M5 Edge Research Articles

On the origin of low-valent uranium oxidation state

The significant interest in actinide bonding has recently focused on novel compounds with exotic oxidation states. However, the difficulty in obtaining relevant high-quality experimental data, particularly for low-valent actinide compounds, prevents a deeper understanding of 5f systems. Here we show X-ray absorption near-edge structure (XANES) measurements in the high-energy resolution fluorescence detection (HERFD) mode at the uranium M4 edge for the UIII and UIV halides, namely UX3 and UX4 (X = F, Cl, Br, I). The spectral shapes of these two series exhibit clear differences, which we explain using electronic structure calculations of the 3d-4f resonant inelastic X-ray scattering (RIXS) process. To understand the changes observed, we implemented crystal field models with ab initio derived parameters and investigated the effect of reducing different contributions to the electron-electron interactions involved in the RIXS process. Our analysis shows that the electron-electron interactions weaken as the ligand changes from I to F, indicative of a decrease in ionicity both along and between the UX3 and UX4 halide series.

A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

ThTi2O6 derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U5+ or U6+, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd3+, Ca2+, Al3+, or Cr3+, on the Th or Ti site. U L3 edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M4 edge XANES evidenced U5+ as the major oxidation state in all compounds, with a minor fraction of U6+ (2–13%). The balance of X-ray and Raman spectroscopy data support uranate, rather than uranyl, as the dominant U6+ speciation in the reported brannerites. It is considered that the U6+ concentration was limited by unfavourable electrostatic repulsion arising from substitution in the octahedral Th or Ti sites, which share two or three edges, respectively, with neighbouring polyhedra in the brannerite structure.

Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO3 and AmVO4.

In search for chemically stable americium compounds with high power densities for radioisotope sources for space applications, AmVO3 and AmVO4 were prepared by a solid-state reaction. We present here their crystal structure at room temperature solved by powder X-ray diffraction combined with Rietveld refinement. Their thermal and self-irradiation stabilities have been studied. The oxidation states of americium were confirmed by the Am M5 edge high-resolution X-ray absorption near-edge structure (HR-XANES) technique. Such ceramics are investigated as potential power sources for space applications like radioisotope thermoelectric generators, and they have to endure extreme conditions including vacuum, high or low temperatures, and internal irradiation. Thus, their stability under self-irradiation and heat treatment in inert and oxidizing atmospheres was tested and discussed relative to other compounds with a high content of americium.

X-ray spectroscopic study of chemical state in uranium carbides.

UC and UMeC2 (Me = Fe, Zr, Mo) carbides were studied by the high-energy-resolution fluorescence-detected X-ray absorption (HERFD-XAS) technique at the U M4 and L3 edges. Both U M4 and L3 HERFD-XAS reveal some differences between UMeC2 and UC; there are differences also between the M4 and L3 edge results for both types of carbide in terms of the spectral width and energy position. The observed differences are attributed to the consequences of the U 5f, 6d-4d(3d) hybridization in UMeC2. Calculations of the U M4 HERFD-XAS spectra were also performed using the Anderson impurity model (AIM). Based on the analysis of the data, the 5f occupancy in the ground state of UC was estimated to be 3.05 electrons. This finding is also supported by the analysis of U N4,5 XAS of UC and by the results of the AIM calculations of the U 4f X-ray photoelectron spectrum of UC.

Synthesis and characterization of homogeneous (U,Am)O2 and (U,Pu,Am)O2 nanopowders.

This paper details the first dedicated production of homogeneous nanocrystalline particles of mixed actinide oxide solid solutions containing americium. The target compositions were U0.75Pu0.20Am0.05O2, U0.90Am0.10O2 and U0.80Am0.20O2. After successful hydrothermal synthesis and chemical characterisation, the nanocrystals were sintered and their structure and behaviour under self-irradiation were studied by powder XRD. Cationic charge distribution of the as-prepared nanocrystalline and sintered U0.80Am0.20O2 materials was investigated applying U M4 and Am M5 edge high energy resolution XANES (HR-XANES). Typical oxidation states detected for the cations are U(iv)/U(v) and Am(iii)/Am(iv). The measured crystallographic swelling was systematically smaller for the as-synthesised nanoparticles than the sintered products. For sintered pellets, the maximal volumetric swelling was about 0.8% at saturation, in line with literature data for PuO2, AmO2, (U,Pu)O2 or (U,Am)O2.

To form or not to form: PuO2 nanoparticles at acidic pH.

