environments surrounding the Fe (Si) sites in the two compounds. Si substitution favors the formation of Sm–Si bonds while minimizing the Si–C and Si–Si interactions. Crystal orbital Hamilton populations and crystal orbital bond index calculations indicate that the partial replacement of Fe with Si strengthens the chemical bonding of Sm–Fe 3 (18f) and Sm–Fe 4 (18h) and improves overall phase stability in Sm 2 (Fe,Si) 17 C 3 . Beyond the dilution effect, Si substitution also reduces the magnetic moments of neighboring Fe atoms, a phenomenon linked to the strong Fe–Si bonding. These findings highlight the dual role of Si in modifying both the structural and magnetic characteristics of Sm 2 Fe 17 -based magnetic compounds.

Chemical bonding↗
DOE OSTI2025

Enhanced Phase Stability of Sm 2 (Fe, Al) 17 C x

Aluminum doping can improve the phase stability of metastable compound Sm 2 Fe 17 C x with a high carbon content (x > 1.5). We investigated the preferential site substitution of Al, chemical bonding, and structural stability in Sm 2 (Fe,Al) 17 C 3 using first-principle calculations. Our results reveal a strong correlation between the preferential substitution of Fe by Al and the atomic site chemical environment, which affects the overall phase stability. Specifically, Al preferentially occupies the 9d site in Sm 2 (Fe,Al) 17 C 3 . At the same time, Al prefers the site 6c in its parent phase Sm 2 (Fe,Al) 17 . Partial replacement of Fe with Al leads to a more negative formation energy, indicating enhanced thermodynamic stability. Crystal Orbital Hamilton Population (COHP) and Crystal Orbital Bond Index (COBI) analysis suggest that insertion of carbon weakens the bonding strength of Sm-Fe (18f) and Sm-Fe (18h), resulting in metastability of Sm 2 Fe 17 C x . Doping Al strengthens Al-Fe, Al-Sm, Sm-Fe (18f, 18h) and Fe–C bonding in Sm 2 (Fe,Al) 17 C 3 , as revealed by calculated COHP and COBI. These effects contribute to improved phase stability in the Al-doped 2:17 interstitial compound.

chemical bonding↗
DOE OSTI2026

Conditional diffusion machine-learning framework for mapping valence electron distribution from convergent beam electron diffraction

Quantitative convergent beam electron diffraction (CBED) enables determination of aspherical valence electron distributions through refinement of low-order structure factors, which are highly sensitive to chemical bonding and charge density variations. However, conventional quantitative CBED (QCBED) requires solving a highly nonlinear inverse problem with many coupled parameters, and computationally intensive dynamical diffraction calculations, making it time-consuming and difficult to apply to complex systems. More broadly, reconstructing charge density and orbital electron distribution from diffraction data has long been a central challenge in both x-ray and electron crystallography. Here, in this study, we introduce an artificial-intelligence (AI)-based framework that replaces traditional refinement with a data-driven inverse solver. Using a large synthetic CBED dataset generated by Bloch-wave simulations, we train a conditional diffusion model to directly infer crystal structural parameters and multipole density formalism parameters, and hence valence electron distributions, from CBED patterns alone. By learning from forward simulations across realistic parameter space, the model effectively solves the inverse problem. Compared with direct regression approaches, the diffusion-based framework provides posterior parameter distributions for rigorous uncertainty quantification while preserving quantitative fidelity and reducing analysis time by orders of magnitude. By eliminating the need for external single-crystal x-ray diffraction data and complex nonlinear refinement, this approach enables practical, high-throughput, and in situ quantitative CBED, enabling real-time mapping of valence electron distributions and their correlation with functional responses in quantum and energy materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electron Paramagnetic Resonance Characteristics and Crystal Structure of a Tutton Salt Analogue: Copper-Doped Cadmium Creatininium Sulfate

Electron paramagnetic resonance and crystallographic studies on copper-doped cadmium creatininium sulfate (CdCrnS) were undertaken to study the characteristics of a copper-hexahydrate complex in an organic analogue of Tutton's salt. X-ray diffraction experiments determined the crystal structure of CdCrnS at both 100 and 298 K. CdCrnS, like Tutton salt, crystallizes in the monoclinic space group P2 1 /n. The unit cell contains two cadmium hexahydrate complexes, four creatininium ions, four sulfates, and four additional solvation waters. Both crystallography and EPR find that the doped copper replaces the cadmium in the structure. Single-crystal EPR measurements at room temperature determined the g and copper hyperfine (A Cu ) tensors (principal values: g = 2.437, 2.134, and 2.080 and A Cu = -327, -84.8, and 7.33 MHz). EPR spectra of the powder at room temperature gave g = 2.448, 2.125, and 2.085 and A Cu = -315, -75.0, and 35.0 MHz and at 110 K gave g = 2.462, 2.116, and 2.077 and A Cu = -340, -30.0, and 35.0 MHz. The room-temperature tensors are close to the "rigid lattice limit" values found in copper-doped Tutton salts but with a higher gmin and weaker A Cu x coupling than average. A small but measurable temperature dependency of the tensors indicated the presence of a dynamic Jahn-Teller (JT) effect. In addition, the EPR line width changed dramatically with temperature, which is like that found in all copper-doped Tutton crystals. Utilizing the model of Silver-Getz for the g-value variation gave an estimate for the energy difference (δ 12 = 640 cm -1 ) between the ground and next highest JT configurations. An empirical correlation appears to exist between δ 12 and g min and A Cu x for the copper hexahydrates studied in similar crystals. Finally, this suggests a relationship between the amount of unpaired spins in the copper d-orbital x lobe and the gap between wells of the adiabatic potential surface.

