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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Spontaneous Hall effect enhanced by local Ir moments in epitaxial Pr 2 Ir 2 O 7 thin films

Rare-earth pyrochlore iridates ( RE 2 Ir 2 O 7 ) consist of two interpenetrating cation sublattices, the RE with highly frustrated magnetic moments, and the iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. While bulk Pr 2 Ir 2 O 7 shows a spontaneous hysteretic Hall effect below 1.5 K, we observe the effect at elevated temperatures up to 15 K in epitaxial thin films on (111) yttria-stabilized zirconia (YSZ) substrates synthesized via solid-phase epitaxy. Similar to the bulk, the lack of observable long-range magnetic order in the thin films points to a topological origin. We use synchrotron-based element-specific x-ray diffraction and x-ray magnetic circular dichroism to compare powders and thin films to attribute the spontaneous Hall effect in the films to localization of the Ir moments. Further, we link the thin-film Ir local moments to lattice distortions absent in the bulklike powders. We conclude that the elevated-temperature spontaneous Hall effect is caused by the topological effect originating either from the Ir or Pr sublattice, with interaction strength enhanced by the Ir local moments. This spontaneous Hall effect with weak net moment highlights the effect of vanishingly small lattice distortions as a means to discover topological phenomena in metallic frustrated magnetic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Strain engineering of the charge and spin-orbital interactions in Sr2IrO4

In the high spin–orbit-coupled Sr 2 IrO 4 , the high sensitivity of the ground state to the details of the local lattice structure shows a large potential for the manipulation of the functional properties by inducing local lattice distortions. We use epitaxial strain to modify the Ir–O bond geometry in Sr 2 IrO 4 and perform momentum-dependent resonant inelastic X-ray scattering (RIXS) at the metal and at the ligand sites to unveil the response of the low-energy elementary excitations. Furthermore, we observe that the pseudospin-wave dispersion for tensile-strained Sr 2 IrO 4 films displays large softening along the [h,0] direction, while along the [h,h] direction it shows hardening. This evolution reveals a renormalization of the magnetic interactions caused by a strain-driven cross-over from anisotropic to isotropic interactions between the magnetic moments. Moreover, we detect dispersive electron–hole pair excitations which shift to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. This behavior shows an intimate coupling between charge excitations and lattice distortions in Sr 2 IrO 4 , originating from the modified hopping elements between the t 2g orbitals. Our work highlights the central role played by the lattice degrees of freedom in determining both the pseudospin and charge excitations of Sr 2 IrO 4 and provides valuable information toward the control of the ground state of complex oxides in the presence of high spin–orbit coupling.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microstructural evolution in doped high entropy alloys NiCoFeCr-3X (X=Pd/Al/Cu) under irradiation

Commonly studied equatomic single-phase FCC high entropy alloys based on 3d transition metals like NiCoFeCr do not provide adequate strength and radiation resistance at high doses for nuclear structural applications. In the current study, the major alloying effects like lattice distortion, ordering and clustering tendencies were investigated by adding low concentration of Pd, Al, or Cu respectively to study the doping effects on the ion irradiation response of NiCoFeCr alloy. The alloys were irradiated with 3 MeV Ni 2+ ions at 500 °C to a fluence of 1 × 10 17 /cm 2 at a beam flux of approximately 2.8 × 10 12 ions/cm 2 /s. The microstructural evolution upon irradiation i.e., formation of dislocation networks, radiation induced segregation and precipitation, and void formation were studied in detail. Further, post-irradiation characterization results showed that a Pd addition leads to a high void nucleation rate but controlled void growth, which may be attributed to increased lattice distortion. In Al added HEA, our microstructural analysis indicates that radiation induced ordered L1 2 precipitates do not affect void swelling significantly. Cu addition led to Cu precipitation that drastically suppressed dislocation density and void swelling of the alloy. Additionally, a model was developed to qualitatively describe the trend in void swelling of typical FCC alloys under ion irradiation. This model was able to qualitatively explain the suppression and reappearance of void swelling in ion irradiated alloys that generally occurs near the region with peak implanted ion concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hierarchical Eutectoid Nano-lamellar Decomposition in an Al 0.3 CoFeNi Complex Concentrated Alloy