The aim of this study is to synthesize PuO2 nanoparticles (NPs) at low pH values and characterize the materials using laboratory and synchrotron-based methods. Properties of the PuO2 NPs formed under acidic conditions (pH 1–4) are explored here at the atomic scale. High-resolution transmission electron microscopy (HRTEM) is applied to characterize the crystallinity, morphology and size of the particles. It is found that 2 nm crystalline NPs are formed with a PuO2 crystal structure. High energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy at the Pu M4 edge has been used to identify the Pu oxidation states and recorded data are analysed using the theory based on the Anderson impurity model (AIM). The experimental data obtained on NPs show that the Pu(iv) oxidation state dominates in all NPs formed at pH 1–4. However, the suspension at pH 1 demonstrates the presence of Pu(iii) and Pu(vi) in addition to the Pu(iv), which is associated with redox dissolution of PuO2 NPs under acidic conditions. We discuss in detail the mechanism that affects the PuO2 NPs synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Hence, the results shown here, together with the first Pu M4 HERFD data on PuF3 and PuF4 compounds, are significant for the colloid facilitated transport governing the migration of plutonium in a subsurface environment.

Advanced x-ray spectroscopy of actinide trichlorides.

The application of core-to-core (3d-to-4f) resonant inelastic x-ray scattering (RIXS) and high-energy-resolution fluorescence-detected x-ray absorption (HERFD-XAS) at actinide M4,5 edges, as techniques with the enhanced sensitivity to changes in the chemical state, was analyzed for trivalent actinide compounds. As an example, a series of actinide chlorides AnCl3 (An = U, Np, Pu, Am, Cm, Bk, and Cf) was used. The crystal-field multiplet formalism was applied to calculate the 3d-4f RIXS maps, and the HERFD-XAS spectra were extracted as cuts of these RIXS maps along the incident energy axis at the constant emitted energy, corresponding to the maximum of the RIXS intensity. A relation between HERFD and conventional XAS methods was also examined. Despite some differences between profiles of the An M5 HERFD and conventional XAS spectra of trivalent actinides, the results of calculations indicate that the HERFD method can be used at the An M5 edge for monitoring even small variations in the An chemical state. As a whole, better agreement between the HERFD and XAS spectra was found for the An M4 edges as compared to the An M5 edges. By using the pointcharge electrostatic model, the dependence of the An M4,5 HERFD-XAS spectra on the An coordination number was studied, which indicates the significant sensitivity of the distribution of the An 5f states to the ligand structural arrangement around the An sites.

Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding

We report here a detailed experimental and theoretical investigation of hexavalent uranium in various local configurations with a high-energy-resolution fluorescence-detected X-ray absorption near-edge structure at the U M4 edge. We show the pronounced sensitivity of the technique to the arrangement of atoms around the absorber and provide a detailed theoretical interpretation revealing the nature of spectral features. Calculations based on density functional theory and on crystal field multiplet theory indicate that for all local configurations analyzed, the main peak corresponds to nonbonding 5f orbitals, and the highest energy peak corresponds to antibonding 5f orbitals. Our findings agree with the accepted interpretation of uranyl spectral features and embed the latter in a broader field of view, which interprets the spectra of a large variety of U6+-containing samples on a common theoretical ground.

Computational and Spectroscopic Tools for the Detection of Bond Covalency in Pu(IV) Materials.

Plutonium is used as a major component of new-generation nuclear fuels and of radioisotope batteries for Mars rovers, but it is also an environmental pollutant. Plutonium clearly has high technological and environmental importance, but it has an extremely complex, not well-understood electronic structure. The level of covalency of the Pu 5f valence orbitals and their role in chemical bonding are still an enigma and thus at the frontier of research in actinide science. We performed fully relativistic quantum chemical computations of the electronic structure of the Pu4+ ion and the PuO2 compound. Using four different theoretical tools, it is shown that the 5f orbitals have very little covalent character although the 5f(7/2) a2u orbital with the highest orbital energy has the greatest extent of covalency in PuO2. It is illustrated that the Pu M4,5 edge high-energy resolution X-ray absorption near-edge structure (Pu M4,5 HR-XANES) spectra cannot be interpreted in terms of dipole selection rules applied between individual 3d and 5f orbitals, but the selection rules must be applied between the total wavefunctions for the initial and excited states. This is because the states cannot be represented by single determinants. They are shown to involve major redistributions on the 5f electrons over the different 5f orbitals. These redistributions could be viewed as shake-up-like excitations in the 5f shell from the lowest orbital energy from J = 5f(5/2) into higher orbital energy J = 5f(7/2). We show that the second peak in the Pu M4 edge and the high-energy shoulder of the Pu M5 edge HR-XANES spectra probe the 5f(7/2) a2u orbital; thus, these spectral features are expected to change upon bond variations. We describe theoretical and spectroscopy tools, which can be applied for all actinide elements in materials with cubic structure.

