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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

First-principles calculation of spin and orbital contributions to magnetically ordered moments in Sr 2 IrO 4

Here, we show how an accurate first-principles treatment of the canted-anti-ferromagnetic ground state of Sr 2 IrO 4 , a prototypical 5d correlated spin-orbit coupled material, can be obtained without invoking any free parameters, such as the Hubbard $\textit{U}$ or tuning the spin-orbit coupling strength. Our theoretically predicted iridium magnetic moment of 0.250$μ_B$, canted by 12.6° off the $\textit{a}$ axis, is in accord with experimental results. By resolving the magnetic moments into their spin and orbital components, we show that our theoretically obtained variation of the magnetic scattering amplitude $\langle M_m \rangle$ as a function of the polarization angle is consistent with recent nonresonant magnetic x-ray scattering measurements. The computed value of the band gap (55 meV) is also in line with the corresponding experimental values. A comparison of the band structure to that of the cuprates suggests the presence of incommensurate charge-density wave phases in Sr 2 IrO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Structural and magnetic transitions in the planar antiferromagnet Ba 4 Ir 3 O 10

We report the structural and magnetic ground state properties of the monoclinic compound barium iridium oxide Ba 4 Ir 3 O 10 using a combination of resonant x-ray scattering, magnetometry, and thermodynamic techniques. Magnetic susceptibility exhibits a pronounced antiferromagnetic transition at T N ≈ 25 K, a weaker anomaly at T S ≈ 142 K, and strong magnetic anisotropy at all temperatures. Resonant elastic x-ray scattering experiments reveal a second order structural phase transition at T S and a magnetic transition at T N . Both structural and magnetic superlattice peaks are observed at L = half integer values. The magnetization anomaly at T S implies the presence of magnetoelastic coupling, which conceivably facilitates the symmetry lowering. Mean field critical scattering is observed above T S . The magnetic structure of the antiferromagnetic ground state is discussed based on the measured magnetic superlattice peak intensity. Furthermore, our study not only presents essential information for understanding the intertwined structural and magnetic properties in Ba 4 Ir 3 O 10 but also highlights the necessary ingredients for exploring novel ground states with octahedra trimers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Charge ordering in Ir dimers in the ground state of Ba 5 AlIr 2 O 11

It has been well established experimentally that the interplay of electronic correlations and spin-orbit interactions in Ir 4+ and Ir 5+ oxides results in insulating J eff = 1/2 and J eff = 0 ground states, respectively. However, in compounds where the structural dimerization of iridium ions is favorable, the direct Ir d-d hybridization can be significant and takes a key role. Here, in this study, we investigate the effects of direct Ir d-d hybridization in comparison with electronic correlations and spin-orbit coupling in Ba 5 AlIr 2 O 11 , a compound with Ir dimers. Using a combination of ab initio many-body wave-function quantum chemistry calculations and resonant inelastic x-ray scattering experiments, we elucidate the electronic structure of Ba 5 AlIr 2 O 11 . We find excellent agreement between the calculated and the measured spin-orbit excitations. Contrary to expectations, the analysis of the many-body wave function shows that the two Ir (Ir 4+ and Ir 5+ ) ions in the Ir 2 O 9 dimer unit in this compound preserve their local J eff character close to 1/2 and 0, respectively. The local point group symmetry at each of the Ir ions plays an important role, significantly limiting the direct d-d hybridization. Our results emphasize that minute details in the local crystal field environment can lead to dramatic differences in the electronic states in iridates and 5d oxides in general.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Ferroelectric control of magnetic anisotropy of 5 d transition metal monolayers

