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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Epitaxial stabilization of ultra thin films of high entropy perovskite

High entropy oxides (HEOs) are a class of materials, containing equimolar portions of five or more transition metal and/or rare-earth elements. We report here about the layer-by-layer growth of HEO [(La 0.2 Pr 0.2 Nd 0.2 Sm 0.2 Eu 0.2 )NiO 3 ] thin films on NdGaO 3 substrates by pulsed laser deposition. The combined characterizations with in situ reflection high energy electron diffraction, atomic force microscopy, and x-ray diffraction affirm the single crystalline nature of the film with smooth surface morphology. The desired +3 oxidation of Ni has been confirmed by an element sensitive x-ray absorption spectroscopy measurement. Temperature dependent electrical transport measurements revealed a first order metal-insulator transition with the transition temperature very similar to the undoped NdNiO 3 . Furthermore, since both these systems have a comparable tolerance factor, this work demonstrates that the electronic behaviors of A-site disordered perovskite-HEOs are primarily controlled by the average tolerance factor.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Combined TDLAS and chemiluminescence imaging in a flat flame burner operated with NH3/H2 blends

Ammonia is a promising hydrogen carrier due to its favorable storage and transport characteristics. However, its direct use in combustion systems is limited by low flammability and potential for high nitrogen oxide emissions. To better understand ammonia combustion, researchers conducted experiments using a flat flame burner and measured species profiles using tunable-diode-laser-absorption-spectroscopy and chemiluminescence imaging. They tested three flame conditions with different ammonia-hydrogen blends and oxygen levels. The results were compared to simulations using various kinetic mechanisms, including one that accounts for excited species chemistry. The goal is to provide direct information about species profiles in a simple system, isolating chemical kinetics from fluid dynamic effects, which can inform the development of more efficient and low-emission combustion systems using ammonia.

ammonia combustion↗
DOE OSTI2024

Effects of dimensionality on the electronic structure of Ruddlesden-Popper chromates Sr n+1 ⁢Cr n ⁢O 3⁢n+1

Transition-metal oxides host a wide variety of electronic phenomena that can be significantly influenced by the effective dimensionality of the system under consideration. These include charge, spin, and orbital orderings, as well as unconventional superconductivity. In this context, the Ruddlesden-Popper chromates Sr n+1 Cr n O 3n+1 emerge as a particularly intriguing series of materials. Formally, the chromium atom displays a rather special 4+ oxidation state throughout the entire series. However, the effective dimensionality changes from quasi-2D to 3D as n increases from 1 to ∞. As a result, the insulating antiferromagnetic behavior observed for n = 1, 2, 3 transforms into itinerant antiferromagnetism with reduced transition temperature for the n = ∞ end member of the series, i.e., the perovskite SrCrO 3 . Further, distinct orbital orderings with exotic singlet states have been predicted for these systems. However, the lack of single-crystal bulk or thin-film samples has made experimental progress difficult. Here we demonstrate the synthesis of thin films of the perovskite SrCrO 3 and the associated layered chromates via oxide molecular beam epitaxy for n = 1 to n = 5. Our electrical transport measurements reveal a gradual evolution from a strongly insulating state in Sr 2 CrO 4 to a metallic state in the end member SrCrO 3 . X-ray absorption spectroscopy measurements demonstrate a varying hybridization strength of the Cr 4+ valence electrons across the series, helping to explain the trend in conduction. Finally, density functional theory calculations further confirm the observed transport trend and identify additional distortions present in the system.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Phase segregation and miscibility of TiO x nanocomposites in Gd-doped ceria solid electrolyte material

