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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Excited States of Crystalline Point Defects with Multireference Density Matrix Embedding Theory

Accurate and affordable methods to characterize the electronic structure of solids are important for targeted materials design. Embedding-based methods provide an appealing balance in the trade-off between cost and accuracy-particularly when studying localized phenomena. Here, we use the density matrix embedding theory (DMET) algorithm to study the electronic excitations in solid-state defects with a restricted open-shell Hartree-Fock (ROHF) bath and multireference impurity solvers, specifically, complete active space self-consistent field (CASSCF) and n-electron valence state second-order perturbation theory (NEVPT2). In this work, we apply the method to investigate the electronic excitations in an oxygen vacancy (OV) on a MgO(100) surface and find absolute deviations within 0.05 eV between DMET using the CASSCF/NEVPT2 solver, denoted as CAS-DMET/NEVPT2-DMET, and the nonembedded CASSCF/NEVPT2 approach. Next, we establish the practicality of DMET by extending it to larger supercells for the OV defect and a neutral silicon vacancy in diamond where the use of nonembedded CASSCF/NEVPT2 is extremely expensive.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Evolution of magnetism, valence, and crystal lattice in EuCd 2 As 2 under pressure

EuCd 2 As 2 has been proposed to be one of the ideal platforms as an intrinsic topological magnetic system, potentially hosting a single pair of Weyl points when it is tuned into the ferromagnetic state with spins aligned out of plane by either external pressure or chemical doping. To investigate the possible realization of an ideal topological state, we have systematically investigated pressure control of the magnetic state, valence, and crystal structure using synchrotron-based time-domain Mössbauer spectroscopy, x-ray absorption spectroscopy, and powder x-ray diffraction. Our experimental results show that the magnetic configuration remains mostly in plane under pressure up to 42.8 GPa and pressure effectively enhances the magnetic ordering temperature. Meanwhile, Eu ions remain divalent when subjected to pressure up to 35.9 GPa, and the trigonal crystal lattice is maintained up to 34.6 GPa. Our work provides valuable experimental data to benchmark future theoretical studies in magnetic topological materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1989

Medium-resolution studies of extreme ultraviolet emission from N2 by electron impact - Vibrational perturbations and cross sections of the c4-prime 1Sigma(+)u and b-prime 1Sigma(+)u states

In a crossed-beam experiment the electron-impact-induced fluorescence spectrum of N2 in the extreme ultraviolet is studied at a spectral resolution of up to 0.03 nm. The optically thin experiment obtained the highest-resolution electron-impact emission spectrum of the Rydberg and valence states of N2. The spectral measurements provide the emission cross sections of each of the vibrational transitions of the Carroll-Yoshino and the Birge-Hopfield-II band systems. Laboratory vibrational-excitation cross sections arising from the mutual perturbation of the c4-prime 1Sigma(+)u and b-prime 1Sigma(+)u states by homogeneous configuration interactions are measured from 10 to 400 eV, and a modified Born approximation analytic model is given for them. The analysis leads to accurate band-system oscillator strengths. The relative emission and excitation cross sections each of the vibrational levels are compared. In addition, low-resolution measurements of the cross section of the atomic dissociation fragments (NI, NII, NIII) from 40 to 102 nm are made, and medium-resolution measurements are made of the emission cross section of the c4 1Pi(u), c5-prime 1Sigma(+)u, c5 1Pi(u), and c6-prime 1Sigma(+)u to X 1Sigma(+)g (0,0) transitions.