The M4,5-NN Auger and M2,3-M4,5N Coster-Kronig spectra of krypton induced by electron impact

The Kr M4,5-NN Auger transitions are studied in the kinetic energy region 24 - 59 eV by using non-monochromatic electron beam and a high resolution electrostatic analyzer. Among the sixty four features observed, fifty five are assigned, while nine stay unassigned. The post-collision interaction (PCI) effects in the M5-N1N1(1S0) and M5-N2,3N2,3(1S0) Auger peaks are studied as a function of electron incident energy approaching the M5 edge (95.04 eV). The PCI shifts are found to be well represented by an exponential function of the excess energy with exponents β = (-0.99±0.05 and -3.08±0.05) for the N2,3N2,3(1S0) and N1N1(1S0) peaks, respectively. The M2,3-M4,5N Coster-Kronig transitions are studied in kinetic energy region 65 - 110 eV and at incident energies from 120 to 2019 eV. Owing to a high resolution twelve well resolved satellites in the region bellow 90 eV are observed, while in the region 90 - 110 eV two broad features are assigned to the M2,3-M4,5N2,3 C-K peaks with intensity ratio (M3/M2 = 2.37) are observed. Comparison is made with available data from literature.

Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser.

Femtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d5/2 core level (M5-edge, 572–577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400 ± 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1–2 ps timescale, which is interpreted as electron–hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.

Characterization of CeOx-decorated Pd/C Catalysts Synthesized By Controlled Surface Reactions for Hydrogen Oxidation in Anion Exchange Membrane Fuel Cells

The high cost and difficult task to eliminate platinum (Pt) catalyst in a Proton Exchange Membrane Fuel Cells (PEM-FCs) are one of the main obstacles preventing wide adoption of fuel cells. The anion exchange membrane fuel cell (AEM-FC) has been proposed as an alternative in which non-Pt metals may be employed. However, obtaining a high-power density Pt-free AEM-FC has been a challenging task because the Hydrogen Oxidation Reaction (HOR) in the electrocatalysis experiences a slow kinetics at the anode1–3. In the present study, with the aim of improving the efficiency of HOR catalysts, various ratios of CeOx/Pd catalysts were deposited onto a carbon support using Controlled Surface Reactions (CSR) process. A homogenous distribution of CeOx preferentially attached to Pd nanoparticles (NPs) was expected to achieve highly active CeOx-Pd/C catalysts for HOR. We present here a comprehensive characterization approach for the synthesized highly active catalyst, and correlate obtained structural/compositional parameters to the performance. The characterization of the catalysts was carried out via Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Transmission Electron Microscopy (STEM) - Energy Dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS), and X-ray Photoelectron Spectroscopy (XPS) to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation state and surface chemical states, respectively. The HRTEM images indicated that Pd NPs were uniformly distributed on the carbon support with only some minor agglomeration. Additionally, the achieved high interfacial contact between CeOx and Pd acquired on single NPs was, for the first time, segmented and calculated using High-resolution STEM-EDS maps and Image J processing program by measuring the overlap intensities between Pd and Ce NPs; the results clearly showed that CeOx NPs were in intimate contact with Pd and their interfacial contact area increased with the addition of CeOx, reached a maximum at a ratio 0.38 CeOx-Pd/C, then decreased due to the formation of large CeOx islands upon the further addition of CeOx. The attained interfacial contact area also seems to be much higher than other previously reported Pd-CeO2 catalysts synthesized by other methods4–6, suggesting that the CSR method resulted in a higher selective deposition of CeOx on Pd.Moreover, the characterized two strong peaks shape resulting from electron transitions12 and the intensity ratio13 of the M4 to M5 edge bands of CeOx obtained from the highest energy resolution (1.1eV) EELS, suggested that most of the CeOx was in the form of CeO2 and Ce4+ was predominantly present in CeOx. However, the presence of Ce3+ cannot be ruled out, specially near the surface of the particles14. The study also found that the already mentioned interfacial contact area was directly correlated to the electrochemical performance reflected on the HOR activity of the CeOx-Pd/C catalysts. From this work, it can be said that when using the CSR method the highest recorded HOR exchange current density, compared to previously reported Pd-CeO2 catalysts, (51.5 mA mg–1 Pd)4–9 can be achieved. AEMFC performance obtained when using CeOx-Pd/C catalyst with Ce/Pd bulk atomic ratio of 0.38 was considerably higher than other cells with analogous HOR catalysts (peak power density of 1169 mW cm–2 mgPd –1 at 333 K)4,5,10,11. This results can be explained by the improved distribution of CeOx onto Pd enhancing OH– spillover from CeOx to Pd, the higher concentration of Pd (IV) sites, and the higher interfacial contact area between CeOx and Pd nanoparticles. REFERENCES: Davydova, E. S., Mukerjee, S., Jaouen, F. & Dekel, D. R. ACS Catal. 8, 6665–6690 (2018).Dekel, D. R. Curr. Opin. Electrochem. 12, 182–188 (2018).Dekel, D. R. J. Power Sources 375, 158–169 (2018).Miller, H. A. et al. Angew. Chemie - Int. Ed. 55, 6004–6007 (2016).Hamish Miller, Francesco Vizza, et al. Nano Energy, Elsevier 293–305 (2017) doi:10.1039/b000000x/1.Yu, H. et al. Nano Energy 57, 820–826 (2019).Yarmiayev, V., Alesker, M., Muzikansky, A., Zysler, M. & Zitoun, D. J. Electrochem. Soc. 166, F3234–F3239 (2019).Bellini, M. et al. ACS Appl. Energy Mater. 2, 4999–5008 (2019).Ralbag, N. et al. Electrochem. Soc. 167, 054514 (2020).Omasta, T. J. et al. J. Electrochem. Soc. 165, J3039–J3044 (2018).Alesker, M. et al. J. Power Sources 304, 332–339 (2016).Goris, B., Turner, S., Bals, S. & Van Tendeloo, G. ACS Nano 8, 10878–10884 (2014).Liu, J. Thesis, Univ. Birmingham (2017).Sims, C. M. et al. Nanotechnology 30, (2019).

