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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Intraligand Charge Transfer Enables Visible‐Light‐Mediated Nickel‐Catalyzed Cross‐Coupling Reactions**

Abstract We demonstrate that several visible‐light‐mediated carbon−heteroatom cross‐coupling reactions can be carried out using a photoactive Ni II precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl 2 ). The activation of this precatalyst towards cross‐coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light‐responsive nickel complexes that undergo metal‐to‐ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl 2 with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross‐coupling reactions. The heterogeneous catalyst shows stable performance in a packed‐bed flow reactor during a week of continuous operation.

Cavedon, Cristian↗
DOE OSTI2026

Estimating the Contribution of the Nickel to Protium Loading of Full-Length Getters

The full-length getter (FLG) is a critical component of the tritium producing burnable absorber rod (TPBAR), designed to capture tritium produced within the lithium aluminate pellets. However, due to the chemical similarity between protium (¹H) and tritium (³H), the FLG readily absorbs protium, reducing its capacity to absorb tritium and increasing the risk of tritium permeation into the reactor coolant. This study evaluates protium produced from neutron irradiation of nickel plating on the FLG through 5?Ni(n,p) reactions, quantifies its contribution to the total protium observed, and informs models of tritium and hydrogen transport within TPBARs. During irradiation, neutron capture by 58Ni results in the formation of 5?Ni, which undergoes neutron bombardment to produce 4He via 5?Ni(n,a) reactions and protium via 5?Ni(n,p) reactions. The measured helium content post-irradiation provides insight into the neutron capture processes within the getter. The 4He measured within the getter may be useful in determining hydrogen produced by the nickel plating on FLG because 59Ni also produces protium in a 59Ni(n, p) reaction. The contribution of 1H from the nickel in FLG contributed less than 1% of the measured H2 gas in PIE, ranging from 0.005% to 0.614%. These findings refine current knowledge of the protium-tritium interplay in TPBARs and support the development of improved transport models for tritium and hydrogen, ultimately aiding in the optimization of TPBAR design and reactor operations.

Arbova, Dana L.↗
DOE OSTI2025

Creep Deformation and Damage Mechanisms in an Advanced High-Temperature Additively Manufactured Nickel-Base Superalloy

Abstract This research investigates the processing–structure–properties–performance relationship in a novel nickel-base superalloy, ABD ® -900AM, designed for extreme environments. Specifically tailored for additive manufacturing (AM), ABD ® -900AM maintains mechanical integrity at high temperatures and is comparable to other nickel-based superalloys with a 30–40% gamma-prime volume fraction. A comprehensive study was conducted using laser-beam powder bed fusion and electron-beam powder bed fusion methods. Factors such as heat treatment, porosity, build orientation, and hot isostatic pressing were evaluated to understand their effects on microstructure and mechanical performance. Microstructural characterization revealed significant differences in grain size and orientation across build processes and heat treatments. High-temperature mechanical testing indicated that grain size, heat treatment, and orientation significantly influence creep behavior. A super-solvus heat treatment led to recrystallization and grain growth, significantly improving creep properties compared to a near-solvus heat treatment. Various creep mechanisms were identified across different conditions, and creep rupture models were developed for each build process. Post-test microstructural analysis showed grain boundary damage, with differences in creep cavitation morphology under varying stress conditions. It was shown that MC carbides grow at the expense of gamma-prime near grain boundaries, leading to precipitate-free zones in specimens tested at higher temperatures. This study fills a significant gap in fundamental research by offering a deeper insight into the high-temperature mechanical behavior of additively manufactured nickel-base superalloys. It also explores critical research questions regarding the role of carbides and the significance of heat treatment. The insights gained enhance confidence in the industry adoption of ABD ® -900AM and similar alloys for high-temperature applications, bridging the knowledge gap and supporting the development of reliable AM processes for extreme environments.