Electro-chemo-mechanical (ECM) coupling refers to mechanical deformation due to electrochemically driven compositional change in a solid. An ECM actuator producing micrometre-size displacements and long-term stability at room temperature was recently reported, comprising a 20 mol% Gd-doped ceria (20GDC), a solid electrolyte membrane, placed between two working bodies made of TiO x /20GDC (Ti-GDC) nanocomposites with Ti concentration of 38 mol%. The volumetric changes originating from oxidation or reduction in the local TiO x units are hypothesized to be the origin of mechanical deformation in the ECM actuator. Studying the Ti concentration-dependent structural changes in the Ti-GDC nanocomposites is therefore required for (i) understanding the mechanism of dimensional changes in the ECM actuator and (ii) maximizing the ECM response. Here, the systematic investigation of the local structure of the Ti and Ce ions in Ti-GDC over a broad range of Ti concentrations using synchrotron X-ray absorption spectroscopy and X-ray diffraction is reported. The main finding is that, depending on the Ti concentration, Ti atoms either form a cerium titanate or segregate into a TiO 2 anatase-like phase. The transition region between these two regimes with Ti(IV) concentration between 19% and 57% contained strongly disordered TiO x units dispersed in 20GDC containing Ce(III) and Ce(IV) and hence rich with oxygen vacancies. As a result, this transition region is proposed to be the most advantageous for developing ECM-active materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Controlling MoO 2 and MoO 3 phases in MoO x /CNTs nanocomposites and their application to anode materials for lithium-ion batteries and capacitors

Molybdenum oxides (MoO 2 and MoO 3 ) are attractive anode materials for Li- and Na- ion batteries. Although there have been extensive studies on them individually, systematic and comparative studies are still lacking. Here, we demonstrate a facile and straightforward synthesis method to control the phase and oxidation state in the MoO x /CNTs nanocomposites via hydrothermal reaction followed by heat-treatment. By changing the gas atmosphere during the annealing process, well-dispersed MoO 2 /CNTs and MoO 3 /CNTs nanocomposites are formed without altering their overall morphology. This strategy enables us to investigate the true structure-property correlation of MoO x /CNTs nanocomposites by comparing the structure and electrochemical properties of MoO 2 /CNTs and MoO 3 /CNTs. When tested as anode materials for lithium-ion batteries, both HT-MoO 2&3 /CNTs electrodes show much-improved cycling stability and rate performance compared to the rod-shaped bulk MoO 3 electrode. In situ Mo K-edge x-ray absorption spectroscopy (XAS) has been further employed to compare and elucidate Li + storage mechanisms of both electrodes. When employed to the negative electrode of a high-power lithium-ion capacitor (LIC), the LIC full-cell composed of HT-MoO 3 /CNTs negative and activated carbon positive electrodes demonstrates impressive energy and power densities (~ 90 Wh kg –1 with 2000 W kg –1 ) and excellent cycling stability (96.8 % capacity retention after 300 cycles), revealing the versatility of the MoO x /CNTs electrodes in energy applications.

25 ENERGY STORAGE↗
DOE OSTI2021

Oxygen vacancy induced electronic structure modification of KTaO 3

The observation of a metallic interface between band insulators LaAlO 3 and SrTiO 3 has led to massive efforts to understand the origin of the phenomenon as well as to search for other systems hosting such two-dimensional electron gases (2-DEGs). However, the understanding of the origin of the 2-DEG is very often hindered as several possible mechanisms such as polar catastrophe, cationic intermixing, and oxygen vacancy (OV) can be operative simultaneously. The presence of a heavy element makes the KTaO 3 (KTO) based 2-DEG a potential platform to investigate spin-orbit coupling driven novel electronic and magnetic phenomena. In this work, we investigate the sole effect of OV in KTO, which makes KTO metallic. O K-edge x-ray absorption spectroscopy measurements find that OV dopes electrons in Ta t * 2g antibonding states. Photoluminescence measurements reveal the existence of a highly localized deep midgap state in oxygen-deficient KTO. Our detailed ab initio calculations demonstrate that such deep midgap state arises due to the linear clustering of OVs around Ta. Finally, our present work emphasizes that we must pay attention to the possible presence of OVs in interpreting emergent behavior of KTO-based heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Growth and characterization of large (Y,La)TiO 3 and (Y,Ca)TiO 3 single crystals

