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Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2
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Ground state in the novel dimer iridate Ba 13 Ir 6 O 30 with Ir 6+ (5$d^3$) ions
We have synthesized and studied an iridate, Ba 13 Ir 6 O 30 , with unusual Ir oxidation states: 2/3 Ir 6+ (5$d^3$) ions and 1/3 Ir 5+ (5$d^4$) ions. Its crystal structure features dimers of face-sharing IrO 6 octahedra, and IrO 6 monomers that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless, Ba 13 Ir 6 O 30 exhibits two transitions at $T_{\text{N}1}$ = 4.7K and $T_{\text{N}2}$ = 1.6 K. This magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K 2 below $T_{\text{N}2}$, signaling a coexisting frustrated/disordered state persisting down to at least 0.05 K. This iridate hosts unusually large $J_{\text{eff}}$ = 3/2 degrees of freedom, which is enabled by strong spin-orbit interactions (SOI) in the monomers with Ir 6+ ions and a joint effect of molecular orbitals and SOI in the dimers occupied by Ir 5+ and Ir 6+ ions. Features displayed by the magnetization and heat capacity suggest that the combination of covalency, SOI, and large effective spins leads to highly frustrated ferrimagnetic ordering, an interesting feature of this high-spin iridate.
Electronic properties of corundum-like Ir 2 O 3 and Ir 2 O 3 -Ga 2 O 3 alloys
In the hexagonal, corundum-like structure, α-Ga 2 O 3 has a bandgap of ~5.1 eV, which, combined with its relatively small electron effective mass, high Baliga's figure of merit, and high breakdown field, makes it a promising candidate for power electronics. Ga 2 O 3 is easy to dope n-type, but impossible to dope p-type, impeding the realization of some electronic device designs. Developing a lattice-matched p-type material that forms a high-quality heterojunction with n-type Ga 2 O 3 would open new opportunities in electronics and perhaps optoelectronic devices. In this work, we studied Ir 2 O 3 as a candidate for that purpose. Using hybrid density functional theory calculations we predict the electronic band structure of α-Ir 2 O 3 and compare that to α-Ga 2 O 3 , and study the stability and electronic properties of α-(Ir x Ga 1–x ) 2 O 3 alloys. We discuss the band offset between the two materials and compare it with recently available experimental data. We find that the Ir d bands that compose the top of the valence band in α-Ir 2 O 3 are much higher in energy than O p bands in α-Ga 2 O 3 , possibly enabling effective p-type doping. Finally, our results provide an insight into using the Ir 2 O 3 or Ir 2 O 3 -Ga 2 O 3 alloys as p-type material lattice-matched to α-Ga 2 O 3 for the realization of p–n heterojunctions.
Resolution Limits of Near IR and IR Thermography Applied to Imaging Thermal Heterogeneities in Nuclear Fuel Surrogate Materials
ABSTRACT The main goal of the project is: To develop an IR thermographic method to monitor fuel pellet cracking inside the hostile environment of the nuclear core. This method will use a high temperature and radiation resistant fiber optic bundle to monitor the fuel pellet inside the core and transmit the information outside to an IR camera. This method will give a visual cue (using IR video) as to when the crack started to form and how it progresses. To achieve this goal, we identified the following as major milestones of the project: 1. Perform 1D modeling of temperature and blackbody radiation intensity across a crack. 2. Image a crack on a fuel cell surrogate using an IR camera with free-space optics and high power laser setup. 3. Simulate cracking in the surrogate sample through inductive heating method, and demonstrate imaging crack formation using the free-space optics setup. 4. Determine parameters for a suitable fiber optic bundle compatible with the IR camera based on the results from the free-space optics experiments. 5. Demonstrate imaging using fiber bundle and integrate viewing system with nuclear core at INL. This report focuses on Goals 1-3. We have successfully achieved Goals 1-3 and are making steady progress towards achieving Goals 4-9. Goals 2 and 4 are very similar and require 2 basic parts: the imaging system and the method to crack the fuel surrogate to replicate the fuel pellet cracking in the nuclear reactor core. The following sections describe the 1D modeling of temperature and blackbody radiation intensity across a crack (Goal 1), procedure to create crack on fuel surrogates and results (Goals 2 and 3), and the IR imaging system and results (Goal 2).