Uranium solubility and speciation in reductive soda-lime aluminosilicate glass melts

Uranium solubility in aluminosilicate melts of the Na2O-CaO-Al2O3-SiO2 system with two different Na/Ca ratios was studied at temperatures of 1250–1400°C and under various redox conditions. A closed thermochemical reactor was used to control the alkali metal activity (sodium oxide content) and the oxygen fugacity imposing the reducing environment on the glass melt (10−5 atm < fO2 < 10−15 atm). The compositions of the quenched glasses were analyzed by scanning electron microscopy and electron probe microanalysis. It appeared that uranium solubility decreased with decreasing oxygen fugacity, elucidating the roles of the different valences of uranium. To account for the respective effects of theses valences, we proposed a method to determine the proportion of each uranium oxidation state in the glass sample. The coexisting UVI, UV, and UIV species have been characterized for the first time in glass samples using U M4 edge high energy resolution X-ray absorption near-edge structure. Results showed that the lowest solubility values, of approximately 1 mol% UO2, were obtained under strongly reducing conditions, and thus with UIV as the main valence. Under higher oxygen fugacity, uranium solubility was controlled and drastically enhanced by the UVI concentration in the melt.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non‐homogeneous samples with NPs less than 3 nm. By combining high‐energy X‐ray scattering (HEXS) and high‐energy‐resolution fluorescence‐detected X‐ray absorption near‐edge structure (HERFD XANES) analysis, we have characterised for the first time both the short‐ and medium‐range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.

3d-4f Resonant Inelastic X-ray Scattering of Actinide Dioxides: Crystal-Field Multiplet Description.