This paper reports a novel eutectoid nano-lamellar (FCC + L1 2 )/(BCC + B2) microstructure that has been discovered in a relatively simple Al 0.3 CoFeNi high entropy alloy (HEA) or complex concentrated alloy (CCA). This novel eutectoid nano-lamellar microstructure presumably results from the complex interplay between Al-mediated lattice distortion (due to its larger atomic radius) in a face-centered cubic (FCC) CoFeNi solid solution, and a chemical ordering tendency leading to precipitation of ordered phases such as L1 2 and B2. This eutectoid microstructure is a result of solid-state decomposition of the FCC matrix and therefore distinct from the commonly reported eutectic microstructure in HEAs which results from solidification. This novel nano-lamellar microstructure exhibits a tensile yield strength of 1074 MPa with a reasonable ductility of 8%. The same alloy can be tuned to form a more damage-tolerant FCC + B2 microstructure, retaining high tensile yield stress (~900 MPa) with appreciable tensile ductility (>20%), via annealing at 700 °C. Such tunability of microstructures with dramatically different mechanical properties can be effectively engineered in the same CCA, by exploiting the complex interplay between ordering tendencies and lattice distortion.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Machine-learning and first-principles investigation of lightweight medium-entropy alloys for hydrogen-storage applications

The transition to a low-carbon economy demands efficient and sustainable energy-storage solutions, with hydrogen emerging as a promising clean-energy carrier and with metal hydrides recognized for their hydrogen-storage capacity. Here, we leverage machine learning (ML) to predict hydrogen-to-metal (H/M) ratios and solution energy by incorporating thermodynamic parameters and local lattice distortion (LLD) as key features. Our best-performing ML model provides improvements to H/M ratios and solution energies over a broad class of medium-entripy alloys (easily extendable to multi-principal-element alloys), such as Ti–Nb-X (X = Mo, Cr, Hf, Ta, V, Zr) and Co–Ni-X (X = Al, Mg, V). Ti–Nb–Mo alloys reveal compositional effects in H-storage behavior, in particular Ti, Nb, and V enhance H-storage capacity, while Mo reduces H/M and hydrogen weight percent by 40–50 %. We attributed results in molybdenum-rich alloys to slow hydrogen kinetics, as validated by our pressure-composition-temperature (PCT) isotherm experiments on pure Ti and Ti 5 Mo 95 alloys. Density functional theory (DFT) and molecular dynamics (MD) simulations also confirm that Ti and Nb promote H diffusion, whereas Mo hinders it, highlighting the interplay between electronic structure, lattice distortions, and hydrogen uptake. Notably, our Gradient Boosting Regression model identifies LLD as a critical factor in H/M predictions. Here, to aid material selection, we present two periodic tables illustrating elemental effects on (a) H 2 wt% and (b) solution energy, derived from ML, and provide a reference for identifying alloying elements that enhance hydrogen solubility and storage.

08 HYDROGEN↗
DOE OSTI2021

Photocarrier Dynamics in MoTe 2 Nanofilms with 2 H and Distorted 1 T Lattice Structures

Molybdenum telluride (MoTe 2 ), an emerging layered two-dimensional (2D) material, possesses excellent phase-changing properties. Previous studies revealed its reversible transition between 2H and 1T' phases with a transition energy as small as 35 meV. Since 1T'-MoTe 2 is metallic, it can serve as an electrical contact for semiconducting 2H-MoTe 2 -based optoelectronic devices. Here, the photocarrier dynamics in MoTe 2 nanofilms synthesized by a one-step method and with coexisting multiple phases are investigated by transient absorption measurements. Both the energy relaxation time and the recombination lifetime of the excitons are shorter in the 1T'-MoTe 2 compared to its 2H phase. Furthermore, these results provide information on the different photocarrier dynamical properties of these two phases, which is important for future 2D optoelectronic and phase-change electronic devices based on MoTe 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2024

Surface triggered stabilization of metastable charge-ordered phase in SrTiO 3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO 3 epitaxial thin films that have the lowest d-electron occupancy i.e., d 1 -d 0 . Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO 3 . Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Interacting spin and charge density waves in the kagome metal FeGe