The Mott-insulating rare-earth titanates ( R TiO 3 , with R being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of Ti 3 + to oxidize to Ti 4 + . We describe our efforts to grow sizable single crystals of YTiO 3 , Y 1 - x La x TiO 3 ( x ≤ 0.25 ), and Y 1 - y Ca y TiO 3 ( y ≤ 0.35 ) with the optical traveling-solvent floating-zone technique. Finally, we present sample characterization via chemical composition analysis, magnetometry, charge transport, neutron scattering, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism to understand macroscopic physical property variations associated with overoxidation. Furthermore, we demonstrate a good signal-to-noise ratio in inelastic magnetic neutron scattering measurements of spin-wave excitations. A superconducting impurity phase, found to appear in Ca-doped samples at high doping levels, is identified as TiO.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Impact of Charge Voltage on Factors Influencing Capacity Fade in Layered NMC622: Multimodal X-ray and Electrochemical Characterization

Ni-rich NMC is an attractive Li-ion battery cathode due to its combination of energy density, thermal stability, and reversibility. While higher delivered energy density can be achieved with a more positive charge voltage limit, this approach compromises sustained reversibility. Improved understanding of the local and bulk structural transformations as a function of charge voltage, and their associated impacts on capacity fade are critically needed. Through simultaneous operando synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) of cells cycled at 3–4.3 or 3–4.7 V, this study presents an in-depth investigation into the effects of voltage window on local coordination, bulk structure, and oxidation state. These measurements are complemented by ex situ X-ray fluorescence (XRF) mapping and scanning electrochemical microscopy mapping (SECM) of the negative electrode, X-ray photoelectron spectroscopy (XPS) of the positive electrode, and cell level electrochemical impedance spectroscopy (EIS). Initially, cycling between 3 and 4.7 V leads to greater delivered capacity due to greater lithium extraction, accompanied by increased structural distortion, moderately higher Ni oxidation, and substantially higher Co oxidation. Continued cycling at this high voltage results in suppressed Ni and Co redox, greater structural distortion, increased levels of transition metal dissolution, higher cell impedance, and 3× greater capacity fade.

capacity fading mechanisms↗
DOE OSTI2021

Spin-liquid behavior of the three-dimensional magnetic system Ba 3 NiIr 2 O 9 with S = 1

The quantum spin liquid (QSL) is an exotic phase of magnetic materials where the spins continue to fluctuate without any symmetry breaking down to zero temperature. Among the handful reports of QSL with spin S ≥ 1, examples with magnetic ions on a three-dimensional (3D) magnetic lattice are extremely rare since both larger spin and higher dimension tend to suppress quantum fluctuations. In this work, we offer a new strategy to achieve 3D QSL with high spin by utilizing two types of transition metal ions; both are magnetically active but located at crystallographically inequivalent positions. Furthermore, we design a 3D magnetic system Ba 3 NiIr 2 O 9 consisting of interconnected corner-shared NiO 6 octahedra and face-shared Ir 2 O 9 dimer, both having triangular arrangements in a-b plane. X-ray absorption spectroscopy measurements confirm the presence of Ni 2+ (S = 1). Furthermore, our detailed thermodynamic and magnetic measurements reveal that this compound is a realization of gapless QSL state down to at least 100 mK. Ab initio calculations find a strong magnetic exchange between Ir and Ni sublattices and in-plane antiferromagnetic coupling between the dimers, resulting in dynamically fluctuating magnetic moments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Giant orbital polarization of Ni 2+ in a square planar environment