Ajello, Joseph M.↗
DOE OSTI2022

Electronically-coupled redox centers in trimetallic cobalt complexes

Synthesis and isolation of molecular building blocks of metal–organic frameworks (MOFs) can provide unique opportunities for characterization that would otherwise be inaccessible due to the heterogeneous nature of MOFs. Herein, we report a series of trinuclear cobalt complexes incorporating dithiolene ligands, triphenylene-2,3,6,7,10,11-hexathiolate (THT) (1 3+ ), and benzene hexathiolate (BHT) (2 3+ ), with 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) employed as the capping ligand. Single crystal X-ray analyses of 1 3+ and 2 3+ display three five-coordinate cobalt centers bound to the triphos and dithiolene ligands in a distorted square pyramidal geometry. Cyclic voltammetry studies of 1 3+ and 2 3+ reveal three redox features associated with the formation of mixed valence states due to the sequential reduction of the redox-active metal centers (Co III/II ). Using this electrochemical data, the comproportionality values were determined for 1 and 2 (log K c = 1.4 and 1.5 for 1, and 4.7 and 5.8 for 2), suggesting strong resonance-stabilized coupling of the metal centers, with stronger electronic coupling observed for complex 2 compared to that for complex 1. Cyclic voltammetry studies were also performed in solvents of varying polarity, whereupon the difference in the standard potentials (ΔE 1/2 ) for 1 and 2 was found to shift as a function of the polarity of the solvent, indicating a negative correlation between the dielectric constant of the electrochemical medium and the stability of the mixed valence species. Spectroelectrochemical studies of in situ generated multi-valent (MV) states of complexes 1 and 2 display characteristic NIR intervalence charge transfer (IVCT) bands, and analysis of the IVCT transitions for complex 2 suggests a weakly coupled class II multi-valent species and relatively large electronic coupling factors (1700 cm –1 for the first multi-valent state of 2 2+ , and 1400 and 4000 cm –1 for the second multi-valent state of 2 + ). Here, density functional theory (DFT) calculations indicate a significant deviation in relative energies of the frontier orbitals of complexes 1 3+ , 2 3+ , and 3 + that contrasts those calculated for the analogous trinuclear cobalt dithiolene complexes employing pentamethylcyclopentadienyl (Cp*) as the capping ligand (Co 3 Cp* 3 THT and Co 3 Cp* 3 BHT, respectively), and may be a result of the cationic nature of complexes 1 3+ , 2 3+ , and 3 + .

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

Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds

In this work, we report the presence of surface-densified phases (β-Ni 5 O 8 , γ-Ni 3 O 4 , and δ-Ni 7 O 8 ) in LiNiO 2 (LNO)- and LiNi 0.8 Al 0.2 O 2 (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These surface phases form upon electrochemical aging at high state of charge corresponding to a fully delithiated state. A unique feature of these phases is the periodic occupancy by Ni 2+ in the Li layer. This periodic Ni occupancy gives rise to extra diffraction reflections, which are qualitatively similar to those of the LiNi 2 O 4 spinel structure, but these surface phases have a lower Ni valence state and cation content than spinel. Furthermore, these experimental results confirm the presence of thermodynamically stable surface phases and provide new insights into the phenomena of surface phase formation in Ni-rich layered structures.

25 ENERGY STORAGE↗
DOE OSTI2024

Importance of Electron Correlation on the Geometry and Electronic Structure of [2Fe–2S] Systems: A Benchmark Study of the [Fe 2 S 2 (SCH 3 ) 4 ] 2–,3–,4– , [Fe 2 S 2 (SCys) 4 ] 2– , [Fe 2 S 2 (S- p -tol) 4 ] 2– , and [Fe 2 S 2 (S- o -xyl) 4 ] 2– Complexes