Bridges, Alex (ORCID:000000030338759X)↗
DOE OSTI2023

Change of Electrical and Transport Properties of Nickel Oxide by Carrier Concentration and Temperature through First-Principle Calculations

Abstract Nickel is typically used as one of the main components in electrical contact devices or connectors. Nickel oxide (NiO) is usually formed on the surfaces of electrodes and can negatively impact system performance by introducing electrical contact resistance. The thermal, electrical, and transport properties of NiO, as a Mott insulator or a p-type semiconductor, can be altered by operating and environmental conditions such as temperature and stress/strain by contact. In this study, we investigate the fundamental material properties of NiO through the first-principle calculations. First, we obtain and compare the lattice parameter, magnetic moment, and electronic structure for NiO via the WIEN2K simulations with four different potentials (i.e., GGA, GGA + U, LSDA, and LSDA + U). Then, using the WIEN2K simulation results with LSDA + U potential that produces a highly accurate bandgap for NiO, we calculate the electrical conductivity and electrical part of the thermal conductivity of nickel and NiO as a function of temperature and carrier concentration through the BoltzTraP simulations. Systematic simulation results revealed that the electrical conductivity relative to the relaxation time for NiO increases with the carrier concentration, while it shows a slightly decreasing trend with temperature under a fixed carrier concentration. By contrast, the electrical part of the thermal conductivity shows an increasing trend considering carrier concentration and temperature.

Shah, Nayem Md. Reza↗
DOE OSTI2024

Atomic Layer Deposition of Nickel Using Ni(dmamb)2 and ZnO Adhesion Layer Without Plasma

Abstract In this study, a novel deposition technique that utilizes diethylzinc (C 4 H 10 ZnO) with H 2 O to form a ZnO adhesion layer was proposed. This technique was followed by the deposition of vaporized nickel(II) 1-dimethylamino-2-methyl-2-butoxide (Ni(dmamb) 2 ) and H 2 gas to facilitate the deposit of uniform layers of nickel on the ZnO adhesion layer using atomic layer deposition. Deposition temperatures ranged from 220 to 300 °C. Thickness, composition, and crystallographic structure results were analyzed using spectroscopic ellipsometry, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), respectively. An average growth rate of approximately 0.0105 angstroms per cycle at 260 °C was observed via ellipsometry. Uniform deposition of ZnO with less than 1% of Ni was displayed by utilizing the elemental analysis function via SEM, thereby providing high-quality images. XPS revealed ionizations consistent with nickel and ZnO through the kinetic and binding energies of each detected electron. XRD provided supplemental information regarding the validity of ZnO by exhibiting crystalline attributes, revealing the presence of its hexagonal wurtzite structure.

Baker, Kaiya↗
DOE OSTI2022

Pro5 is not essential for the formation of ‘Ni-hook’ in nickel superoxide dismutase

The N-terminus of nickel-dependent superoxide dismutase (NiSOD) forms a structural motif known as the "Ni-hook," where the peptide wraps around the metal to bring cysteine-2 and cysteine-6 into spatial proximity, allowing these residues to coordinate in a cis-geometry. A highly conserved proline-5 residue in the Ni-hook adopts a cis-conformation that is widely considered important for its formation. Herein, we investigate this role by point mutation of Pro5 to alanine. The results obtained show that the variant exhibits wild-type-like redox catalysis and features a Ni(III) center very similar to that found in enzyme. Structural analysis using x-ray absorption spectroscopy of the nickel sites in as-isolated P5A-NiSOD reveals changes in the variant and are consistent with a six-coordinate Ni site with (N/O) 4 S 2 coordination. These changes are attributed to changes in the Ni(II) site structure. Nickel-binding studies using isothermal titration calorimetry reveal two binding events with K d = 25(20) nM, and 250(60) nM. These events are attributed to i) Ni(II) binding to a preformed Ni-hook containing cis-Pro5 and ii) the combination of trans- to cis- isomerization upon Ni(II) binding, respectively. Finally, the higher-affinity binding event is absent in P5A-NiSOD, an observation attributed to the low abundance of the cis-Ala5 isomer in the apo-protein.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

The effect of varying powder feedstock chemistry and printing atmosphere on the microstructure of additively manufactured nickel-based ODS alloys: Role on stabilization of cellular structures vs. oxide dispersion formation