Finding large orbital polarization in Ni-based oxides has become a topic of paramount interest in recent years due to the prospect of finding superconductivity. In this study, we investigate the electronic structure of single-crystalline samples of Sr 2 CuO 3 and Ni-doped Sr 2 CuO 3 containing Cu/NiO 4 square planar units. Our polarization-dependent x-ray absorption spectroscopy experiments reveal extremely large orbital polarization (~63%) for Ni 2+ . Cluster calculations and ab initio calculations find that the giant orbital polarization arises due to the low-spin (S=0) configuration with two holes in the Ni 3d x 2 -y 2 orbital, contrary to the expected high-spin (S=1) state from Hund's first rule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

In-situ polarization modulation IRRAS investigation of ammonia electrooxidation on Pt-Ir and Pt-Ru nanoparticles prepared on engineered catalyst supports

The catalytic activity and surface reactivity of monometallic Pt and bimetallic Pt-Ir and Pt-Ru nanoparticles, supported on two distinct Engineered Catalyst Supports (ECSs), were investigated for the Ammonia Electrooxidation Reaction (AmER) in alkaline media. XRD measurements confirmed alloy formation between Pt-Ir and Pt-Ru nanoparticles, as indicated by the shift of the (111) reflection to higher 2θ values. Cyclic voltammetry, linear sweep voltammetry, and chronoamperometry experiments were conducted to assess the catalytic activity of the Pt, Pt-Ir, and Pt-Ru electrocatalysts. All bimetallic catalysts exhibited lower onset potentials compared to Pt. The differing Tafel slopes between Pt (74 mV dec⁻¹), Pt-Ir (152 mV dec⁻¹), and Pt-Ru (118–197 mV dec⁻¹) suggest that alloying Pt with Ir or Ru alters the reaction mechanisms. Furthermore, the bimetallic Pt-Ir and Pt-Ru catalysts demonstrated greater tolerance for concentrated ammonia solutions relative to Pt. In-situ Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) provided insights into the formation of N-H species, azide anions (N₃⁻), and N-O compounds. For the Pt-Ru catalyst, an additional peak around ~3600 cm⁻¹ was observed, corresponding to OH⁻ species. The PM-IRRAS results align with the Gerischer–Mauerer mechanism, indicating that partially dehydrogenated ammonia adsorbates act as active intermediates in the oxidation of ammonia over Pt-Ir and Pt-Ru catalysts.

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

Unoccupied electronic structure of actinide dioxides

Ligand field density functional theory calculations of the dioxides of thorium, uranium, and plutonium have been combined with high-energy-resolution fluorescence detection (HERFD) in x-ray absorption spectroscopy and inverse photoelectron spectroscopy (IPES) measurements to provide powerful insight into the underlying composition of the unoccupied 5f electronic structure in these 5f localized systems. Fine structure in the 5f 5/2 transitions in HERFD can be directly correlated with the fine structure in the leading edge of the IPES. The shapes, intensities, and systematics in HERFD and IPES are explained in a consistent and rigorous fashion in terms of the j-specific 5f electronic structure.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Area and thickness dependence of Auger recombination in nanoplatelets

The ability to control both the thickness and the lateral dimensions of colloidal nanoplatelets offers a test-bed for area and thickness dependent properties in 2D materials. An important example is Auger recombination, which is typically the dominant process by which multiexcitons decay in nanoplatelets. In this report, we uncover fundamental properties of biexciton decay in nanoplatelets by comparing the Auger recombination lifetimes based on interacting and noninteracting formalisms with measurements based on transient absorption spectroscopy. Specifically, we report that electron-hole correlations in the initial biexcitonic state must be included in order to obtain Auger recombination lifetimes in agreement with experimental measurements and that Auger recombination lifetimes depend nearly linearly on the lateral area and somewhat more strongly on the thickness of the nanoplatelet. We also connect these scalings to those of the area and thickness dependencies of single exciton radiative recombination lifetimes, exciton coherence areas, and exciton Bohr radii in these quasi-2D materials.