A theoretical overview of the core-to-core (3d-4f) resonant inelastic X-ray scattering (RIXS) spectra of actinide dioxides AnO2 (An = Th, U, Np, Pu, Am, Cu, Bk, Cf) is provided. The 3d-4f RIXS maps were calculated using crystal-field multiplet theory and turned out to be significantly different at the An M5 vs M4 edges, because of selection rules and final state effects. The results of the calculations allowed for a general analysis of so-called high-energy-resolution fluorescence-detected X-ray absorption (HERFD-XAS) spectra. The cuts of the calculated RIXS maps along the incident energy axis at the constant emitted energy, corresponding to the maximum of the RIXS intensity, represented the HERFD spectra and provided their comparison with calculated conventional X-ray absorption (XAS) spectra with a reduced core-hole lifetime broadening at the An M5 and M4 edges. Although the An M5 HERFD profiles were found to depart from the X-ray absorption cross-section, in terms of appearing additional transitions, the results of calculations for the An M4 edges demonstrate overall better agreement between the HERFD and XAS spectra for most dioxides, keeping in mind a restricted HERFD resolution, because of the core–hole lifetime broadening in the final state. The results confirm the utility of HERFD for the An chemical state determination and indicate the importance of calculating the entire RIXS process in order to interpret the HERFD data correctly.

The X-ray emission of cerium oxide

Using a combination of spectral simulation with FEFF and experimental data for metallic Ce, it has been possible to extract the Cerium Oxide-only M4,5 X-ray Emission Spectrum (XES) from an experimental result, which had been produced utilizing a combined CeOxide/Ce sample. It is shown that the XES emission for CeOxide-only is significantly different for the M4 and M5 edges, with the M4 XES consistent with normal XES and the M5 XES exhibiting a continued enhancement associated with a threshold effect, connected to Resonant Inverse Photoelectron Spectroscopy (RIPES).

Synthesis and characterization of nanocrystalline U1-xPuxO2(+y) mixed oxides

We report here the first synthesis of mixed oxide U1-xPuxO2(+y) nanoparticles. The obtained nanopowders were characterized by X-ray diffraction, thermal ionization mass spectrometry, transmission electron microscopy, Raman spectroscopy, and U M4 edge high-energy-resolution X-ray absorption near edge structure (HR-XANES). The HR-XANES spectra give evidence for the partial oxidation of UIV to UV. This novel route toward the formation of actinide–actinide solid solution opens research opportunities that are not accessible using bulk materials. We give details on the X-ray diffraction study on plutonium oxalate hexahydrate, as a reagent for the synthesis of such nanoparticles.

Ru doping effect on the structural, electronic, transport, optical and dye degradation properties of layered Li2MnO3

Ru doped Li2MnO3 compositions show the presence of redox couples Mn+3–Mn+4 and Ru+4–Ru+5. Due to the presence of these redox couples in Li2Mn1−xRuxO3 (x = 0.05, 0.1), Ru doped compositions show (i) change in structure (ii) large decrease in impedance(~ 107 to ~ 105 Ohm) (iii) degrade methyl orange and methylene blue solution (pH-6) in quick time in presence of tungsten (W) bulb and sun light. Synchrotron X-ray powder diffraction studies show the change in lattice parameters with Ru doping at Mn site in Li2Mn1−xRuxO3 (x = 0.05, 0.1). Raman and Infrared spectroscopic studies show the shift in Mn–O stretching mode of Li2MnO3 towards lower wave number with Ru doping. Soft X-ray absorption spectroscopic studies show the presence of mixed valences of Mn+3, Mn+4, Ru+4 and Ru+5 in Li2Mn1−xRuxO3 (x = 0.05, 0.1) using L2,3 and M4 edge. UV–Vis diffuse reflectance spectroscopic studies show the optical band gap in visible light range in Ru doped Li2Mn1−xRuxO3 (x = 0.05, 0.1) compositions. The presence of 5 mg of Li2Mn0.9Ru0.1O3 degrades 5 ml of methyl orange solution (10 mg/L) of pH 6 in 5 min and methylene blue solution (10 mg/L) of pH 6 in 3 min of exposure in W bulb light.

The missing pieces of the PuO2 nanoparticle puzzle.

The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 nanoparticles (NPs) at the atomic level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L3 edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L3 and M4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidation states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analyzed with state-of-the-art theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of ∼2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(iv) oxidation state in all NPs, reported here for the first time, indicate that the structure of PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.

Selective probing of magnetic order on Tb and Ir sites in stuffed Tb2Ir2O7 using resonant x-ray scattering

We study the magnetic structure of the ‘stuffed’ (Tb-rich) pyrochlore iridate Tb2+xIr2−xO7−y (x ∼ 0.18), using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir L3 and Tb M5 edges, which provide selective sensitivity to Ir 5d and Tb 4f magnetic moments, respectively. At the Ir L3 edge, we found the onset of long-range magnetic order below K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of at 5 K. At the Tb M5 edge, long-range magnetic order appeared below ∼ K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the M5 edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb2Ir2O7, for which K and , indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.