Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO 4– x : Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature

Murphy, Gabriel L.; Wang, Chun-Hai; Kimpton, Justin A.; Beridze, George; Avdeev, Maxim; Zhang, Zhaoming; Kowalski, Piotr; Kennedy, Brendan J.

doi:10.1021/acs.inorgchem.8b00463

Journal Article

FZJ-2018-03395

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO 4– x : Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature

Murphy, G. L. ; Wang, C.-H. ; Beridze, G. ; Zhang, Z. ; Kimpton, J. A. ; Avdeev, M. ; Kowalski, P.FZJ* ; Kennedy, B. J. (Corresponding author)

2018
American Chemical Society Washington, DC

Inorganic chemistry 57(10), 5948 - 5958 (2018) [10.1021/acs.inorgchem.8b00463]

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Please use a persistent id in citations: 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.

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