Order/Disorder and in Situ Oxide Defect Control in the Bixbyite Phase YPrO_3+δ (0 ≤ δ < 0.5)

Abstract

The YPrO3+δ system is a nearly ideal model system for the investigation of oxide defect creation and annihilation in oxide ion conductor related phases with potential applications as solid state electrolytes in solid oxide fuel cells. The formation, structure, high temperature reactivity, and magnetic susceptibility of phase pure YPrO3+δ (0 ≤ δ ≤ 0.46) are reported. The topotactic reduction and oxidation of the YPrO3+δ system was investigated by powder X-ray in situ diffraction experiments and revealed bixbyite structures (space group: Ia3̅) throughout the series. Combined neutron and X-ray data clearly show oxygen uptake and removal. The research provides a detailed picture of the Y(3+)/Pr(3+)/Pr(4+) sublattice evolution in response to the redox chemistry. Upon oxidation, cation site splitting is observed where the cation in the ((1)/4, (1)/4, (1)/4) position migrates along the body diagonal to the (x, x, x) position. Any oxygen in excess of YPrO3.0 is located in the additional 16c site without depopulating the original 48e site. The in situ X-ray diffraction data and thermal gravimetric analysis have revealed the reversible topotactic redox reactivity at low temperatures (below 425 °C) for all compositions from YPrO3 to YPrO3.46. Magnetic susceptibility studies were utilized in order to further confirm praseodymium oxidation states. The linear relation between the cubic unit cell parameter and oxygen content allows for the straightforward determination of oxygen stoichiometry from X-ray diffraction data. The different synthesis strategies reported here are rationalized with the structural details and the reactivity of YPrO3+δ phases and provide guidelines for the targeted synthesis of these functional materials.

Oxygen trapping and cation site-splitting in Y(2−x)PrxO3+δ (0.0≤x<2.0 and δ≤1.0)

“…Y2O316 is stable as a bixbyite phase but Pr2O3 17 forms a trigonal structure. Since all reduced Y2-xPrxO3 samples were prepared from the oxidized bixbyite and fluorite phases by topotactic reduction in dilute hydrogen only reduced bixbyite phases were obtained.The "as-prepared" samples are oxidized phases with +4 being the predominant oxidation state for praseodymium as determined by TGA and confirmed with magnetic measurements 9. The oxidized samples also show alinear increase of V/Z as a function of the praseodymium concentration for the bixbyite and fluorite structures.…”

“…For a Y:Pr ratio of approximately 1:1 that the anion 16c position will be almost half filled, which will pull the cation towards the filled position, and away from the hole. 9 For praseodymium richer samples more oxide anions are added which will fill the remaining vacancies on the 16c anion site therefore pulling the cation back towards the ideal 8b position.…”

“…The reduction -oxidation reactions are also fully reversible. Furthermore the synthesis of the x = 1 material reported by Lussier, et al 9 is well understood, employing a sol-gel synthesis utilizing the citrate method. Understanding the oxygen loss at high temperatures led to a reliable synthesis method for the full solid solution.…”

Inflexible stoichiometry in bulk pyrite FeS2 as viewed by in situ and high-resolution X-ray diffraction

“…If the stoichiometry were to vary across the range of sulfur content, as in TiS 2 , the lattice constants should change linearly with respect to anion occupancy due to Vergard's law in the single-phase regime (Benard & Jeannin, 1963;Vegard, 1921). These large concentrations of vacancies, as claimed by Birkholz, should be observable, especially in high-resolution data, as demonstrated by Lussier's study on oxygen content in YPrO 3À (Birkholz et al, 1991;Lussier et al, 2016). On the other hand, no deviations in lattice parameter or relative peak intensities would be observed for a line compound -rather, the appearance of new secondary phases would signal crossing from the FeS 2 + S to the FeS 2 + Fe 1À S two-phase region.…”

Structural Competition and Reactivity of Rare-Earth Oxide Phases in YxPr2–xO3 (0.05 ≤ x ≤ 0.80)

“…Bulk samples of the parent oxidized materials Y x Pr 2– x O 3+δ ( x = 0.05, 0.10, 0.15, 0.20, 0.40, 0.60, and 0.80) were prepared via the citrate sol–gel method using stoichiometric amounts of calcined Y 2 O 3 (Cerac, 99.999% purity) and Pr 6 O 11 (Alfa Aesar, 99.996% purity) in molten citric acid monohydrate (Aldrich, 99% purity) with final annealing in air at 1200 °C as previously outlined by Lussier et al , The resulting oxidized cubic phases were topotactically reduced at 500 °C for 12 h in 3–5% H 2 (balance N 2 ). All samples were confirmed to be phase pure cubic bixbyite (C) phases by powder X-ray diffraction (section 2.2.1).…”