Unveiling the interplay between spin density wave (SDW) and charge density wave (CDW) orders in correlated electron materials is important in obtaining a comprehensive understanding of their electronic, structural, and magnetic properties. Kagome lattice materials are interesting because their flat electronic bands, Dirac points, and Van Hove singularities can enable a variety of exotic electronic and magnetic phenomena. The kagome metal FeGe (the B35 phase), which exhibits a CDW order deep within an A-type antiferromagnetic (AFM) phase, was found to respond dramatically to postgrowth annealing—with the ability to tune the CDW repeatedly from long-range order to negligible order. Additionally, neutron scattering studies suggest that incommensurate magnetic peaks that onset at 𝑇 Canting = 𝑇 SDW ≈ 60 K in the system arise from a SDW order instead of the AFM double-cone structure. Here, in this study, we use inelastic neutron scattering to show that two distinct spin excitations exist below 𝑇 Canting corresponding to two coexisting magnetic orders in the system in both sets of annealed samples with and without CDW. While CDW order or negligible order can dramatically affect the onset temperature of 𝑇 Canting and elastic incommensurate magnetic scattering, its impact on low-energy spin fluctuations is more limited. In both samples, a pair of gapless incommensurate spin excitations arising from the SDW order wave vector coexist with gapped commensurate spin waves from the A-type AFM order across 𝑇 Canting . The low-energy spin excitations for both samples couple dynamically to the lattice through enhanced magnetic scattering intensity on cooling below 𝑇 CDW , regardless of the status of the static long-range CDW order. The incommensurate SDW order in the long-range CDW ordered sample also induces a tiny in-plane lattice distortion of the kagome lattice that is absent in the negligible CDW ordered sample, in a way that is different from the previously known SDW and CDW ordering materials.

charge density waves↗
DOE OSTI2021

Light-driven permanent transition from insulator to conductor

The transition from insulator to conductor can be realized in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a different mechanism the lattice may adopt to induce such a transition. Experiments showed the surprising finding that limited exposure to subband-gap light caused a permanent transition from an insulator state to a conductor state in the insulating oxide Ga 2 O 3 , with a nine orders of magnitude increase in electronic conduction. Furthermore, annealing up to 400 °C did not suppress or decrease the induced conductivity. Photoexcitation by light-induced modification in the charge state of defects and subsequent lattice distortion around them was suggested to be the underlying mechanism behind this transition. Density functional theory calculations confirmed that modifying the charge state of defects leads to redistribution of the localized electrons and massive structural distortion in the surrounding lattice, causing large shifts in the density of states and introducing new states with shallower energy levels. Both experimental and theoretical results revealed the introduction of stable shallow energy levels, explaining the mechanism behind the transition from an insulator to a conductor state by light. We suggest that this mechanism may occur in other wide band-gap metal oxides leading to drastic modification in their electronic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Efficient CO2 and Natural Gas Refinery for Ethylene Production - Catalyst Development for Oxidative Coupling of Methane (OCM)

We designed a new electro-catalyst for oxidative coupling of methane (OCM) reaction, the structure of the Sc doped La0.3Sr0.7TiO3-d (LaSrTi) Sc doping to LaSrTi leading to lattice expansion and more distorted lattice, Investigated the effect of doping on the oxygen vacancy formation energy DFT calculations shows that Sc doping facilitate oxygen vacancy formation. Quantifying the surface basicity of the sample before and after Sc doping Sc doped sample exhibits high surface basicity.

catalysis↗
DOE OSTI2026

A strong and ductile Super Kovar alloy via fully coherent nanoprecipitates

Emerging high-precision technologies demand materials with exceptional dimensional stability and mechanical robustness, as even microscopic thermomechanical deformation can cause functional failure. However, a fundamental trilemma exists: High strength, ductility, and low thermal expansion are mutually exclusive, as strengthening-induced lattice distortions compromise the spin-lattice coupling, which is essential for low thermal expansion. Here, we overcome this trilemma by designing “Super Kovar,” an Fe-Ni-Co-Al-Ta alloy, featuring fully coherent nanoprecipitates. It unifies a 1.0-gigapascal ultimate tensile strength and ∼39% elongation with a Kovar-grade thermal expansion of 3.81 × 10−6 K−1 (100 to 410 K). The key is a dense dispersion of L12 nanoprecipitates that form an almost strain-free coherent interface with the ferromagnetic matrix. Beyond providing precipitation strengthening, these coherent interfaces suppress intrinsic phonons of nanoprecipitates via elastic coupling while avoiding magnetic domain pinning to preserve the Invar effect of the matrix. This reduces the thermal expansion of the precipitates by 56% and achieves a fourfold enhancement in the strength-ductility product, establishing a paradigm for dimensionally stable, ultrastrong alloys.

