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@ARTICLE{Murphy:862402,
      author       = {Murphy, Gabriel and Wang, Chun-Hai and Zhang, Zhaoming and
                      Kowalski, Piotr and Beridze, George and Avdeev, Maxim and
                      Muransky, Ondrej and Brand, Helen E. A. and Gu, Qin-Fen and
                      Kennedy, Brendan J.},
      title        = {{C}ontrolling {O}xygen {D}efect {F}ormation and {I}ts
                      {E}ffect on {R}eversible {S}ymmetry {L}owering and
                      {D}isorder-to-{O}rder {P}hase {T}ransformations in
                      {N}onstoichiometric {T}ernary {U}ranium {O}xides},
      journal      = {Inorganic chemistry},
      volume       = {58},
      number       = {9},
      issn         = {1520-510X},
      address      = {Washington, DC},
      publisher    = {American Chemical Society},
      reportid     = {FZJ-2019-02730},
      pages        = {6143-6154},
      year         = {2019},
      abstract     = {In situ synchrotron powder X-ray diffraction measurements
                      have demonstrated that the isostructural AUO4–x (A =
                      alkaline earth metal cation) oxides CaUO4–x and
                      α-Sr0.4Ca0.6UO4–x undergo a reversible phase
                      transformation under reducing conditions at high
                      temperatures associated with the ordering of in-plane oxygen
                      vacancies resulting in the lowering of symmetry. When
                      rhombohedral (space group R3̅m) CaUO4–x and
                      α-Sr0.4Ca0.6UO4–x are heated to 450 and 400 °C,
                      respectively, in a hydrogen atmosphere, they undergo a
                      first-order phase transformation to a single phase structure
                      which can be refined against a triclinic model in space
                      group P1̅, δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x, where the
                      oxygen vacancies are disordered initially. Continued heating
                      results in the appearance of superlattice reflections,
                      indicating the ordering of in-plane oxygen vacancies.
                      Cooling ordered δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x to
                      near room temperature results in the reformation of the
                      disordered rhombohedral phases. Essential to the
                      transformation is the generation of a critical amount of
                      oxygen vacancies. Once these are formed, the transformation
                      can be accessed continuously through thermal cycling,
                      showing that the transformations are purely thermodynamic in
                      origin. Stoichiometric structures of both oxides can be
                      recovered by heating oxygen deficient CaUO4–x and
                      α-Sr0.4Ca0.6UO4–x under pure oxygen to high temperatures.
                      When heated in air, the amount of oxygen vacancy defects
                      that form in CaUO4–x and α-Sr0.4Ca0.6UO4–x are found to
                      correlate with the A site composition. The inclusion of the
                      larger Sr2+ cation on the A site reduces defect–defect
                      interactions, which increases the amount of defects that can
                      form and lowers their formation temperature. The relative
                      difference in the amount of defects that form can be
                      understood on the basis of oxygen vacancy and U5+
                      disordering as shown by both ab initio calculations and
                      estimated oxygen vacancy formation energies based on
                      thermodynamic considerations. This difference in
                      defect–defect interactions consequently introduces
                      variations in the long-range ordered anionic lattice of the
                      δ phases despite the isostructural relationship of the α
                      structures of CaUO4–x and Sr0.4Ca0.6UO4–x. These results
                      are discussed with respect to the influence the A site
                      cation has upon anion defect formation and ordering and are
                      also compared to δ-SrUO4–x, the only other material known
                      to be able to undergo a reversible symmetry lowering and
                      disorder-to-order transformation with increasing
                      temperature},
      cin          = {IEK-6 / JARA-HPC},
      ddc          = {540},
      cid          = {I:(DE-Juel1)IEK-6-20101013 / $I:(DE-82)080012_20140620$},
      pnm          = {161 - Nuclear Waste Management (POF3-161) / Atomistic
                      modeling of radionuclide-bearing materials for safe
                      management of high level nuclear waste. $(jiek61_20181101)$
                      / Investigation of the new materials for safe management of
                      high level nuclear waste. $(jara0038_20121101)$},
      pid          = {G:(DE-HGF)POF3-161 / $G:(DE-Juel1)jiek61_20181101$ /
                      $G:(DE-Juel1)jara0038_20121101$},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:30964275},
      UT           = {WOS:000467351100086},
      doi          = {10.1021/acs.inorgchem.9b00406},
      url          = {https://juser.fz-juelich.de/record/862402},
}