Investigation of thermally induced phase transitions in uranium perovskites

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.

Transformation from pyrochlore to fluorite by diffraction and X-ray spectroscopy

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.

Perovskites in low dimensional multi-layer structure types

This study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi₅PbTi₃O₁₄Cl [1] with charge balancing by removing Pb²⁺ for Bi³⁺. A statistical 1:2 distribution of M³⁺ and Ti⁴⁺ across all three perovskite layers was found in Bi₆Ti₂MO₁₄Cl, M = Cr³⁺, Mn³⁺, Fe³⁺, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi₅PbTi₃O₁₄Cl) and pronounced spin-glass behavior below T_irr(0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi₆Ti₂FeO₁₄Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi₃Sr₂Nb₂O₁₁Br, based on Bi₃Pb₂Nb₂O₁₁Cl [2], was synthesized by simultaneously replacing Pb²⁺ with Sr²⁺ and Cl^- with Br^-. Inter-layer mismatch prevented the formation of Bi₃Sr₂Nb₂O₁₁Cl and Bi₃Pb₂Nb₂O₁₁Br. Sr²⁺ doping reduces the impact of the stereochemically active 6s² lone pair found on Pb²⁺ and Bi³⁺, resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br^-. X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi₃Sr₂Nb₂O₁₁Br compared to Bi₃Pb₂Nb₂O₁₁Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.

Studying the structural and electronic effects of substituted (Bi_0.5Na_0.5)TiO₃

Significant efforts have been made in the development of (Bi0.5Na0.5)TiO3 ferroelectrics as an alternative to the lead-based industry standard PbTi1-xZrxO3.[1] It has also been shown that doping the A- and B-site of (Bi0.5Na0.5)TiO3 can greatly improve the ferroelectric behavior of these materials,[2] possibly due to the formation of two or more ferroelectric phases at a morphotropic phase boundary (MPB). As such, there is a significant interest in understanding the structural changes in (Bi0.5Na0.5)TiO3-based solid solutions. (Bi0.5Na0.5)TiO3 was originally described as adopting a rhombohedral structure in space group R3c, However, the accuracy of this description has been greatly debated. It was recently suggested that (Bi0.5Na0.5)TiO3 actually adopts a monoclinic structure in space group Cc.[3] Given this recent controversy, we investigated the structural evolution of (Bi0.5Na0.5)TiO3-based solid solutions, particularly the (Bi0.5Na0.5)Ti1-xZrxO3 and (1-x)(Bi0.5Na0.5)TiO3-xBiFeO3 solid solutions., using both diffraction and spectroscopy techniques. Diffraction measurements on (Bi0.5Na0.5)TiO3 confirm that both monoclinic Cc and rhombohedral R3c phases are present at room temperature. Diffraction analysis showed that doping (Bi0.5Na0.5)TiO3 with a small amount of (Bi0.5Na0.5)ZrO3 and BiFeO3 can stabilizes the rhombohedral phase. The Ti/Fe K-edge and Zr L3-edge XANES spectra analysis was performed to determine the effects doping has on the local displacement of the B-site cations.