2D materials↗
DOE OSTI2023

Highly durable fuel cell electrocatalyst with low-loading Pt-Co nanoparticles dispersed over single-atom Pt-Co-N-Graphene nanofiber

The limited durability of Pt electrocatalysis toward cathodic oxygen reduction reaction remains challenging, yet crucial for the development of Proton Exchange Membrane Fuel Cell. Here, we present a rational design of a robust catalyst consisting of PtCo nanoparticles supported on Pt-Co-N-graphene nanofiber prepared through electrospun Cobalt-Metal-Organic-Framework. The catalyst delivers unprecedented mass activity of 2.48 A·mgPt -1 , and retains 80% of initial value after 60,000 Accelerated-Stress-Test cycles. Operando X-ray absorption spectroscopies show that the electronic configurations of Pt sites in PtCo and Co sites in Co-N4 in the hybrid catalyst are modified toward high catalytic activities. Density Functional Theory unveils that the enhanced curvature of the substrate induced by the morphology engineering lowers the reaction thermodynamic barrier on Co-N4 sites, favoring the formation of H2O and suppressing that of H2O2. This result along with the strong affinity of PtCo nanoparticles to the Pt-Co-N-graphene fiber endows the catalyst an exceptional durability.

acidic electrolyte↗
DOE OSTI2023

Multiple and nonlocal cation redox in Ca–Ce–Ti–Mn oxide perovskites for solar thermochemical applications

Modeling-driven design of redox-active off-stoichiometric oxides for solar thermochemical H 2 production (STCH) seldom has resulted in empirical demonstration of competitive materials. Here, we report the theoretical prediction and experimental evidence that the perovskite Ca 2/3 Ce 1/3 Ti 1/3 Mn 2/3 O 3 is synthesizable with high phase purity, stable, and has desirable redox thermodynamics for STCH, with a predicted average neutral oxygen vacancy (VO) formation energy, E v = 3.30 eV. Flow reactor experiments suggest potentially comparable or greater H 2 production capacity than recent promising Sr–La–Mn–Al and Ba–Ce–Mn metal oxide perovskites. Utilizing quantum-based modeling of a solid solution on both A and B sub-lattices, we predict the impact of nearest-neighbor composition on E v and determine that A-site Ce 4+ reduction dominates the redox-activity of Ca 2/3 Ce 1/3 Ti 1/3 Mn 2/3 O 3 . X-ray absorption spectroscopy measurements provide evidence that supports these predictions and reversible Ce 4+ -to-Ce 3+ reduction. Our models predict that Ce 4+ reduces even when it is not nearest-neighbor to the V O , suggesting that refinement of Ce stoichiometry has the possibility of further enhancing performance.

08 HYDROGEN↗
DOE OSTI2021

Polyamorphism in a solute-lean Al–Ce metallic glass

Polyamorphism discovered in lanthanide-rich metallic glasses (MGs) has been attributed to the electronic transition of the lanthanide element as a solvent element. In this work, we report that pressure-induced polyamorphism still exists in a Ce-poor Al 93 Ce 7 binary MG where the 4f electron element serves as a solute and solute–solute avoidance is expected. The polyamorphic transition, observed by in situ high-pressure synchrotron x-ray diffraction, is accompanied by a volume collapse of ~0.78% and occurs over a narrow pressure range from ~0.8 to ~1.8 GPa. Further synchrotron Ce L 3 -edge x-ray absorption spectroscopy measurements reveal that pressure-induced 4f electron delocalization underlies the polyamorphic transition. Molecular dynamics simulations confirm that the Ce atoms in the MG are completely isolated by the solvent Al atoms. This result demonstrates that 4f element-bearing alloys with extremely dilute concentrations can also exhibit polyamorphic states originating from electronic transitions, extending the compositional space of polyamorphism of MGs into very dilute regions. Our work suggests that tunable properties under compressive stress could be achieved in MGs by even minor doping of elements prone to electronic transitions.

36 MATERIALS SCIENCE↗