Electric field control of magnetism using ferroelectric materials offers promising applications in low-power spintronics. We have conducted systematic density functional theory calculations to investigate the magnetic properties of epitaxial 5 d transition metal monolayers on a ferroelectric substrate: PbTi⁢O 3 (PTO). Our study reveals that the magnetocrystalline anisotropy energy of the osmium monolayer is significantly enhanced to 18.1 meV/Os in the upward polarization state and, moreover, the anomalous Hall coefficient of osmium/PTO film is tunable by reversing the electric polarization of the PTO substrate. Additionally, the electric polarization reversal in PTO rotates the easy magnetization axis of the iridium and platinum monolayers by 90°, which is attributed to the rearrangement of interfacial charges. In conclusion, our findings suggest an efficient approach to control the magnetization direction of monoatomic layers and provide valuable insights for the development of low-energy spintronics devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Stabilization of collinear ferromagnetic order in Ir-doped triple-layer ruthenate Sr 4 Ru 3 O 10

The triple-layer Sr 4 Ru 3 O 10 is a spin-orbit-coupled metal featuring an intriguing combination of ferromagnetic and metamagnetic states. Here we report a comprehensive study of single-crystal neutron and x-ray diffraction on Sr 4 Ru 3 O 10 with 4% iridium doping. The system crystallizes in an orthorhombic Bbem structure in which two triple layers are related by a base centering symmetry operation within one unit cell. The 4% Ir doping at the Ru site sensitively alters the structural balance between the RuO 6 layers, as such the two outer-layer octahedra rotate 6.7° about the c axis, while the central-layer one rotates in the opposite direction by 11.0°. The increased lattice distortion stabilizes a collinear ferromagnetic state with magnetic moments fully aligned along the c axis. The emerging perpendicular magnetic anisotropy naturally increases the critical field for the inplane metamagnetic transition to 4.5 Tesla. All results highlight the critical linkage between the magnetic properties and underlying lattice through orbital degrees of freedom, as well as the enhanced spin orbit coupling due to heavier 5d ions. Our observation provides a viable route for an efficient manipulation of the magnetic states with functionalities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Spin frustration and an exotic critical point in ferromagnets from nonuniform opposite g factors

Here, it is demonstrated that novel spin frustration can be induced in ferromagnets with nonuniform opposite Landé g-factors. The frustrated state is characterized by a mutual interplay of typical ferromagnetic (FM) and antiferromagnetic (AF) features, such as the zero-field susceptibility being FM-like at low temperatures but AF-like at high temperatures. It is also found to contain an exotic zero-temperature 'half fire, half ice' critical point at which the spins on one sublattice are fully disordered and on the other one are fully ordered. We suggest that such frustration may occur in a number of copper-iridium oxides such as Sr 3 CuIrO 6 . We also anticipate a realization of the frustration and 'partial fire, partial ice' states in certain antiferromagnets, lattice gas, and neuron systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Single crystal growth of iridates without platinum impurities

Iridates have attracted much interest in the last decade for their novel magnetism emerging in the limit of strong spin-orbit coupling and possible unconventional superconductivity. A standard for growing iridate single crystals has been the flux method using platinum crucibles. Here, in this work, we show that this widely used method compromises the sample quality by inclusion of platinum impurities. We find that Sr 2 ⁢IrO 4 single crystals grown in iridium crucibles show remarkable differences from those grown in platinum crucibles in their sample characterizations using Raman spectroscopy, resistivity, magnetization, optical third harmonic generation, resonant x-ray diffraction, and resonant inelastic x-ray scattering measurements. In particular, we show that several peaks of sizable intensities disappear in the Raman spectra of samples free of platinum impurities, and a significantly larger activation energy is extracted from the resistivity data compared to previously reported values. Furthermore, we find no evidence of the previously reported glide-symmetry-breaking structural distortions and confirm the I⁢ 4 1 / acd space group of the lattice symmetry. Although the platinum impurities are not apparent in the magnetic properties and thus went unnoticed in the stoichiometric insulating phase for a long time, their effects can be much more detrimental to transport properties in chemically doped compounds. Therefore, our result suggests using growth methods that avoid platinum impurities for an investigation of the intrinsic physical properties of iridates, and possible superconducting phases.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Structure transition and zigzag magnetic order in Ir/Rh-substituted honeycomb lattice α - RuCl 3