Nickel-based alloys have a wide variety of structural applications due to their high corrosion resistance and mechanical strength which depend on solid solution strengthening, or the formation of oxides and/or intermetallic precipitation for their properties. In this study, the microstructure of six Ni–Cr–Y–Ti–Al powder batches designed for the production of oxide dispersion strengthened nickel were compared. These batches varied in chemistry and atomization technique used which included Gas Atomization Reactive Synthesis (GARS). The batches of powder were then consolidated via Additive Manufacturing (AM) Powder Bed Fusion using Laser Beam (PBF-LB) and characterized via transmission electron microscopy to elucidate the influence of powder feedstock (i.e. synthesis methodology and chemistry) on the PBF-LB microstructure. The study investigates (i) how the amount of yttrium and titanium additions in the powder feedstock and the addition of oxygen during the processing (through GARS) affect the microstructure of the powder itself and the AM printed microstructure, and (ii) how the control of oxygen addition in the printing atmosphere during the PBF-LB printing process itself is another important parameter for achieving the formation of the wanted oxide dispersion versus the stabilization of the cellular structure (often observed in AM processed alloys). Microstructural characterization of both powder particles and additively manufactured nickel alloys in this study provide important insights into the movement of yttrium within the material upon solidification, particularly along cell boundaries, and how yttrium behaves depending on alloy chemistry. When a threshold of yttrium content is reached within the system, yttrium consistently reacts to form an intermetallic along cell boundaries instead of forming oxide nanoparticles.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Rare Earth Nickelate Electrodes Containing Heavily Doped Ceria for Reversible Solid Oxide Fuel Cells

The electrochemical performance of composite rare-earth nickelate-rare-earth doped ceria oxygen electrodes, with a high level of rare-earth doping in ceria are reported. Additionally, the chemical stability of these compositions is reported at both the sintering (1240 ?C) and operating temperature (800 ?C). Specifically, a lanthanum nickelate La2NiO4+d (LNO) ?? 50 mol% lanthanum doped ceria (LDC50) oxygen electrode and a neodymium nickelate Nd2NiO4+d (NNO) ?? 50 mol% neodymium doped ceria (NDC50) oxygen electrode are tested in solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes and compared to a composite (La0.75Sr0.25)0.95MnO3±d (LSM)-8 mol% yttria stabilized zirconia (YSZ) electrode. The LNO–LDC50 oxygen electrode reaches a current density which is approximately three times that of the LSM-YSZ electrode in SOFC mode at 0.7 V and approximately two times the LSM-YSZ electrode at 1.2 V in SOEC mode. Similarly the NNONDC50 oxygen electrode reaches a current density which is approximately two times and approximately one and a half times that of LSM-YSZ at 0.7 V and 1.2 V respectively. Oxygen surface exchange results for LNO and NNO are also reported which show different oxygen exchange kinetics during oxidation versus reduction steps.

solid oxide electrolysis, solid oxide fuel cell (S↗
DOE OSTI2022

Delineating the Roles of Mn, Al, and Co by Comparing Three Layered Oxide Cathodes with the Same Nickel Content of 70% for Lithium-Ion Batteries

High-nickel layered oxides continue to prevail in the energy storage market as the frontmost cathode candidates for next-generation lithium-ion batteries. Demand and development of LiNi 1–x–y Mn x Co y O 2 (NMC) and LiNi 1–x–y Co x Al y O 2 (NCA) cathodes are rampantly increasing, particularly for the electric vehicle (EV) industry. However, the continued presence of cobalt in NMC and NCA cathodes raises global concerns due to geopolitical and ethical issues attributed to Co sourcing. We herein introduce a novel cobalt-free, high-nickel cathode LiNi 0.7 Mn 0.25 Al 0.05 O 2 (NMA70) and benchmark it against Co-containing LiNi 0.7 Mn 0.15 Co 0.15 O 2 (NMC70) as well as Co- and Al-free LiNi 0.7 Mn 0.3 O 2 (NM70) cathodes with equivalent 70% Ni contents that are all synthesized in-house. NMA70 displays a high initial C/10 capacity of 210 mA h g –1 , matching that of NMC70 in half cells with a cutoff voltage of 4.5 V. NMA70 also exhibits an impressive high-voltage full cell cycling performance with a cutoff voltage of 4.4 V with a nearly identical capacity retention of 83% compared to that of 82% for NMC70 after 300 cycles. Postmortem X-ray photoelectron spectroscopy (XPS), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and electron energy loss spectroscopy (EELS) analyses indicate a thinner cathode–electrolyte interface (CEI) developed in NMA70 compared to that in NM70 and unveil a more robust solid-electrolyte interface (SEI) passivation on the graphite anode among all samples. The benefits of Al doping are additionally highlighted with enhanced high-voltage CEI and thermal stabilities in NMA70. Furthermore, this work assesses the roles of Mn, Al, and Co to demonstrate both the practicality and feasibility of synthesizing cobalt-free, high-nickel cathodes that are promising alternatives to current NMC- and NCA-based cathodes.