AFM-IR and s-SNOM-IR measurements of chemically addressable monolayers on Au nanoparticles
The performance of catalysts depends on their nanoscale properties, and local variations in structure and composition can have a dramatic impact on the catalytic reactivity. Therefore, probing the localized reactivity of catalytic surfaces using high spatial resolution vibrational spectroscopy, such as infrared (IR) nanospectroscopy and tip-enhanced Raman spectroscopy, is essential for mapping their reactivity pattern. Two fundamentally different scanning probe IR nanospectroscopy techniques, namely, scattering-type scanning near-field optical microscopy (sSNOM) and atomic force microscopy-infrared spectroscopy (AFM-IR), provide the capabilities for mapping the reactivity pattern of catalytic surfaces with a spatial resolution of ~20 nm. Herein, we compare these two techniques with regard to their applicability for probing the vibrational signature of reactive molecules on catalytic nanoparticles. For this purpose, we use chemically addressable self-assembled molecules on Au nanoparticles as model systems. We identified significant spectral differences depending on the measurement technique, which originate from the fundamentally different working principles of the applied methods. While AFM-IR spectra provided information from all the molecules that were positioned underneath the tip, the s-SNOM spectra were more orientation-sensitive. Due to its field-enhancement factor, the s-SNOM spectra showed higher vibrational signals for dipoles that were perpendicularly oriented to the surface. The s-SNOM sensitivity to the molecular orientation influenced the amplitude, position, and signal-to-noise ratio of the collected spectra. Ensemble-based IR measurements verified that differences in the localized IR spectra stem from the enhanced sensitivity of s-SNOM measurements to the adsorption geometry of the probed molecules.
Spontaneous Hall effect enhanced by local Ir moments in epitaxial Pr 2 Ir 2 O 7 thin films
Rare-earth pyrochlore iridates ( RE 2 Ir 2 O 7 ) consist of two interpenetrating cation sublattices, the RE with highly frustrated magnetic moments, and the iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. While bulk Pr 2 Ir 2 O 7 shows a spontaneous hysteretic Hall effect below 1.5 K, we observe the effect at elevated temperatures up to 15 K in epitaxial thin films on (111) yttria-stabilized zirconia (YSZ) substrates synthesized via solid-phase epitaxy. Similar to the bulk, the lack of observable long-range magnetic order in the thin films points to a topological origin. We use synchrotron-based element-specific x-ray diffraction and x-ray magnetic circular dichroism to compare powders and thin films to attribute the spontaneous Hall effect in the films to localization of the Ir moments. Further, we link the thin-film Ir local moments to lattice distortions absent in the bulklike powders. We conclude that the elevated-temperature spontaneous Hall effect is caused by the topological effect originating either from the Ir or Pr sublattice, with interaction strength enhanced by the Ir local moments. This spontaneous Hall effect with weak net moment highlights the effect of vanishingly small lattice distortions as a means to discover topological phenomena in metallic frustrated magnetic materials.
Determining the 193 m Ir/ 192 Ir ratio from high-resolution X-ray measurements
Two pellets of iridium salt were irradiated at the center of the Flattop critical assembly, one at low neutron flux and the other at high flux. After that two counting samples were prepared from each pellet following a given chemical procedure. Measurements of the four counting samples were carried out with two high-resolution silicon drift detectors for over a year. Pulse height spectra of the L→M X-rays from 193 m Ir and 192 Ir were approximated with a Voigt function and the ratio of produced 193 m Ir and 192 Ir nuclei obtained. The 193 m Ir/ 192 Ir ratios from each measured spectrum are compared to identify possible systematic deviations.
Dual‐IR Window/Electrode Operando Attenuated Total Reflection‐IR Absorption Spectroscopy for Battery Research
Abstract We report a novel dual‐IR window/electrode operando infrared (IR) spectroscopy method for battery research. In addition to keeping all the features of the in‐situ/operando IR spectroscopic methodologies for batteries reported in the literature so far, the dual‐IR window/electrode configuration reported herein offers a unique advantage of accessing both, anode and cathode processes quasi‐simultaneously by rotating the dual‐window/electrode IR battery cell 180° step‐wise in the same operando real‐battery‐like cell. The proof‐of‐concept studies demonstrated clearly its investigative advantages in gaining broader insights into the chemistry of batteries during charging/discharging and beyond.
