I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Open Access
Abstract: The crystallization of a new series of A-site substituted lanthanum ferrite materials (La1–xREx)FeO3 was explored by the hydrothermal method at 240 °C, for rare earth (RE) = Nd, Sm, Gd, Ho, Er, Yb, and Y, with 0 ≤ x ≤ 1. The effect of elemental substitution on the morphological, structural, and magnetic properties of the materials was studied using high-resolution powder X-ray diffraction, energy dispersive spectroscopy (EDS) on the scanning electron microscope, Raman spectroscopy, and SQUID magnetometry. If the radius of the La3+ and the substituent ions is similar, such as for Nd3+, Sm3+, and Gd3+, homogeneous solid solutions are formed, with the orthorhombic GdFeO3-type structure, and a continuous evolution of Raman spectra with composition and distinct magnetic behavior from the end members. When the radius difference between substituents and La3+ is large, such as for Ho3+, Er3+, Yb3+, and Y3+, then instead of forming solid solutions, crystallization in separate phases is found. However, low levels of element mixing are found and intergrowths of segregated regions give composite particles. In this case, the Raman spectra and magnetic behavior are characteristic of mixtures of phases, while EDS shows distinctive elemental segregation. A-site replacement induces an evolution in the crystallite shape with an increasing amount of substituent ions and this is most evident for RE = Y from cube-shaped crystals seen for LaFeO3 to multipodal crystals for (La1–xYx)FeO3, providing evidence for a phase-separation-driven evolution of morphology.
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Mar 2023
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[19053]
Open Access
Abstract: A reverse Monte Carlo analysis of neutron and x-ray total scattering data from two ceria-zirconia samples of composition Ce0.75Zr0.25O2 is performed to analyze the distribution of cations and to examine the possibility of oxide-ion disorder. For the first material, heated in air under moderate conditions (800 °C), the structure is a single-phase solid-solution with the statistical distribution of cations, but a local tetragonal symmetry is found, consistent with the different coordination preferences of Ce and Zr. For the second material, heated under H2 at 1050 °C followed by reoxidation at 400 °C, the structure shows a considerable disorder, with evidence for oxygen interstitials (Frenkel-ion defects) and a non-statistical distribution of cations with significantly higher concentrations of like–like cation nearest neighbors, highlighting the existence of cation-rich nano-domains. The results highlight the dynamic nature of this solid-solution, with structural evolution upon thermal treatment, which is of relevance to understanding its stability under redox catalytic conditions in practical applications.
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Mar 2023
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B18-Core EXAFS
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Diamond Proposal Number(s):
[14239]
Abstract: Terbium-doped YVO4 has been considered a nonluminescent solid since the first classic studies on rare-earth-doped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO4:Tb3+ nanoparticles overcomes the metal–metal charge transfer (MMCT) process which is responsible for the quenching of the Tb3+ luminescence. Tetragonal (Y1–xTbx)VO4 nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb3+ luminescence can be excited by VO43– → Tb3+ energy transfer and by direct population of the 5D4 state in YVO4. We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO4:Tb3+ can be reclassified as a UV-excitable luminescent material.
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Apr 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[11646, 8986]
Abstract: Iridate pyrochlore oxides (Na,Ca)2-x(Ir2-yMy)O6·nH2O (M = Sb, Zr, Ru, Rh) are studied as resilient electrocatalysts for the oxygen evolution reaction under acid conditions. The materials crystallise from aqueous solution under alkali hydrothermal conditions with 10-40 nm crystallite size. Refinement of their crystal structures using both powder neutron and X-ray diffraction determined the composition of the materials, and Ir LIII-edge XANES spectroscopy shows the average Ir oxidation state to be close to 4.5 in all materials, consistent with bond valence sums. All materials show high electrocatalytic activity for the oxygen evolution reaction and the electrocatalyst which best maintains activity on cycling is the sodium-free Ca2-xIr2O6·nH2O, while the (Na,Ca)2-xIr2O6·nH2O material shows highest activity when normalised for surface area. In membrane electrode assemblies, carbon corrosion is minimised, making the materials suitable for use in catalyst layers in proton exchange membrane devices, such as electrolysers and fuel cells. Under strongly acidic conditions it is proved that while A-site Ca and Na are readily leached, the average pyrochlore structure is maintained, as is electrocatalytic activity, with charge balance achieved by inclusion of protons in the pyrochlore structure in the form of bridging hydroxyls, as seen using inelastic neutron scattering spectroscopy.