25 ENERGY STORAGE↗
DOE OSTI2022

Surface Stabilization with Fluorine of Layered Ultrahigh-Nickel Oxide Cathodes for Lithium-Ion Batteries

High-nickel layered oxide cathodes are key to meet the demands of the electric vehicle industry because of their high specific capacity. However, commercialization of these materials is hindered by critical challenges, such as phase transitions, particle cracking, aggressive surface reactivity, and thermal instability. Cation doping along with surface coating has proven to be an effective way to circumvent some of these issues to a large extent. Herein, fluorine coating is employed on a high-nickel Li[Ni 0.95 Mn 0.015 Co 0.02 Al 0.01 Mg 0.005 ]O 2 (NMCAM) cathode via a solution route. Detailed structural and electrochemical analyses indicate fluorine largely decorates the surface at low enough calcination temperatures. The cathode with 1 mol % fluorine coating exhibits a capacity retention of 71% after 500 cycles as compared to 59% for the control sample when cycled to a high cutoff voltage of 4.3 V in a full cell configuration with graphite anode. Post-mortem analysis of cycled electrodes reveals that surface reactivity is a major contributor to capacity fade as compared to particle cracking. Fluorine coating reduces surface reactivity and the depth to which rock-salt phase is formed on the surface during cycling. The thermal stability is also enhanced after fluorine coating as the material shows less heat release at high states of charge. Furthermore, this work demonstrates an effective, economical, and scalable way to stabilize the surface with fluorine and enhance the electrochemical performance of high-nickel cathodes.

25 ENERGY STORAGE↗
DOE OSTI2022

Effect of Redox-Inactive Metal Ion–Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

We report Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe 4 ) 2 [Ni- II (HMPAB)] (1) and (NMe 4 )[Ni III (HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (M n+ = Ca 2+ , Mg 2+ , Zn 2+ , Y 3+ , and Sc 3+ ) with 2 resulted in 2-M n+ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca 2+ coordinated to Ni(III) complexes, 2Ca 2+ T, was determined and exhibited an average Ni-Ca distance of 3.1253 angstrom, close to the metal ions' covalent radius. XAS analysis of 2-Ca 2+ and 2-Y 3+ in solution further revealed an additional coordination to Ca and Y in the 2-M n+ adducts with shortened Ni-M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-M n+ species, moreover, displayed rhombic signals with g av > 2.12 for all complexes, supporting the +III oxidation state of Ni. The Ni III /Ni II redox potential of 2 and 2-M n+ species was determined, and a plot of E 1/ 2 of 2-M n+ versus pK a of [M(H 2 O) n ] m+ exhibited a linear relationship, implying that the Ni III /Ni II potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-M n+ with different 4-X-2,6-ditert-butylphenol (4-XDTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pK a value of 24.2 was estimated for 1a. The reaction of 2-M n+ species was examined with 4-OMe-DTBP, and it was observed that the k 2 values of 2-M n+ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k 2 values for 2-M n+ species are much lower compared to the k 2 value for 2. Such a variation of PCET reactivity between 2 and 2-M n+ species may be attributed to the interactions between Ni(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Topochemical Oxidation of Ruddlesden–Popper Nickelates Reveals Distinct Structural Family: Oxygen-Intercalated Layered Perovskites

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, in this study, we report a new family of layered perovskites realized through topochemical oxidation of La n+1 Ni n O 3n+1+δ (n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c-axis expansion─17.8% for La 2 NiO 4+δ (n = 1)─that monotonically decreases with increasing n. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