Imidazole and 1-Methylimidazole Hydrogen Bonding and Nonhydrogen Bonding Liquid Dynamics: Ultrafast IR Experiments
Dynamics of imidazole (IM) and 1-methylimidazole (1-MeIM) in the liquid phase at 95 °C were studied by IR polarization selective pump-probe (PSPP) and two-dimensional IR (2D IR) spectroscopies. The two molecules are very similar structurally except that IM can be simultaneously a hydrogen bond donor and acceptor and therefore forms extended hydrogen bonded networks. The broader IR absorption spectrum and shorter vibrational lifetime of the vibrational probe, selenocyanate anion (SeCN – ), in IM vs. 1-MeIM indicate that stronger hydrogen bonding exists between SeCN – and IM. Molecular dynamics (MD) simulations support the strong hydrogen bond formation between SeCN – and IM via the HN moiety. SeCN – makes two H-bonds with IM; it is inserted in the IM H-bonded chains rather than being a chain terminator. The strong hydrogen bonding influenced the reorientation dynamics of SeCN – in IM, leading to more restricted short time angular sampling (wobbling-in-a-cone). The complete orientational diffusion time in IM is 1.7 times slower than in 1-MeIM, but the slowdown is less than expected, considering the 3-fold larger viscosity of IM. The jump reorientation mechanism accounts for the anomalously fast orientational relaxation in IM, and the MD simulations determined the average jump angle of the probe between hydrogen bonding sites. Spectral diffusion time constants obtained from the 2D IR experiments are only modestly slower in IM than in 1-MeIM in spite of the significant increase in viscosity. Here, the results indicate that the spectral diffusion sensed by the SeCN – has IM hydrogen bond dynamics contributions not present in 1-MeIM.
Mathematical Modeling of the Potential and Time Dependence of Ir Dissolution from Hydrous Ir Oxide Oxygen Evolution Catalysts
One of the main degradation mechanisms of hydrous iridium oxide acidic oxygen evolution reaction (OER) catalysts is dissolution and loss into the acidic membrane. While degradation models have been proposed, there is a gap in understanding the potential and time dependence of the iridium dissolution reaction and its mechanistic underpinnings. In this work, Ir dissolution rates measured as a function of time and potential via time-resolved inductively-coupled plasma mass spectrometry (ICP-MS) in aqueous acidic electrolyte are used to establish a mathematical model for Ir dissolution. The mathematical model is generated using proposed formation and dissolution reactions for Ir species. Through comparison with the ICP-MS data and existing information on the potential-dependent Ir phase, we find that the potential and time-dependence of dissolution can be modeled as dissolution of an oxide phase, here represented as IrO 2 , with potential dependent kinetics and formation of a passivating species, a process with a rate-limiting step that is not potential dependent. This understanding of the potential dependence of dissolution and passivation kinetics using aqueous electrolyte half-cell measurements can be used to predict the degradation of Ir oxide in operating energy conversion devices relying on the OER, such as proton-exchange membrane water electrolyzers.
Optical parametric oscillator: status of tunable radiation in mid-IR to IR spectral range based on ZnGeP{sub 2} crystal pumped by solid state lasers
In this review fundamentals of optical parametric oscillator and different phase matching situation have been discussed. Requirement of nonlinear crystal and properties of some mid-IR and IR transmitting crystal have been discussed. Present status of generation of tunable radiation in mid-IR to IR spectral range by OPO based on ZnGeP{sub 2} crystal and other techniques and its applications has been discussed.
High-Resolution X-ray Photoelectron Spectroscopy of an IrO2(110) Film on Ir(100)
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Nonlinear post-compression in multi-pass cells in the mid-IR region using bulk materials
We numerically investigate the regime of nonlinear pulse compression at mid-IR wavelengths in a multi-pass cell (MPC) containing a dielectric plate. This post-compression setup allows for ionization-free spectral broadening and self-compression while mitigating self-focusing effects. We find that self-compression occurs for a wide range of MPC and pulse parameters and derive scaling rules that enable its optimization. We also reveal the solitonic dynamics of the pulse propagation in the MPC and its limitations and show that spatiotemporal/spectral couplings can be mitigated for appropriately chosen parameters. In addition, we reveal the formation of spectral features akin to quasi-phase matched degenerate four-wave mixing. Finally, we present two case studies of self-compression at 3-μm and 6-μm wavelengths using pulse parameters compatible with driving high-field physics experiments. The simulations presented in this paper set a framework for future experimental work using few-cycle pulses at mid-IR wavelengths.
