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Title: Unexpected crystallographic phase transformation in nonstoichiometric SrUO4–x: Reversible oxygen defect ordering and symmetry lowering with increasing temperature
Authors: Murphy, GL
Wang, CH
Beridze, G
Zhang, Z
Kimpton, JA
Avdeev, M
Kowalski, PM
Kennedy, BJ
Keywords: Oxygen
Crystal defects
Phase transformations
X-ray diffraction
Issue Date: 1-May-2018
Publisher: American Chemical Society
Citation: Murphy, G. L., Wang, C.- H., Beridze, G., Zhang, Z., Kimpton, J. A., Avdeev, M., Kowalski, P. M., & Kennedy, B. J. (2018). Unexpected crystallographic phase transformation in nonstoichiometric SrUO4–x: Reversible oxygen defect ordering and symmetry lowering with increasing temperature. Inorganic Chemistry, 57(10), 5948–5958. doi:10.1021/acs.inorgchem.8b00463
Abstract: In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4–x, in space group R3̅m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4–x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4–x. Cooling δ-SrUO4–x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4–x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4–x to enable the transformation to δ-SrUO4–x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4–x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4–x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4–x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations. © 2018 American Chemical Society
ISSN: 1520-510X
Appears in Collections:Journal Articles

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