Ferenc Segedin, Dan [Harvard Univ., Cambridge, MA ↗
DOE OSTI2020

Intertwined density waves in a metallic nickelate

Nickelates are a rich class of materials, ranging from insulating magnets to superconductors. But for stoichiometric materials, insulating behavior is the norm, as for most late transition metal oxides. Notable exceptions are the 3D perovskite LaNiO 3 , an unconventional paramagnetic metal, and the layered Ruddlesden-Popper phases R 4 Ni 3 O 10 , (R = La, Pr, Nd). The latter are particularly intriguing because they exhibit an unusual metal-to-metal transition. Here, we demonstrate that this transition results from an incommensurate density wave with both charge and magnetic character that lies closer in its behavior to the metallic density wave seen in chromium metal than the insulating stripes typically found in single-layer nickelates like La 2-x Sr x NiO 4 . We identify these intertwined density waves as being Fermi surface-driven, revealing a novel ordering mechanism in this nickelate that reflects a coupling among charge, spin, and lattice degrees of freedom that differs not only from the single-layer materials, but from the 3D perovskites as well.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Magnetic excitations in strained infinite-layer nickelate PrNiO 2 films

Strongly correlated materials respond sensitively to external perturbations such as strain, pressure, and doping. In the recently discovered superconducting infinite-layer nickelates, the superconducting transition temperature can be enhanced via only ~ 1% compressive strain-tuning with the root of such enhancement still being elusive. Using resonant inelastic x-ray scattering (RIXS), we investigate the magnetic excitations in infinite-layer PrNiO 2 thin films grown on two different substrates, namely SrTiO 3 (STO) and (LaAlO 3 ) 0.3 (Sr 2 TaAlO 6 ) 0.7 (LSAT) enforcing different strain on the nickelates films. The magnon bandwidth of PrNiO 2 shows only marginal response to strain-tuning, in sharp contrast to the enhancement of the superconducting transition temperature T c in the doped superconducting samples. These results suggest the bandwidth of spin excitations of the parent compounds is similar under strain while T c in the doped ones is not, and thus provide important empirics for the understanding of superconductivity in infinite-layer nickelates.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Absence of 3$a_0$ charge density wave order in the infinite-layer nickelate NdNiO 2

A hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a0 charge density wave order in infinite-layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both cuprates and nickelates. Here we present X-ray scattering and spectroscopy measurements on a series of NdNiO 2+x samples, which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO 2 . The 3a 0 superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with three-unit-cell periodicity. The absence of any observable charge density wave order in NdNiO 2 highlights a crucial difference between the phase diagrams of cuprate and nickelate superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Intrinsic magnetism in superconducting infinite-layer nickelates

The discovery of superconductivity in Nd 0.8 Sr 0.2 NiO 2 introduced a new family of layered nickelate superconductors that has now been extended to include a range of Sr-doping, Pr or La in place of Nd, and the 5-layer Nd 6 Ni 5 O 12 . A number of studies indicate that electron correlations are strong in these materials, and hence a central question is whether or not magnetism is present as a consequence of these interactions. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. In all cases we observe an intrinsic magnetic ground state, regardless of the rare earth ion or doping, arising from local moments on the nickel sublattice. Further, the coexistence of magnetism - which is likely to be antiferromagnetic and short-range ordered - with superconductivity is reminiscent of some iron pnictides and heavy fermion compounds, and qualitatively distinct from the doped cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Antiferromagnetic metal phase in an electron-doped rare-earth nickelate

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in non-collinear or non-centrosymmetric spin structures. The rare-earth nickelate NdNiO 3 is known to be a non-collinear antiferromagnet in which the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. In this work, we find that for low electron doping, the magnetic order on the nickel site is preserved, whereas electronically, a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of rare-earth nickelates and may enable spintronics applications in this family of correlated oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Excited-state structural dynamics of nickel complexes probed by optical and X-ray transient absorption spectroscopies: insights and implications

Excited states of nickel complexes undergo a variety of photochemical processes, such as charge transfer, ligation/deligation, and redox reactions, relevant to solar energy conversion and photocatalysis. The efficiencies of the aforementioned processes are closely coupled to the molecular structures in the ground and excited states. The conventional optical transient absorption spectroscopy has revealed important excited-state pathways and kinetics, but information regarding the metal center, in particular transient structural and electronic properties, remains limited. These deficiencies are addressed by X-ray transient absorption (XTA) spectroscopy, a detailed probe of 3d orbital occupancy, oxidation state and coordination geometry. The examples of excited-state structural dynamics of nickel porphyrin and nickel phthalocyanine have been described from our previous studies with highlights on the unique structural information obtained by XTA spectroscopy. Furthermore, we close by surveying prospective applications of XTA spectroscopy to active areas of Ni-based photocatalysis based on the knowledge gained from our previous studies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