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

Unraveling Adsorbate-Induced Structural Evolution of Iron Carbide Nanoparticles

Iron carbide (Fe x C y ) nanoparticles (NPs) are promising candidates for replacing platinum group metals in industrial applications, such as high-temperature Fischer–Tropsch synthesis. However, due to their amorphous nature, characterization of the active sites has been challenging experimentally and computationally. Here, using a combined density functional theory (DFT), neural network interatomic potential-assisted global optimization, and ensemble learning study, we evaluate dynamic surface changes associated with syngas (H and CO) interactions. For this purpose, we have developed a general procedure that we use to model an experimentally relevant 270-atom Fe 182 C 88 NP using the neural network-assisted stochastic surface walk global optimization algorithm (SSW-NN). Once generated, the Fe 182 C 88 NP active sites and particle morphology are thoroughly characterized before the effects of syngas adsorbate interactions are explored by using DFT and molecular dynamics simulations. Lastly, we explore correlations between geometric and electronic features of the active sites and the adsorption of H (H ads ), using a regularized random forest machine learning algorithm. In doing so, we identified the Fe–C coordination number and p orbital occupancy as the most important descriptors affecting H ads . Furthermore, using a combined ML and quantum chemistry approach, our work demonstrates a general and efficient procedure for generating and probing complex surface phenomena on binary nanoparticles.

Adsorption↗
DOE OSTI2026

Luminescent N-heterocyclic carbene Cu( I ) complexes with N^O chelating ligands exhibit microsecond lifetimes and photocatalytic activity

To replace precious noble metal-based photosensitizers in applications involving photoinduced charge separation, energy transfer, or photocatalysis, Cu(I) complexes are considered to be cost-effective, earth-abundant, and sustainable alternatives. An emerging and effective design principle in Cu(I) photosensitizers involves heteroleptic structures where the HOMO and LUMO are spatially separated over two different ligands. In the present work, we introduce a complementary class of heteroleptic, three-coordinate copper photosensitizers that pairs variable N^O chelating ligands (8-hydroxyquinoline and 10-hydroxybenzo[h]quinoline) with a bulky 2,6-diisopropylphenyl-substituted N-heterocyclic carbene (NHC). In this design, both frontier orbitals are localized on the same ligand, the N^O chelate, such that structural modulation of the electron-rich N^O-chelates can substantially tune the energy levels of the HOMO and LUMO, thereby controlling the photoluminescence properties. Detailed photophysical and electrochemical experiments as well as DFT calculations suggest charge-transfer transitions with intra-ligand charge transfer (ILCT) character, involving the N^O ligands. This strategy successfully produced long triplet excited-state lifetimes (up to 44 µs) in compounds that are strong photoreductants (E([Cu] + /*[Cu] as negative as −2.0 V vs. the ferrocenium/ferrocene couple). These properties allow these photosensitizers to be used as photocatalysts in various transformations of organic compounds, such as hydrogenation of substituted benzophenones, hydrodehalogenation of aryl/alkyl halides (including challenging C–Cl bond activation) and E/Z isomerization of (E)-stilbene (an example of triplet–triplet energy transfer).

Chakraborty, Soumi [University of Houston, TX (Uni↗
DOE OSTI2025

Dust-void evolution driven by turbulent dust flux can induce runaway migration of Earth-mass planets

Torques from asymmetric dust structures (so-called dust-void and filamentary structures) formed around low-mass planets embedded in a nonturbulent dust-gas disk can exceed the torques produced by the gas disk component and then go on to dominate the planet’s orbital dynamics. Here, we investigate how these structures (hence the dust torque) change when the effect of turbulent dust diffusion and dust feedback are included, along with the direct implications on the migration of Earth-like planets. Using the FARGO3D code, we performed 2D and 3D multifluid hydrodynamic simulations, focusing on a non-migrating planet with a mass of M p = 1.5 M ⊕ in 2D and on migrating planets with M p ∈ [1.5, 12] M ⊕ in 3D. We varied the δ-dimensionless diffusivity parameter in the range [0, 3 × 10 −3 ] and considered three different Stokes numbers, St = {0.04, 0.26, 0.55}, which are representative of the gas-dominated, the transitional, and the gravity-dominated regimes, respectively. In our 2D models, we find that turbulent diffusion of dust prevents the formation of the dust-void and filamentary structures when δ > 3 × 10 −4 . Otherwise, dust structures survive turbulent diffusion flow. However, dust and total torques become positive only in transitional and gravity-dominated regimes. In our 3D models, we find that the dust-void is drastically modified and the high-density ring-shaped barrier delineating the dust-void disappears if δ ≳ 10 −4 , due to the effect of dust turbulent diffusion along with the back-reaction of the dust. For all values of δ, the filament in front of the planet is replaced by a low-density trench. Remarkably, as we have allowed the planets to migrate, the evolving dust-void can drive either runaway migration or outward (inward) oscillatory-torque migration. Our study thus suggests that low-mass Earth-like planets can undergo runaway migration in dusty disks.

79 ASTRONOMY AND ASTROPHYSICS↗