Topotactic Solid-State Metal Hydride Reductions of Sr₂MnO₄

We report novel details regarding the reactivity and mechanism of the solid-state topotactic reduction of Sr₂MnO₄ using a series of solid-state metal hydrides. Comprehensive details describing the active reducing species are reported and comments on the reductive mechanism are provided, where it is shown that more than one electron is being donated by H^–. Commonly used solid-state hydrides LiH, NaH, and CaH_2, were characterized in terms of reducing power. In addition the unexplored solid-state hydrides MgH₂, SrH₂, and BaH₂ are evaluated as potential solid-state reductants and characterized in terms of their reductive reactivities. These 6 group I and II metal hydrides show the following trend in terms of reactivity: MgH₂ < SrH₂ < LiH ≈ CaH₂ ≈ BaH₂ < NaH. The order of the reductants are discussed in terms of metal electronegativity and bond strengths. NaH and the novel use of SrH₂ allowed for targeted synthesis of reduced Sr₂MnO_4–<i>x</i> (0 ≤ <i>x</i> ≤ 0.37) phases. The enhanced control during synthesis demonstrated by this soft chemistry approach has allowed for a more comprehensive and systematic evaluation of Sr₂MnO_4–<i>x</i> phases than previously reported phases prepared by high temperature methods. Sr₂MnO_3.63(1) has for the first time been shown to be monoclinic by powder X-ray diffraction and the oxidative monoclinic to tetragonal transition occurs at 450 °C

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

The YPrO_3+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 YPrO_3+δ (0 ≤ δ ≤ 0.46) are reported. The topotactic reduction and oxidation of the YPrO_3+δ system was investigated by powder X-ray <i>in situ</i> diffraction experiments and revealed bixbyite structures (space group: <i>Ia</i>3̅) throughout the series. Combined neutron and X-ray data clearly show oxygen uptake and removal. The research provides a detailed picture of the Y³⁺/Pr³⁺/Pr⁴⁺ 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 (<i>x</i>, <i>x</i>, <i>x</i>) position. Any oxygen in excess of YPrO_3.0 is located in the additional 16<i>c</i> site without depopulating the original 48<i>e</i> site. The <i>in situ</i> 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 YPrO_3.46. 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 YPrO_3+δ phases and provide guidelines for the targeted synthesis of these functional materials

Structure Evolution and Reactivity of the Sc_{(2–<i>x</i>)}V_<i>x</i>O_3+δ (0 ≤ <i>x</i> ≤ 2.0) System

Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that directly convert chemical energy of fuels into electricity with high efficiency. Because of their fuel flexibility, low emissions, high conversion efficiency, no moving parts, and quiet operation, they are considered as a promising energy conversion technology for low carbon future needs. Solid-state oxide and proton conducting electrolytes play a crucial role in improving the performance and market acceptability of SOFCs. Defect fluorite phases are some of the most promising fast oxide ion conductors for use as electrolytes in SOFCs. We report the synthesis, structure, phase diagram, and high-temperature reactivity of the Sc_{(2–<i>x</i>)}V_<i>x</i>O_3+δ (0 ≤ <i>x</i> ≤ 2.00) oxide defect model system. For all Sc_{(2–<i>x</i>)}V_<i>x</i>O_3.0 phases with <i>x</i> ≤ 1.08 phase-pure bixbyite-type structures are found, whereas for <i>x</i> ≥ 1.68 phase-pure corundum structures are reported, with a miscibility gap found for 1.08 < <i>x</i> < 1.68. Structural details obtained from the simultaneous Rietveld refinements using powder neutron and X-ray diffraction data are reported for the bixbyite phases, demonstrating a slight V³⁺ preference toward the 8b site. In situ X-ray diffraction experiments were used to explore the oxidation of the Sc_{(2–<i>x</i>)}V_<i>x</i>O_3.0 phases. In all cases ScVO₄ was found as a final product, accompanied by Sc₂O₃ for <i>x</i> < 1.0 and V₂O₅ when <i>x</i> > 1.0; however, the oxidative pathway varied greatly throughout the series. Comments are made on different synthesis strategies, including the effect on crystallinity, reaction times, rate-limiting steps, and reaction pathways. This work provides insight into the mechanisms of solid-state reactions and strategic guidelines for targeted materials synthesis

Zero Thermal Expansion in ZrMgMo₃O₁₂: NMR Crystallography Reveals Origins of Thermoelastic Properties

The coefficient of thermal expansion of ZrMgMo₃O₁₂ has been measured and was found to be extremely close to zero over a wide temperature range including room temperature (αl = (1.6 ± 0.2) × 10^–7 K^–1 from 25 to 450 °C by X-ray diffraction (XRD)). ZrMgMo₃O₁₂ belongs to the family of AMgM₃O₁₂ materials, for which coefficients of thermal expansion have previously been reported to range from low-positive to low-negative. However, the low thermal expansion property had not previously been explained because atomic position information was not available for any members of this family of materials. We determined the structure of ZrMgMo₃O₁₂ by nuclear magnetic resonance (NMR) crystallography, using ⁹¹Zr, ²⁵Mg, ⁹⁵Mo, and ¹⁷O magic angle spinning (MAS) and ¹⁷O multiple quantum MAS (MQMAS) NMR in conjunction with XRD and density functional theory calculations. The resulting structure was of sufficient detail that the observed zero thermal expansion could be explained using quantitative measures of the properties of the coordination polyhedra. We also found that ZrMgMo₃O₁₂ shows significant ionic conductivity, a property that is also related to its structure