Here, we report magnetization and neutron diffraction studies on crystal and magnetic structures of Ir- and Rh-substituted honeycomb lattice α-RuCl 3 . The iridium or rhodium atoms are distributed at the Ru site with little structural modification. Both systems undergo a room-temperature monoclinic C2/m to low-temperature trigonal $R\bar{3}$ phase transformation with a large recoverable hysteresis. At low temperature, a zigzag spin order is observed with the same characteristic wave vector (0,0.5,1) as in the parent α-RuCl 3 . Detailed magnetic structure refinement reveals an ordered moment of 0.32(5)µB/Ru and an upper boundary of canting angle of 15(4)º away from the basal plane at 5 K for the 10% Ir-substituted α-RuCl 3 , which is different from the 0.45–0.73 µB/Ru and 32°–48° canting angle reported in the parent compound α-RuCl 3 . The observation of unchanged RuCl 6 local octahedral environment, reduced ordered magnetic moment size and canting angle compared to previously reported highlights the potential to study quantum spin-liquid behavior through nonmagnetic ion doping.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Strain-dependent insulating state and Kondo effect in epitaxial SrIrO 3 films

The large spin-orbit coupling in iridium oxides plays a significant role in driving novel physical behaviors, including emergent phenomena in the films and heterostructures of perovskite and Ruddlesden-Popper iridates. Here, in this Letter, we study the role of epitaxial strain on the electronic behavior of thin SrIrO 3 films. We find that compressive epitaxial strain leads to metallic transport behavior, but a slight tensile strain shows gapped behavior. Temperature-dependent resistivity measurements are used to examine different behaviors in films as a function of strain. We find Kondo contributions to the resistivity, with stronger effects in films that are thinner and under less compressive epitaxial strain. These results show the potential to tune SrIrO 3 into Kondo insulating states and open possibilities for a quantum critical point that can be controlled with strain in epitaxial films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Multielectrode electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering

A multielectrode-based electrochemical cell allows the structural characterization of an amorphous thin-film water oxidation catalyst under various electrochemical potentials using high-energy X-ray scattering and atomic pair distribution function (PDF) techniques. A multielectrode with five electrodes provides a sufficiently low background signal to enable high-energy X-ray scattering (HEXS) measurements and amplifies the extremely low HEXS signals from samples for high-resolution PDF analysis of in situ data from thin-film catalysts. Glassy carbon (GC) creates a relatively low intensity HEXS pattern and is used as a working electrode. Instead of a three-dimensional (3D) porous electrode architecture, the flat geometry of the electrode enables various deposition techniques to be used for the preparation of a highly conductive metal oxide layer. PDF analysis demonstrates high spatial resolution for a 230 nm thick amorphous iridium oxide film deposited on two roughened 60 µm thick GC electrodes. The PDF analysis resolves the domain size and distinguishes changes in fine structure which are directly correlated with the structure and function of the catalysts. In conclusion, the results bring the opportunity to analyze the structure of nanometre-scale amorphous thin-film catalysts in an electrolyte-compatible and compact 3D-printed electrochemical cell in a three-electrode configuration.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electromagnetic Properties of Aluminum-based Bilayers for Kinetic Inductance Detectors

The complex conductivity of a superconducting thin film is related to the quasiparticle density, which depends on the physical temperature and can also be modified by external pair breaking with photons and phonons. This relationship forms the underlying operating principle of Kinetic Inductance Detectors (KIDs), where the detection threshold is governed by the superconducting energy gap. We investigate the electromagnetic properties of thin-film aluminum that is proximitized with either a normal metal layer of copper or a superconducting layer with a lower T-C, such as iridium, in order to extend the operating range of KIDs. Using the Usadel equations along with the Nam expressions for complex conductivity, we calculate the density of states and the complex conductivity of the resulting bilayers to understand the dependence of the pair breaking threshold, surface impedance, and intrinsic quality factor of superconducting bilayers on the relative film thicknesses. The calculations and analyses provide theoretical insights in designing aluminum-based bilayer kinetic inductance detectors for detection of microwave photons and athermal phonons at the frequencies well below the pair breaking threshold of a pure aluminum film.