Yu, Chengyi [University of Science and Technology ↗
DOE OSTI2023

How Atomic Bonding Plays the Hardness Behavior in the Al–Co–Cr–Cu–Fe–Ni High Entropy Family

A systematic study on a face-centered cubic-based compositionally complex alloy system Al–Co–Cr–Cu–Fe–Ni in its single-phase state is carried out, where a mother senary compound Al 8 Co 17 Cr 17 Cu 8 Fe 17 Ni 33 and five of its suballoys, obtained by removing one element at a time, are investigated and exhaustively analyzed determining the contribution of each alloying element in the solid solution. The senary and the quinaries are compared using experimental techniques including X-ray absorption spectroscopy, X-ray diffraction, transmission electron microscopy, and first principles hybrid Monte Carlo/molecular dynamics simulations. Chemical short-range order and bond length distances have been determined both at the experimental and computational level. Electronic structure and local atomic distortions up to 5.2 Å have been correlated to the microhardness values. A linear regression model connecting hardness with local lattice distortions is presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnetically driven phonon instability enables the metal–insulator transition in h-FeS

Hexagonal iron sulfide exhibits a fascinating coexistence of metal–insulator, structural and magnetic transitions, reflecting an intimate interplay of its spin, phonon and charge degrees of freedom. Here, we show how a subtle competition of energetic and entropic free-energy components governs its thermodynamics and the sequence of phase transitions it undergoes upon cooling. By means of comprehensive neutron and X-ray scattering measurements, and supported by first-principles electronic structure simulations, we identify the critical role of the coupling between antiferromagnetic ordering and instabilities of anharmonic phonons in the metallic phase in driving the metal–insulator transition. The antiferromagnetic ordering enables the emergence of two zone-boundary soft phonons, whose coupling to a zone-centre mode drives the lattice distortion opening the electronic bandgap. Simultaneously, spin–lattice coupling opens a gap in the magnon spectrum that controls the entropy component of the metal–insulator transition free energy. Furthermore, these results reveal the importance of spin–phonon coupling to tune anharmonic effects, thus opening new avenues to design novel technologically important materials harbouring the metal–insulator transition and magnetoelectric behaviours.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Effects of rhodium doping on dislocation nucleation in a [001] SrNi 2 P 2 single crystal under spherical nanoindentation

Nanoindentation was performed on SrNi 2 P 2 single crystals and their dilute solid solutions, Sr(Ni 1–x Rh x ) 2 P 2 (x = 0 ~ 0.055), under [001] loading to investigate the effects of elemental doping on Young’s modulus and dislocation nucleation stress. The results show that Young’s modulus and the dislocation nucleation stress decrease as the Rh content increases, and their Rh content dependence also varies with the Rh content. Electron diffraction analysis and the anisotropic lattice distortion calculations revealed that a local structural transition from the orthorhombic superstructure to the tetragonal structure and local compressive residual stresses could be the main reasons for the decrease in Young’s modulus and dislocation nucleation stress, respectively. The overlap of stress–strain fields between Rh atoms and the associated local structural transition contribute to their Rh content dependence. Here, the results also indicate that elemental doping does not necessarily strengthen materials if the incipient plasticity is controlled by dislocation nucleation, unlike conventional solid solution strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Phase‐Dependent Band Gap Engineering in Alloys of Metal‐Semiconductor Transition Metal Dichalcogenides

Abstract Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)‐electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe 2 with metallic NbSe 2 , two structural phases of Mo 0.5 Nb 0.5 Se 2 , the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps in the range of 0.42–0.58 at 77 K. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions that result in an uncommon 4 × 4 supercell, rotated approximately 4° from the lattice. Density‐functional‐theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in 1T ‐Mo 0.5 Nb 0.5 Se 2 , but not in the 2H phase. This work expands the boundaries of alloy‐based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying.