Iron–sulfur clusters are crucial for biological electron transport and catalysis. Obtaining accurate geometries, energetics, manifolds of their excited electronic states, and reduction energies is important to understand their role in these processes. Here, using a [2Fe–2S] model complex with Fe II and Fe III oxidation states, which leads to different charges, i.e., [Fe 2 S 2 (SMe) 4 ] 2–,3–,4– , we benchmarked a variety of computational methodologies ranging from density functional theory (DFT) to post-Hartree–Fock methods, including complete active space self-consistent field (CASSCF), multireference configuration interaction, the second-order N-electron valence state perturbation theory (NEVPT2), and the linearized integrand approximation of adiabatic connection (AC0) approaches. Additionally, we studied three experimentally well-characterized complexes, [Fe 2 S 2 (SCys) 4 ] 2– , [Fe 2 S 2 (S-o-tol) 4 ] 2– , and [Fe 2 S 2 (S-o-xyl) 4 ] 2– , via DFT methods. We conclude that the dynamic electron correlation is important for accurately predicting the geometry of these complexes. Broken symmetry (BS) DFT correctly predicts experimental geometries of low-spin multiplicity, while CASSCF does not. However, BS-DFT significantly overestimates the difference between the low- and high-spin electronic states for a given oxidation state. At the same time, CASSCF underestimates it but provides relative energies closer to the reference NEVPT2 results. Finally, AC0 provides energetics of NEVPT2 quality with the additional advantage of being able to use large CASSCF sizes. NEVPT2 gives the best estimates of the Fe III /Fe III → Fe II /Fe III (4.27 eV) and Fe II /F III → Fe II /F II (7.72 eV) reduction energies. The results provide insight into the electronic structure of these complexes and assist in the understanding of their physical properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1990

Matrix elements in the coupled-cluster approach - With application to low-lying states in Li

A procedure is suggested for evaluating matrix elements of an operator between wavefunctions in the coupled-cluster form. The use of the exponential ansatz leads to compact exponential expressions also for matrix elements. Algorithms are developed for summing all effects of one-particle clusters and certain chains of two-particle clusters (containing the well-known random-phase approximation as a subset). The treatment of one-particle perturbations in single valence states is investigated in detail. As examples the oscillator strength for the 2s-2p transition in Li as well as the hyperfine structure for the two states are studied and compared to earlier work.

Martensson-Pendrill, Ann-Marie↗
DOE OSTI2021

Incorporation of Ti in epitaxial Fe2TiO4 thin films

The titanomagnetites (Fe2-xTixO4, x = 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe2TiO4 were deposited by molecular beam epitaxy on MgO(001) at 250 – 375°C. The homogeneous incorporation of Ti, Fe valence, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe3O4 and rutile TiO2. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe3+, Fe2+, Fe0). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe2+. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe2+ was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe2+ was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the restriction of the deposition conditions to low temperature by the MgO substrate limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe2TiO4 by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on temperature-stable substrates.

Kaspar, Tiffany C.↗
DOE OSTI2025

Electrical Transport Interplay with Charge Density Waves, Magnetization, and Disorder Tuned by 2D van der Waals Interface Modification via Elemental Intercalation and Substitution in ZrTe 3 , 2H-TaS 2 , and Cr 2 Si 2 Te 6 Crystals

Electrical transport in 2D materials exhibits unique behaviors due to reduced dimensionality, broken symmetries, and quantum confinement. It serves as both a sensitive probe for the emergence of coherent electronic phases and a tool to actively manipulate many-body correlated states. Exploring their interplay and interdependence is crucial but remains underexplored. This review integratively cross-examines the atomic and electronic structures and transport properties of van der Waals-layered crystals ZrTe 3 , 2H-TaS 2 , and Cr 2 Si 2 Te 6 , providing a comprehensive understanding and uncovering new discoveries and insights. A common observation from these crystals is that modifying the atomic and electronic interface structures of 2D van der Waals interfaces using heteroatoms significantly influences the emergence and stability of coherent phases, as well as phase-sensitive transport responses. In ZrTe 3 , substitution and intercalation with Se, Hf, Cu, or Ni at the 2D vdW interface alter phonon–electron coupling, valence states, and the quasi-1D interface Fermi band, affecting the onset of CDW and SC, manifested as resistance upturns and zero-resistance states. We conclude here that these phenomena originate from dopant-induced variations in the lattice spacing of the quasi-1D Te chains of the 2D vdW interface, and propose an unconventional superconducting mechanism driven by valence fluctuations at the van Hove singularity, arising from quasi-1D lattice vibrations. Short-range in-plane electronic heterostructures at the vdW interface of Cr 2 Si 2 Te 6 result in a narrowed band gap. The sharp increase in in-plane resistance is found to be linked to the emergence and development of out-of-plane ferromagnetism. The insertion of 2D magnetic layers such as Mn, Fe, and Co into the vdW gap of 2H-TaS 2 induces anisotropic magnetism and associated transport responses to magnetic transitions. Overall, 2D vdW interface modification offers control over collective electronic behavior, transport properties, and their interplays, advancing fundamental science and nanoelectronic devices.