complex conductivity↗
DOE OSTI2023

La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis

Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing the cost for green hydrogen production. Here we report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Tantalum-stabilized ruthenium oxide electrocatalysts for industrial water electrolysis

The iridium oxide (IrO 2 ) catalyst for the oxygen evolution reaction used industrially (in proton exchange membrane water electrolyzers) is scarce and costly. Although ruthenium oxide (RuO 2 ) is a promising alternative, its poor stability has hindered practical application. Here, we used well-defined extended surface models to identify that RuO 2 undergoes structure-dependent corrosion that causes Ru dissolution. Tantalum (Ta) doping effectively stabilized RuO 2 against such corrosion and enhanced the intrinsic activity of RuO 2 . In an industrial demonstration, Ta-RuO 2 electrocatalyst exhibited stability near that of IrO 2 and had a performance decay rate of ~14 microvolts per hour in a 2800-hour test. At current densities of 1 ampere per square centimeter, it had an overpotential 330 millivolts less than that of IrO 2 .

Zhang, Jiahao [Sichuan Univ., Chengdu (China); Uni↗
DOE OSTI2021

Multiple Reaction Pathways for the Oxygen Evolution Reaction May Contribute to IrO 2 (110)’s High Activity

Density functional theory calculations in conjunction with statistical mechanical arguments are performed on the rutile IrO 2 (110) facet in order to characterize multiple reaction pathways on the surface at the highest active limit (the stoichiometric surface with all metal sites available) and at the lowest active limit (the oxygen-terminated surface). Alternative pathways to the oxygen evolution reaction (OER) are found, with multiple pathways determined at each step of the four proton-coupled electron transfer reaction. Of particular interest is the detailed characterization of a co-adsorption pathway utilizing neighboring, adsorbed O, OH species in order to evolve oxygen; activation energies of this pathway are <0.5 eV and therefore easily surmountable at the high operating potentials of OER. We also determined that surface Ir atoms can potentially participate in deprotonating an OOH* intermediate; the activation energy to this is 0.67 eV on the oxygen-terminated surface. These theoretical findings explain in part the high activity present in iridium oxide catalysts and also provide insight into the mechanistic pathways available on metal oxide catalysts, which may require the concerted interaction of nearest neighbor co-adsorbates to produce chemicals of interest.

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

Electrolyzer Performance Loss from Accelerated Stress Tests and Corresponding Changes to Catalyst Layers and Interfaces

Stress tests are developed for proton exchange membrane electrolyzers that utilize low catalyst loading, elevated potential, and frequent cycling with square- and triangle-waves to accelerate anode catalyst layer degradation during intermittent operation. Kinetics drive performance losses (ohmic/transport secondary) and are accompanied by decreasing exchange current density, decreasing cyclic voltammetric capacitance, and increasing polarization resistance. Decreased kinetics are likely due to a combination of iridium (Ir) migration into electrochemically inaccessible locations in the anode or membrane, Ir particle growth (supported by X-ray scattering), changes in the extent of the Ir oxidation state (supported by X-ray absorption spectroscopy), and anode catalyst layer reordering. Decreasing catalyst/transport layer contact and catalyst/membrane interfacial tearing may add contact resistances and account for increasing ohmic losses. Performance losses for low and moderate catalyst loading, as well as from accelerated and model wind/solar cycling protocols, were likewise dominated by kinetics but vary in severity. Additionally, accelerated cycling (1 cycle per minute) appears to reasonably accelerate relevant loss mechanisms and can be used to project electrolyzer lifetime from anode deterioration. Ongoing accelerated stress test development and studies into performance loss mechanisms will continue to be critical as electrolysis shifts to intermittent power and low-cost applications.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Degradation Effects at the Porous Transport Layer/Catalyst Layer Interface in Polymer Electrolyte Membrane Water Electrolyzer