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Nov 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[8708]
Open Access
Abstract: Iridium-containing NaTaO3 is produced using a one-step hydrothermal crystallisation from Ta2O5 and IrCl3 in an aqueous solution of 10 M NaOH in 40 vol% H2O2 heated at 240 °C. Although a nominal replacement of 50% of Ta by Ir was attempted, the amount of Ir included in the perovskite oxide was only up to 15 mol%. The materials are formed as crystalline powders comprising cube-shaped crystallites around 100 nm in edge length, as seen by scanning transmission electron microscopy. Energy dispersive X-ray mapping shows an even dispersion of Ir through the crystallites. Profile fitting of powder X-ray diffraction (XRD) shows expanded unit cell volumes (orthorhombic space group Pbnm) compared to the parent NaTaO3, while XANES spectroscopy at the Ir LIII-edge reveals that the highest Ir-content materials contain Ir4+. The inclusion of Ir4+ into the perovskite by replacement of Ta5+ implies the presence of charge-balancing defects and upon heat treatment the iridium is extruded from the perovskite at around 600 °C in air, with the presence of metallic iridium seen by in situ powder XRD. The highest Ir-content material was loaded with Pt and examined for photocatalytic evolution of H2 from aqueous methanol. Compared to the parent NaTaO3, the Ir-substituted material shows a more than ten-fold enhancement of hydrogen yield with a significant proportion ascribed to visible light absorption.
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Jun 2021
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B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[14239, 18786]
Open Access
Abstract: The cerium(III) hydroxide chloride Ce(OH)2Cl crystallises directly as a polycrystalline powder from a solution of CeCl3·7H2O in poly(ethylene) glycol (Mn = 400) heated at 240 °C and is found to be isostructural with La(OH)2Cl, as determined from high-resolution synchrotron powder X-ray diffraction (P21/m, a = 6.2868(2) Å, b = 3.94950(3) Å, c = 6.8740(3) Å, β = 113.5120(5)°). Replacement of a proportion of the cerium chloride in synthesis by a second lanthanide chloride yields a set of materials Ce1−xLnx(OH)2Cl for Ln = La, Pr, Gd, Tb. For La the maximum value of x is 0.2, with an isotropic expansion of the unit cell, but for the other lanthanides a wider composition range is possible, and the lattice parameters show an isotropic contraction with increasing x. Thermal decomposition of the hydroxide chlorides at 700 °C yields mixed-oxides Ce1−xLnxO2−δ that all have cubic fluorite structures with either expanded (Ln = La, Gd) or contracted (Ln = Pr, Tb) unit cells compared to CeO2. Scanning electron microscopy shows a shape memory effect in crystal morphology upon decomposition, with clusters of anisotropic sub-micron crystallites being seen in the precursor and oxide products. The Pr- and Tb-substituted oxides contain the substituent in a mixture of +3 and +4 oxidation states, as seen by X-ray absorption near edge structure spectroscopy at the lanthanide LIII edges. The mixed oxide materials are examined using temperature programmed reduction in 10%H2 in N2, which reveals redox properties suitable for heterogeneous catalysis, with the Pr-substituted materials showing the greatest reducibility at lower temperature.