2D van der Waals interfaces↗
DOE OSTI2025

Selective reduction in epitaxial SrFe 0.5 Co 0.5 O 2.5 and its reversibility

Oxygen-vacancy engineering in transition metal oxides enables programmable functionalities by modulating the valence states and local coordination of constituents. Here, we report the selective reduction of cobalt ions in epitaxial SrFe 0.5 Co 0.5 O 2.5 thin films under reducing gas environments, while iron ions remain unchanged. X-ray absorption spectroscopy reveals an absorption edge shift of 1.65 eV in the Co L-edge upon reduction, and multiplet simulations estimate a decrease in the average Co valence from Co 2.91+ to Co 2.00+ . This site- and element-specific reduction leads to the formation of a structurally distinct oxygen-deficient phase stabilized by oxygen vacancies at tetrahedral sites, as confirmed by density functional theory. Optical spectroscopy reveals an increase in the bandgap from 2.47 eV to 3.04 eV, accompanied by enhanced transparency. Furthermore, simultaneous in situ diffraction and transport measurements confirm fully reversible redox-driven transitions among three phases: reduced defective perovskite, brownmillerite, and oxygen-rich perovskite phases. These findings demonstrate that selective redox control in multi-cation oxides enables the realization of chemically and functionally distinct oxygen-deficient phases.

Fuel cells↗
DOE OSTI2021

Epitaxial stabilization of thin films of the frustrated Ge-based spinels

Frustrated magnets can host numerous exotic many-body quantum and topological phenomena. GeNi 2 O 4 is a three-dimensional S=1 frustrated magnet with an unusual two-stage transition to the two-dimensional antiferromagnetic ground state, while GeCu 2 O 4 is a high-pressure phase with a strongly tetragonally elongated spinel structure and magnetic lattice formed by S=1/2 CuO 2 linear chains with frustrated interchain exchange interactions and exotic magnetic behavior. Here, we report on the thin-film epitaxial stabilization of these two compounds. The developed growth mode, surface morphology, crystal structure, and copper valence state were characterized by in situ reflection high-energy electron diffraction, atomic force microscopy, x-ray reflectivity, x-ray diffraction, x-ray photoelectron spectroscopy, and resonant x-ray absorption spectroscopy. Our results pave an alternative route to the comprehensive investigation of the puzzling magnetic properties of these compounds and the exploration of emergent features driven by strain.

36 MATERIALS SCIENCE↗
NASA NTRS2017

Diffusion of Redox-Sensitive Elements in Basalt at Different Oxygen Fugacities

The terrestrial planets and moons of our solar system have differentiated over a range of oxygen fugacity conditions. Basalts formed from magmas on the Earth cover a range of more oxidized states (from approximately IW (iron wustite) plus 2 to approximately FMQ (fayalite-magnetite-quartz) plus 3) than crustal rocks from Mars (IW to approximately IW plus 3), and basalts from the Moon are more reduced than both, ranging from IW to IW minus 2. The small body Vesta differentiated around IW minus 4. Characterization of redox sensitive elements' diffusivities will offer insight into behavior of these elements as a function of f (fugacity of) O2 for these planetary bodies. Here, we report a systematic study of the diffusion of redox-sensitive elements in basaltic melts with varying oxygen fugacities (fO2) for trace elements, V, Nb, W, Mo, La, Ce, Pr, Sm, Eu, Gd, Ta, and W. Since fO2 is an intensive variable that is different for the reservoirs of various planets and moons in our solar system, it is important to characterize how changes in redox states will affect diffusion. We conducted experiments in a piston cylinder device at 1300 degrees Centigrade and 1 gigapascal, at the University of Rochester and NASA Johnson Space Center. We buffered some experiments at Ru-RuO2 (FMQ plus 6.00), and conducted other experiments within either a graphite or Mo capsule, which corresponds to fO2s of either FMQ minus1.2, or FMQ minus 3.00, respectively. Characterizing the diffusivities of redox sensitive elements at different fO2s is important because some elements, like Eu, have varying valence states, such as Eu (sup 2 plus) and Eu (sup 3 plus). Differences in charge and ion radii may lead to differences in diffusivities within silicate melts. This could, lead to formation of a Eu anomaly by diffusion, the magnitude of which may be controlled by the fO2. Characterization of trace element diffusion is also important in understanding trace element fractionation. We found, during the course of our investigation, that not only did the diffusivities of the redox sensitive elements change with fO2, but that the diffusivities of all other analyzed elements also changed. This indicates that not only do changes in valence influence trace elements diffusivities but that the structure of melt may have changed with varying oxygen fugacity, probably due to changes in the speciation of the major element Fe.

