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Acta Cryst. (2014). A70, C71
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The physical properties of inorganic solids are intimately related to their crystal structures and there is increasing awareness of the potential importance of metastable structures that exist over a limited temperature and/or pressure range. For the renaissance of nuclear energy to continue it is vital to improve the efficiency and safety of the nuclear fuel process. In order to do this, a comprehensive knowledge of the fundamental chemical, structural, and thermodynamic properties of uranium compounds is required. Compounds of the type A2BUO6 (A = Ba, Sr; B = Ba, Sr, Ca) have been prepared and characterised using neutron and X-Ray diffraction techniques as well as X-ray absorption spectroscopy. For the first time the high temperature behaviour of these complex oxides has been investigated, and as illustrated by Ba2SrUO6, heating such oxides can induce a sequence of phase transitions with the structure of Ba2SrUO6 changing from monoclinic in P21/n at room temperature to cubic in Fm-3m above 1200 K. [1] The compounds Ba2CaUO6 and BaSrCaUO6 were also found to undergo a series of thermally induced phase transitions from the P21/n monoclinic structure. In order to elucidate the structural changes involved in each system, a combination of diffraction techniques was required. We also utilised symmetry-mode analysis in which the structures are refined in terms of the fundamental tilting modes. This elegant way of tracking phase transitions provided vital insight when comparing and contrasting the thermal behaviour of these complex uranium oxides.
Keywords: perovskite.

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Acta Cryst. (2014). A70, C234
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We have studied the long-range average and local structures in a number of zirconium containing materials of the type A2B2O7 ( A = Ln or Y; B = Zr, Hf or Sn) using synchrotron X-ray and neutron powder diffraction and X-ray absorption spectroscopy. Studies of the system Gd2-xTbxZr2O7 include neutron diffraction data, obtained at λ ≍ 0.497 Å to minimise 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. In general we observe that while the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at specific compositions corresponding to a critical ionic radius ratio of the A and B cations (rA/rB) x ~ 1.0-1.2, X-ray absorption near-edge structure (XANES) results reveal a gradual structural evolution across the compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. Where possible the experimental results were supplemented by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures were predicted, and the trends agree well with the experimental XANES results. 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 defect fluorites.
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