Structure Evolution and Reactivity of the Sc_(2-x)V_xO_3+δ (0 ≤ x ≤ 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-x)V_xO_3+δ (0 ≤ x ≤ 2.00) oxide defect model system. For all Sc_(2-x)V_xO_3.0 phases with x ≤ 1.08 phase-pure bixbyite-type structures are found, whereas for x ≥ 1.68 phase-pure corundum structures are reported, with a miscibility gap found for 1.08 < x < 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-x)V_xO_3.0 phases. In all cases ScVO₄ was found as a final product, accompanied by Sc₂O₃ for x < 1.0 and V₂O₅ when x > 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.