Szumila, I.↗
DOE OSTI2025

Unraveling structural, electronic, and magnetic ambiguities in P⁢b 1−𝛿⁢ Cr⁢O 3 with an insulating charge-transfer band structure

As a recently identified Mott system, PbCr⁢O 3 remains largely unexplored, especially for its band structure, leading to many contentious issues on its structural, electronic, and magnetic properties. Here we present a comprehensive study of two different P⁢b 1−𝛿⁢ Cr⁢O 3 (δ = 0 and 0.15) samples involving atomic deficiency prepared under pressure. By means of state-of-the-art diffraction techniques, the crystal structure of PbCr⁢O 3 is definitively determined to adopt the pristine 𝑃𝑚⁢$\overline{3}$𝑚 symmetry, rather than other previously misassigned structures of M2-Pm$\overline{3}$⁢𝑚 and Pmnm. The two materials exhibit a similar charge-transfer-type insulating band structure, and the charge-transfer effect splits both Cr 2⁢𝑝 and Pb 4⁢𝑓 orbitals, rationalizing doublet splitting of the associated spectral lines. Nearly identical nominal cationic valence states of C⁢r 4+ and P⁢b 2+ are identified for this oxide system, hence calling into question the validity of recently proposed charge disproportionation mechanisms. In addition, P⁢b 0.85 ⁢Cr⁢O 3 exhibits an anomalously higher Néel temperature of ∼240 K than that of PbCr⁢O 3 (i.e., ∼200 K), likely due to the deficiency-induced enhancements of Cr3𝑑–O⁢2⁢𝑝 orbital overlap and magnetic exchange. In conclusion, these findings provide much solid evidence to look into the fundamental properties of this important material system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Manipulation of the Electronic State of Mott Iridate Superlattice through Protonation Induced Electron-Filling

Spin-orbit-coupled Mott iridates show great similarity with parent compounds of superconducting cuprates, attracting extensive research interest especially for their electron-doped states. However, previous experiments have been largely limited within a small doping range due to the absence of effective dopants, and therefore the electron-doped phase diagram remains elusive. Here, an ionic-liquid-gating-induced protonation method is utilized to achieve electron-doping into a 5d Mott-insulator built with a SrIrO 3 /SrTiO 3 superlattice (SL), and a systematic mapping of its electron-doped phase diagram is achieved with the evolution of the iridium valence state from 4+ to 3+, equivalent to doping of one electron per iridium ion. Along increasing doping level, the parent Mott-insulator is first turned into a localized metallic state with gradually suppressed magnetic ordering, and then further evolves into a nonmagnetic band insulating state. This work forms an important step forward for the study of electron-doped Mott iridate systems, and the strategy of manipulating the band filling in an artificially designed SL structure can be readily extended into other systems with more exotic states to explore.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states

The spin- 1 2 kagome antiferromagnet is considered an ideal host for a quantum spin liquid ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu 4 (OH) 6 FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin- 1 2 kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel va- lence bond crystal (VBC) state instead of a quantum spin liquid (QSL). The presence of interlayer spins eventually leads to an interesting pinwheel q=0 magnetic order. Partially Zn-substituted bar- lowite (Cu 3.44 Zn 0.56 (OH) 6 FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is simi- lar to that of herbertsmithite, even though the Cu 2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