Wang, Shuxi↗
DOE OSTI2026

The Modulated Crystal Structure of K 2 V 3 O 8

Weak lattice distortions can tune exchange pathways and magnetic interactions in square-lattice quantum magnets. K 2 V 3 O 8 is a mixed valence (V 4+ /V 5+ ) fresnoite oxide that exhibits strong spin–lattice coupling at low temperature. Here, we combine single-crystal neutron diffraction (90 K) and laboratory X-ray diffraction (50 K) to solve the low-temperature structure as an orthorhombic (3 + 1)D incommensurately modulated phase in superspace group Cmm2(β,0,1/2)0s0 [No. 196]. What initially appeared as two independent modulation vectors, q 1 = 0.3132(6)[$110$] + 1/2c* and q 2 = 0.3132(6)[$1\bar{10}$] + 1/2c*, are more naturally described as a single one-dimensional modulation wave q = 0.626(1)a* + 1/2c* in a C-centered orthorhombic lattice, related to the parent tetragonal cell by the transformation a + b, −a + b, c. Refinement with a 4-fold rotational twin inherited from the P4bm parent structure solves oxygen-dominated framework distortions and K + displacements. A de Wolff section (t = 0.40) enables a symmetry-mode decomposition, identifying three dominant mm2 (C 2v ) modes: GM3 for framework tilt, A5 for interlayer shear, and Z5 for c-axis breathing. The mode-resolved structure provides a unified, symmetry-based explanation for reported low-temperature Raman and IR anomalies and clarifies the structural origin of the spin–lattice coupling in the S = 1/2 two-dimensional quantum spin compound.

K2V3O8↗
DOE OSTI2020

Hierarchical Defect Engineering for LiCoO 2 through Low-Solubility Trace Element Doping

Real-world industry-relevant battery composite electrodes are hierarchically structured. Their structural and chemical complexity is featured by ubiquitous multi-scale porosity and cracks, solid-solid and solid-liquid interfaces, compositional and redox heterogeneity, as well as lattice disordering and deformation. In particular for the active cathode particles, which are the fundamental building blocks for the energy reservoir, it is a consensus that these structural and chemical defects could have a profound impact on the battery performance. An in-depth understanding of the underlying mechanisms could critically inform the cathode material engineering, which would have a tremendous potential but remains a daunting challenge at present. In this work, we tackle this question by studying LiCoO 2 (LCO) with trace doping of Ti, which exhibits a low solubility in the LCO layered lattice. Additionally, we observed the spontaneous and heterogeneous segregation of the dopant (Ti) across a wide range of length scales. In addition to the modification of the particle surface and the buried grain boundaries within the particle, we reveal that the Ti doping has induced a significant amount of lattice distortions, which, in turn, promotes the robustness of the LCO lattice at high state of charge (above 4.5V). Our result formulates a multi-scale defect engineering strategy that could be applicable to the synthesis of a broad range of energy materials for applications in batteries and beyond.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ionic‐Liquid‐Gating‐Induced Hydrogenation in Epitaxial Strontium Ferrite

Abstract Incorporating hydrogen into complex oxides holds promise for discovering exotic phenomena and novel functionalities by promoting couplings between ion and lattice/charge/spin/orbital degrees of freedom. Here, electrolyte gating‐driven hydrogenation is investigated in epitaxial brownmillerite SrFeO 2.5 thin films in which the hydrogenation‐induced lattice distortion is qualitatively different from its counterpart SrCoO 2.5 . The achievable lattice expansion in hydrogenated SrFeO 2.5 is weaker than in SrCoO 2.5 and primarily occurs along the normal of the stacked octahedral FeO 6 and tetrahedral FeO 4 layers. Upon the substitution of Fe with Co, the lattice expansion monotonically increases with increase of Co/Fe ratio, implying an intrinsic difference in accommodating hydrogen between Fe‐ and Co‐based brownmillerites. Moreover, a net magnetic moment in hydrogenated SrFeO 2.5 films is observed above room temperature, which gradually weakens with the increase of Co substitution, suggesting a stronger canted magnetism in Fe‐based hydrogenated brownmillerites. This work clarifies the electrolyte gating‐driven hydrogenation mechanisms in brownmillerite SrFeO 2.5 epitaxial thin films and those with Co substitution, particularly the deterministic role of Co/Fe ratio in the evolution of structure and properties upon hydrogenation.

Chemistry↗