Emily Reynolds

Does local disorder occur in the pyrochlore zirconates

The zirconates Ln(2)Zr(2)O(7) (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques.

Peroxide defect formation in zirconate perovskites

Atomic scale modelling suggests that excess oxygen can be accommodated in the group II perovskite zirconates by the formation of peroxide ion defects. This is unprecedented given the lack of charge compensating defects required for standard excess oxygen accommodation. The solution energy of O2 was predicted to be close to zero for BaZrO3, accommodating the peroxide ion defect more easily than in SrZrO3 or CaZrO3. This was experimentally examined by exposing SrZrO3 and BaZrO3 to hydrogen peroxide solution and then carrying out Raman spectroscopy measurements to look for a peak indicative of peroxide ions. A peak was observed at ∼1000 cm−1 in both compositions, suggesting the theoretically predicted peroxide ion is present.

Structural and Magnetic Properties of the Iridium Double Perovskites Ba(2-x)Sr(x)YIrO6.

The crystal structures of the series of ordered double perovskites Ba(2-x)Sr(x)YIrO6 (0 ≤ x ≤ 2) were refined using a combination of high-resolution synchrotron X-ray and high-intensity neutron diffraction data. The materials displayed a sequence of structures Fm3̅m(a(0)a(0)a(0)) (x = 0.6)--> I4/m(a(0)a(0)c(-)) (x = 1.0)--> I2/m(a(-)a(-)c(0)) (x = 1.4)--> P2(1)/n(a(-)a(-)c(+)) associated with increased tilting of the corner-sharing octahedra induced by increasing amount of the smaller Sr cation present. A similar sequence of transitions was induced by heating selected samples. Magnetic susceptibility measurements between 2 and 300 K showed no evidence for long-range magnetic ordering, an observation that was supported by neutron diffraction measurements, and rather strong spin-orbit coupling results in a Jeff = 0 ground state.

Anion Disorder in Lanthanoid Zirconates Gd2–xTbxZr2O7

The pyrochlore-defect fluorite order-disorder transition has been studied for a series of oxides of the type Gd(2-x)Tb(x)Zr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at λ ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore-defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order-disorder transition across the Gd(2-x)Tb(x)Zr2O7 series.

Studies of the antiferrodistortive transition in EuTiO3

Structural studies of EuTiO3, conducted using synchrotron x-ray powder diffraction, reveal the sensitivity of this material to defects. The presence of a single tetragonal-cubic (I4/mcm-Pm-3m) transition is confirmed. Neutron diffraction measurements show EuTiO3 to have a G-type antiferromagnetic arrangement. Examinations of the symmetry-adapted tetragonal strains demonstrate that the stoichiometry impacts on the continuous nature of this. The impact of defects on the electrical conductivity and magnetodielectric effect is also described.

Key Role of Bismuth in the Magnetoelastic Transitions of Ba3BiIr2O9 and Ba3BiRu2O9 As Revealed by Chemical Doping

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La(3+) and Ce(4+). Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials.

Synchrotron X-Ray Powder Diffraction Studies of Structural Phase Transitions in Perovskite Oxides

The utilization of the high resolution powder diffractometer at the Australian Synchrotron to obtain accurate and precise structures of some perovskite-type oxides is described. The structure of CdTiO3 has been studied from room temperature to 1000°C by high-resolution synchrotron X-ray powder diffraction. It was found that CdTiO3 remains orthorhombic in the Pbnm space group over the entire temperature range, with the expansion in the cell volume well fitted to the expression . The magnitudes of the TiO6 tilts are estimated from the refined structural parameters and these progressively reduce as the temperature is increased. The effect of Sr content on the room temperature structure of the double perovskites Ba2–xSrxInTaO6 is also described. At room temperature Ba2InTaO6 crystallizes in a cubic structure in space group . Doping with Sr results in tilting of the corner sharing octahedra with a concurrent lowering of symmetry with the sequence of structures being

Thermal Expansion Behavior in TcO2. Toward Breaking the Tc–Tc Bond

The structure of TcO2 between 25 and 1000 °C has been determined in situ using X-ray powder diffraction methods and is found to remain monoclinic in space group P21/c. Thermal expansion in TcO2 is highly anisotropic, with negative thermal expansion of the b axis observed above 700 °C. This is the result of an anomalous expansion along the a axis that is a consequence of weakening of the Tc-Tc bonds.

An unconventional method for measuring the Tc L3‐edge of technetium compounds

Tc L3-edge XANES spectra have been collected on powder samples of SrTcO3 (octahedral Tc(4+)) and NH4TcO4 (tetrahedral Tc(7+)) immobilized in an epoxy resin. Features in the Tc L3-edge XANES spectra are compared with the pre-edge feature of the Tc K-edge as well as other 4d transition metal L3-edges. Evidence of crystal field splitting is obvious in the Tc L3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L3 absorption edge energy difference between SrTcO3 (Tc(4+)) and NH4TcO4 (Tc(7+)) shows that the energy shift at the Tc L3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L3-edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K-edge.

Correlated disorder in metal–organic frameworks containing asymmetric linkers

Defects within materials, particularly oxides, are well-known to play an important role in their observed physical and chemical properties. In contrast, the different types of defects and disorder in framework materials are not as well understood, although through the creation of open metal sites and mesopore formation defects are key to accessing properties useful for catalysis and adsorption.[1] Linker and/or metal cluster vacancies have been observed in a variety of metal-organic frameworks (MOFs) including UiO-66, NU-125, MOF-505, and HKUST-1.[1] In these cases, the defective materials exhibit different properties but retain the average structure of the parent material, and as a result, conventional powder diffraction techniques can provide little insight into the location and nature of the defects.