Understanding the Interplay of Vacancy, Cation, and Charge Ordering in the Tunable Sc₂VO_5+δ Defect Fluorite System

We report the synthesis, structure, and redox behavior of the cation-ordered tetragonal Sc₂VO_5+δ defect fluorite superstructure previously thought to be the oxygen precise A³⁺₂B⁴⁺O₅ phase. Four synthesis routes in oxidative, reductive, and inert atmospheres are demonstrated. Ex situ and in situ powder X-ray and neutron diffraction analyses reveal vanadium disproportionation reactions. The structure-reaction map illustrates the oxygen-dependent competition between the tetragonal cation and anion ordered Sc₂VO_5+δ and the disordered cubic Sc₂VO_5+δ' (δ < δ' ≤ 0.5) phases as a function of temperature. Oxidation states and oxide stoichiometries were determined with DC magnetometry and XANES experiments. The tetragonal cation ordered Sc₂VO_5+δ phase with δ = -0.15(2) for as-synthesized samples reveals vanadium charge ordering. V³⁺ and V⁴⁺ cations occupy octahedral sites, whereas V⁵⁺ predominantly occupies a tetrahedral site. The paramagnetic ⁸{V^3+/4+}₄ clusters are isolated by diamagnetic ²V⁵⁺ cations. At temperatures below 500 °C the ⁸{V^3+/4+}₄ clusters can be topotactically fine-tuned with varying V³⁺/V⁴⁺ ratios. Above 600 °C the tetragonal structure oxidizes to the cubic Sc₂VO_5+δ' fluorite phase-its disordered competitor. The investigation of the cation- and anion-ordered Sc-V-O phases, their formation, and thermal stability is important for the design of low-temperature solid state oxide ion conductors and vacancy structures.