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Oct 2020
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B18-Core EXAFS
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18786]
Open Access
Abstract: Zirconium tetramandelate (2-hydroxy-2-phenylacetate) has been used for selective gravimetric analysis of zirconium for over 70 years. Herein its crystal structure is reported from synchrotron powder X-ray diffraction and 13C solid-state NMR. The complex is a rare example of isolated zirconium cations, rather than the clusters prevalent in aqueous solutions.
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Aug 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[14239]
Abstract: The rutiles (M,Ru)O2 (M = Mg, Zn, Co, Ni, Cu) are formed directly under hydrothermal conditions at 240 °C from potassium perruthenate and either peroxides of zinc or magnesium, or poorly crystalline oxides of cobalt, nickel or copper. The polycrystalline powders consist of lath-shaped crystallites, tens of nanometres in maximum dimension. Powder neutron diffraction shows that the materials have expanded a axis and contracted c axis compared to the parent RuO2, but there is no evidence of lowering of symmetry to other AO2-type structures, supported by Raman spectroscopy. Rietveld refinement shows no evidence for oxide non-stoichiometry and provides a formula (MxRu1-x)O2 with 0.14 < x < 0.2, depending on the substituent metal. This is supported by energy-dispersive X-ray analysis on the transmission electron microscope, while Ru K-edge XANES spectroscopy shows that upon inclusion of the substituent the average Ru oxidation state is increased to balance charge. Variable temperature magnetic measurements provide evidence for atomic homogeneity of the mixed metal materials, with suppression of the high temperature antiferromagnetism of RuO2 and increased magnetic moment. The new rutiles all show enhanced electrocatalysis compared to reference RuO2 materials for oxygen evolution in 1 M H2SO4 electrolyte at 60 °C, with higher specific and mass activity (per Ru) than a low surface area crystalline RuO2, and with less Ru dissolution over 1000 cycles compared to an RuO2 with a similar surface area. Magnesium substitution provides the optimum balance between stability and activity, despite leaching of the Mg2+ into solution, and this was proved in membrane electrode assemblies.
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Jun 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[11646]
Open Access
Abstract: Pyrochlore iridates (Na,Ca)2−xIr2O6·H2O are acid-stable electrocatalysts that are candidates for use in electrolysers and fuel cells. Ir LIII-edge X-ray absorption fine structure spectroscopy in 1 M H2SO4 at oxygen evolution conditions suggests the involvement of the electrons from the conduction band of the metallic particles, rather than just surface iridium reacting.
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May 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[13841]
Abstract: The oxyhydroxide Ga2.52V2.48O7.33(OH)0.67 is prepared by reaction between Ga metal and Na3VO4 in a 1:1 monoethanolamine:water mixture at 240 °C. Powder neutron diffraction shows the material to be isostructural with the minerals nolanite and akdalaite, with cations occupying tetrahedral and octahedral interstitial sites in a hexagonal close-packed array of oxide/hydroxide (P63mc, a = 5.7906(2) Å, c = 9.2550(5) Å). Rietveld refinement against the data shows that Ga preferentially occupies tetrahedral sites, as well as some octahedral sites, and hence all V is octahedrally coordinated. The oxidation state of vanadium is confirmed as close to V3+ using V K-edge X-ray absorption near-edge structure spectroscopy, consistent with the refined chemical composition. The material is metastable, dehydrating around 300 °C and then decomposing above 500 °C, as shown by thermogravimetric analysis and thermodiffraction. The oxide Ga2.52V2.48O8 produced after dehydration at 300 °C is shown to contain a larger proportion of V4+ than the parent oxyhydroxide, to ensure charge balance, but the essential hexagonal structure is maintained. Variable temperature magnetisation measurements show that although both materials appear to obey the Curie-Weiss Law at high temperatures, at low temperatures the inverse susceptibility curves are non-linear. There is, however, no evidence for strong magnetic exchange and the extracted effective moments are consistent with the presence of more V3+ in the oxyhydroxide compared to the oxide.
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Apr 2020
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