The porous transport layer (PTL)/catalyst layer (CL) interface plays a crucial role in the achievement of high performance and efficiency in polymer electrolyte membrane water electrolyzers (PEMWEs). This study investigated the effects of the PTL/CL interface on the degradation of membrane electrode assemblies (MEAs) during a 4000 h test, comparing the MEAs assembled with uncoated and Ir-coated Ti PTLs. Our results show that compared to an uncoated PTL/CL interface, an optimized interface formed when using a platinum group metal (PGM) coating, i.e., an iridium layer at the PTL/CL interface, and reduced the degradation of the MEA. The agglomeration and formation of voids and cracks could be found for both MEAs after the long-term test, but the incorporation of an Ir coating on the PTL did not affect the morphology change or oxidation of IrO x in the catalyst layer. In addition, our studies suggest that the ionomer loss and restructuring of the anodic MEA can also be reduced by Ir coating of the PTL/CL interface. Optimization of the PTL/CL interface improves the performance and durability of a PEMWE.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Catalyst Layer Resistance and Utilization in PEM Electrolysis

Improving utilization, performance, and stability of low iridium (Ir)-loaded anodes is a key goal to enable widespread adoption of polymer electrolyte membrane water electrolysis (PEMWE) for clean hydrogen production. A potential limitation is high ionic or electronic resistance of the anode catalyst layer, which leads to poor catalyst utilization, increased voltage losses, and high local overpotentials that can accelerate degradation. While catalyst layer resistance is relatively well-understood in fuel cells and other porous electrode systems, characterization of these effects is not as well established in PEMWE research. Here we present in-situ methods for measuring catalyst layer resistance in electrolysis cells using a non-faradaic H 2 /H 2 O condition as well as methods for calculating the associated voltage losses. These methods are applied to anode catalyst layers based on IrO 2 nanoparticles as well as dispersed nano-structured thin film (NSTF) Ir catalysts. Trends with anode catalyst loading and interactions between the porous transport layer and catalyst layer are investigated for IrO 2 anodes. Post-mortem microscopic analysis of durability-tested anodes is also presented, showing uneven degradation of the catalyst layer caused by catalyst layer resistance.

08 HYDROGEN↗
DOE OSTI2025

Durability Optimization of CO 2 Electrolyzers for Syngas Evolution

Recently, there has been an increased interest in mitigating anthropogenic CO 2 emissions through the electrochemical conversion of CO 2 into fuels and fuel feedstocks, including hydrogen gas (H 2 ), carbon monoxide (CO), and mixtures of the two to yield syngas. Commercial applications of these systems require high catalytic selectivity for the desired products, while exhibiting operational lifetimes exceeding thousands of hours. Advancements in this field have produced systems that display high selectivity of the desired products at faradaic efficiencies exceeding 95%. Despite the advancements made in CO 2 electrolysis, system durability remains a standing challenge in the field. CO 2 electrolyzer lifetimes are often limited by carbonate fouling, catalyst degradation, detrimental flooding of electrode microporous layers and anion exchange membrane (AEM) failures. In this report, a 5 cm 2 membrane electrode assembly (MEA) device is used to investigate potential failure modes and to optimize AEM CO 2 electrolyzer operation. Key findings of this study include the importance of CO 2 flow rate, use of a thin PiperION PTFE-reinforced membrane, optimizing compression to enhance contact under 40 in-lb compression, and the effect of more compressible, commercial iridium oxide anodes on system durability.

Abouremeleh, Mohammed H. [Lawrence Berkeley Nation↗