K 3 Ir 2 O 6 and K 16.3 Ir 8 O 30 , Low-Dimensional Iridates with Infinite IrO 6 Chains
A previously unreported 1D iridate, K 3 Ir 2 O 6 , has been grown by a flux method in O 2 -rich environment and its crystal structure determined via single crystal structural analysis. It exhibits straight chains of face-sharing [IrO 6 ] octahedra, which are arranged along the crystallographic c axis, separated by nonmagnetic K ions. No magnetic transitions are observed during measured range, and the material is electrically insulating. Potentially interesting electronic behavior for K 3 Ir 2 O 6 is supported by electronic structure calculations. A structurally related material, K 16.3 Ir 8 O 30 , which displays similar fundamental geometric units but in a different spatial arrangement – zigzag chains based on edge and face sharing [IrO 6 ] octahedra, is also reported. Both materials are of interest for probing the properties of a 1D system with strong spin-orbit coupling
Short-range ferromagnetic order due to Ir substitutions in single-crystalline Ba(Co 1-x Ir x ) 2 As 2 (0 ≤ x ≤ 0.25)
The ternary-arsenide compound BaCo 2 As 2 was previously proposed to be in proximity to a quantum-critical point where long-range ferromagnetic (FM) order is suppressed by quantum fluctuations. In this paper we report the effect of Ir substitutions for Co on the magnetic and thermal properties of Ba(Co 1-x Ir x ) 2 As 2 (0 ≤ x ≤ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr 2 Si 2 structure with space group I4/mmm. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x ≥ 0.11 below a nearly composition-independent characteristic temperature T cl ≈ 13 K. The small variation of T cl with x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versus T, the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations <0.01 μ B /f.u. together indicate that the FM response arises from short-range FM ordering of FM spin clusters as previously inferred to occur in Ca(Co 1–x Ir x ) 2–y As 2 . Heat-capacity Cp(T) data do not exhibit any clear feature around Tcl, consistent with the very small moments of the FM clusters. The Cp(T) in the paramagnetic temperature regime 25–300 K is well described by the sum of a Sommerfeld electronic contribution and Debye and Einstein lattice contributions where the latter lattice contribution suggests the presence of low-frequency optic modes associated with the heavy Ba atoms in the crystals.
Correlation between antiferromagnetic and Mott states in spin orbit coupled Sr 2 Ir O 4 : A study of Sr 2 Ir 1 – x M x O 4 ( M = Fe or Co)
Sr 2 IrO 4 is an archetypal spin-orbit-coupled Mott insulator with an antiferromagnetic state below 240 K. Here we report results of our study on single crystals of Sr 2 Ir 1–x Fe x O 4 (0 ≤ x < 0.32) and Sr 2 Ir 1–x Co x O 4 (0 ≤ x < 0.22). Fe doping retains the antiferromagnetic state but simultaneously precipitates an emergent metallic state whereas Co doping causes a rapid collapse of both the antiferromagnetic and Mott states, giving rise to a confined metallic state featuring a pronounced linearity of the basal-plane resistivity up to 700 K. The results indicate tetravalent Fe 4+ (3d 4 ) ions in the intermediate spin state with S = 1 and Co 4+ (3d 5 ) ions in the high spin state with S = 5/2 substituting for Ir 4+ (5d 5 ) ions in Sr 2 IrO 4 , respectively. The effective magnetic moment closely tracks the Néel temperature as doping increases, suggesting that the spin state of the dopant predominately determines the magnetic properties in doped Sr 2 IrO 4 . Furthermore, all relevant properties including charge-carrier density (e.g., 10 28 /m 3 ), Sommerfeld coefficient (e.g., 19 mJ / mole K 2 ), and Wilson ratio (e.g., 2.6) consistently demonstrate an emergent metallic state that is both robust and highly correlated in the two systems, arising from the percolation of bound states and the weakening of structural distortions. Furthermore, this study strongly indicates that the antiferromagnetic and Mott states are correlated in an unconventional manner in Sr 2 IrO 4 .
Phosphor for the Near-IR and Short-Wave IR Ranges Based on a Garnet Structured Cubic Modification of Lithium–Lanthanum Niobate
Cubic garnets activated by neodymium and holmium, Li{sub 6}CaLa{sub 2}Nb{sub 2}O{sub 12}:Nd{sup 3+},Ho{sup 3+}, were obtained by solid-state method. The main regularities of changes in the luminescence properties of Li{sub 6}C-aLa{sub 2}Nb{sub 2}O{sub 12}:Nd{sup 3+},Ho{sup 3+} solid solutions in the visible, near-IR, and short-wave IR ranges under 808 nm laser diode radiation are determined. The energy transfer mechanism between active centers, involving the participation of Nd{sup 3+} ions as sensitizers for the luminescence of Ho{sup 3+} ions, is proposed. Low phonon energy, high luminescence intensity in the range of 2.0–3.0 μm, and weak up-conversion luminescence in the region of 450–780 nm make it possible to consider the cubic lithium–lanthanum niobates Li{sub 6}CaLa{sub 2}Nb{sub 2}O{sub 12}:Nd{sup 3+},Ho{sup 3+} as promising phosphors of the short-wave IR range.