Understanding the Interplay of Vacancy, Cation, and Charge Ordering in the Tunable ScVO Defect Fluorite System.

We report the synthesis, structure, and redox behavior of the cation-ordered tetragonal ScVO defect fluorite superstructure previously thought to be the oxygen precise ABO phase. Four synthesis routes in oxidative, reductive, and inert atmospheres are demonstrated. and 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 ScVO and the disordered cubic ScVO (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 ScVO phase with δ = -0.15(2) for as-synthesized samples reveals vanadium charge ordering. V and V cations occupy octahedral sites, whereas V predominantly occupies a tetrahedral site. The paramagnetic {V} clusters are isolated by diamagnetic V cations. At temperatures below 500 °C the {V} clusters can be topotactically fine-tuned with varying V/V ratios. Above 600 °C the tetragonal structure oxidizes to the cubic ScVO 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.