Browsing by Author "Mitchell, MR"
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- Item119Sn MAS NMR and first-principles calculations for the investigation of disorder in stannate pyrochlores(Royal Society of Chemistry, 2011-01-14) Mitchell, MR; Reader, SW; Johnston, KE; Pickard, CJ; Whittle, KR; Ashbrook, SEThe local structure and cation disorder in Y2Ti2−xSnxO7 pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is studied using 119Sn (I = 1/2) NMR spectroscopy. NMR provides an excellent probe of disorder, as it is sensitive to the atomic scale environment without the need for any long-range periodicity. However, the complex and overlapping spectral resonances that often result can be difficult to interpret. Here, we demonstrate how 119Sn DFT calculations can be used to aid the spectral interpretation and assignment, confirming that Sn occupies only the six-coordinate pyrochlore B site, and that the Sn chemical shift is sensitive to the number of Sn/Ti on the neighbouring B sites. Although distinct resonances are resolved experimentally when the Ti content is low, there is significant spectral overlap for Ti-rich compositions. We establish that this is a result of two competing contributions to the Sn chemical shift; an upfield shift resulting from the incorporation of the more polarizing Ti4+ cation onto the neighbouring B sites, and a concomitant downfield shift arising from the decrease in unit cell size. Despite the considerably easier spectral acquisition, the lower resolution in the 119Sn spectra hinders the extraction of the detailed structural information previously obtained using 89Y NMR. However, the spectra we obtain are consistent with a random distribution of Sn/Ti on the pyrochlore B sites. Finally, we consider whether an equilibrium structure has been achieved by investigating materials that have been annealed for different durations. © 2011, Royal Society of Chemistry
- ItemNew insights into phase distribution, phase composition and disorder in Y2(Zr,Sn)2O7 ceramics from NMR spectroscopy(Royal Society of Chemistry, 2015-03-02) Ashbrook, SE; Mitchell, MR; Sneddon, S; Moran, RF; de los Reyes, M; Lumpkin, GR; Whittle, KRA combination of 89Y and 119Sn NMR spectroscopy and DFT calculations are used to investigate phase evolution, local structure and disorder in Y2Zr2−xSnxO7 ceramics, where a phase change is predicted, from pyrochlore to defect fluorite, with increasing Zr content. The ability of NMR to effectively probe materials that exhibit positional and compositional disorder provides insight into the atomic-scale structure in both ordered and disordered phases and, by exploiting the quantitative nature of the technique, we are able to determine detailed information on the composition of the phase(s) present and the average coordination number (and next-nearest neighbour environment) of the cations. In contrast to previous studies, a more complex picture of the phase variation with composition emerges, with single-phase pyrochlore found only for the Sn end member, and a single defect fluorite phase only for x = 0 to 0.6. A broad two-phase region is observed, from x = 1.8 to 0.8, but the two phases present have very different composition, with a maximum of 13% Zr incorporated into the pyrochlore phase, whereas the composition of the defect fluorite phase varies throughout. Preferential ordering of the anion vacancies in the defect fluorite phase is observed, with Sn only ever found in a six-coordinate environment, while remaining vacancies are shown to be more likely to be associated with Zr than Y. Our findings are then discussed in the light of those from previous studies, many of which utilize diffraction-based approaches, where, in most cases, a single phase of fixed composition has been assumed for the refinement procedure. The significant and surprising differences encountered demonstrate the need for complementary approaches to be considered for a detailed and accurate picture of both the long- and short-range structure of a solid to be achieved. © Royal Society of Chemistry 2017
- ItemThe pyrochlore to defect fluorite phase transition in Y2Sn2-xZrxO7(Royal Society of Chemistry, 2013-01-30) de los Reyes, M; Whittle, KR; Zhang, ZM; Ashbrook, SE; Mitchell, MR; Jang, LY; Lumpkin, GRThe system Y2Sn2−xZrxO7 (0.0 ≤ x ≤ 2.0) undergoes a phase transformation from ordered pyrochlore (Fdm) to defect fluorite (Fmm) actuated by the substitution of Zr for Sn. X-ray diffraction patterns map the retention of the pyrochlore structure up to x = 1.2. For samples with x = 1.4–2.0 the structure can be described as defect fluorite in broad terms. Electron diffraction patterns are consistent with this interpretation; however, they also demonstrate that the defect fluorite phase exhibits a strain driven compositional/displacive modulation that changes gradually with increasing Zr content. Raman spectra are consistent with gradual anion disorder up to x = 1.0 and highly disordered anion distributions inferred for x > 1.4, but the spectra also suggest the presence of residual order due to the modulated structure. The phase transformation in this system occurs at a higher Zr content than predicted by classical radius ratio models, consistent with the covalent character of Sn–O bonding. An unusual finding of this work comes from 119Sn MAS NMR and Sn L3-edge XANES analyses, indicating that Sn4+ prefers to occupy lattice sites with a 6-fold local coordination environment throughout the series. These results suggest that the incorporation of Sn or other metal cations having significant covalent bonding or a strong preference for octahedral coordination in pyrochlore-based materials may have a detrimental effect on ionic conductivity. © 2013, The Royal Society of Chemistry.
- ItemPyrochlore-defect fluorite phase transitions and stability in the Y2Sn2-xZrxO7 system(Committee of Asia-Pacific Societies of Microscopy, 2012-02-07) de los Reyes, M; Whittle, KR; Elliman, RG; Zaluzec, NJ; Ashbrook, SE; Mitchell, MR; Lumpkin, GRThe Y2Sn2-xZrxO7 pyrochlore series undergoes a phase transformation from a cubic pyrochlore structure type (Fd3m) to defect fluorite (Fm3m) actuated by an increase in Zr content, coupled with thermal annealing above 1500 °C. X-ray diffraction analysis reveals the onset of a pyrochlore to defect fluorite transition at Y2Sn0.8Zr1.2O7 with the loss of long range ordering. This is confirmed further by selected area electron diffraction (SAED) illustrating shorter range ordering in the defect fluorite phase incommensurate with unit cell size. This transformation however, occurs at a much higher Zr content than that predicted by classical radius ratio models. The diffuse scattering features observed in electron diffraction patterns of defect fluorite phases indicate some form of shorter range ordering involving compositional-displacive structural modulation. The behaviour of these materials during irradiation will be discussed and linked with the observed structural parameters (diffuse scattering, unit cell size).
- ItemPyrochlore-fluorite transition in Y2Sn2-xZrxO7 - implications for stability(Australian Institute of Physics, 2011-02-02) de los Reyes, M; Whittle, KR; Mitchell, MR; Ashbrook, SE; Lumpkin, GRThe pyrochlore-fluorite transition is an important factor in determining how materials behave under conditions of irradiation, whether it be as a waste form or as a nuclear material, e.g. ODS additive. Yttrium based materials are often added as oxides to metallic systems, e.g. oxide dispersion strengthened (ODS) steels, which have a wide range of applications. As part of a large programme of research investigating and developing materials which show a high degree of radiation damage resistance, materials based on Y2Sn2-xZrxO7 have been studied. The materials have been examined to determine the order-disorder transition (pyrochlorefluorite), and how this effects the radiation damage resistance, particularly as both end members have previously been shown to be resistant to damage/amorphisation. Results are presented from diffraction and spectroscopic studies showing the degree of order/disorder within the system.