Vanadium Valence in MgAl2O4 Spinels at Reducing Conditions (IW to IW-5)

Vanadium is commonly a major or minor element in spinel structured oxides from a wide range of planetary materials [1-3]. Vanadium is stable in multiple valence states of 5+, 4+, 3+, and 2+ in natural systems, and spinel-structured oxides are known to host vanadium in 4+, 3+, and 2+ [4-6]. However, the understanding of 2+ stability at lower fO2 conditions has been hindered by lack of experiments at or below IW-1 conditions [5]. Insufficient experimental data is available due to the difficulty in controlling fO2 at low conditions in general, and the lack of appropriate standards for comparison to natural materials. Our progress on controlling fO2 in high pressure experimental samples has allowed us to create reducing conditions that are appropriate to studying V valence in spinels at fO2 relevant to natural reducing systems [7]. Here we extend this approach to study V in reduced conditions, at high temperatures. After at-tempting shorter durations and lower temperatures, we demonstrated the need for equilibration times > 6 hours at 1600 °C in order to approach equilibrium. Only then can we produce highly equilibrated samples that provide new insights into V valence at low fO2.

oxygen fugacity↗
DOE OSTI2026

Fingerprinting Uranium Oxides with Electron Energy Loss Spectroscopy Supported by Theoretical Computations

Uranium oxides occur in a variety of phases that differ in crystal structure and uranium oxidation states. Electron energy loss spectroscopy (EELS) is one of the few techniques that has sufficient spatial resolution and sensitivity to electronic structure to distinguish amongst phases at the nanoscale. However, beam-sensitive materials such as uranium oxides are subject to spectral modification due to interactions with the electron beam. Therefore, theory support is essential to reliably exclude the impact of beam damage and generate true reference datasets. Here we use a comparison of theoretical and experimental spectra to probe the impact of beam damage on O K-edge and U N-edge (N6,7 and N4,5) EELS spectra of various single-valent and mixed-valence uranium oxide bulk phases. Using a low-dose experimental set-up, we show that the O K-edge theoretical spectra are in excellent agreement with experiment for both peak positions and relative intensities of respective peaks. In contrast, U N-edge features are less distinguishing due to the partially localized nature of the U 5f orbitals and overlapping multiplet and spin–orbit coupling effects. This work demonstrates that O K-edge EELS is sufficiently diagnostic to distinguish a wide range of uranium oxides and that the experimental approach used here minimizes beam damage and allows valence state discrimination across the U(IV), U(V) and U(VI) series. When combined with imaging modes available in electron mi-croscopy, the work enables detailed investigation and characterization of uranium redox transformations at the nanoscale.

Carbone, Jacopo↗
DOE OSTI2025

Single-Crystallization of O3-Type Layered Oxide Cathode for Na-Ion Battery

The development of high-energy-density Na-ion batteries places significant demands on single-crystal layered oxide cathodes, especially for further high-voltage, solid-state battery scenarios. In the O3-type structure, due to the original sluggish Na ion diffusion kinetics (approximately 1 order of magnitude lower than that of Li-ion), and further hindrance against diffusion kinetics caused by single-crystal architecture, these inherent defects lead to the decline in the electrochemical performance. Herein, we demonstrated that the single crystallization of O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathode (d 50 = 5.04 μm) aggravates surface-to-bulk phase inhomogeneity distribution, which is attributed to the uneven Na ions extraction. Moreover, the Na-depletion of the surface/shell region not only aggravates Na ion diffusion resistance but also leads to a higher valence state of transition-metal elements (e.g., Ni/Fe) near the surface of the single-crystal particle, which further compromises the cathode-electrolyte interface stability. Furthermore, not limited to revealing the challenges, tuning the particle size and moderating quasi-single-crystal strategies have been proven to effectively mitigate the negative uneven distributions of Na ions, phases, and valence/oxidative states, resulting in efficient modification for single crystallization of Na-layered oxide cathodes.

36 MATERIALS SCIENCE↗