Solid oxide fuel cells (SOFCs) are considered a promising energy conversion technology for the future. These devices show fuel flexibility, high conversion efficiency, and low emissions. Solid-state electrolytes play a crucial role in improving the performance of SOFCs. Currently, defect fluorites are most commonly used as electrolytes for commercial SOFCs. This thesis focuses primarily on fluorite and fluorite-related structures with the goal of furthering the understanding of oxide defect model systems. The Y2-xPrxO3+δ and Sc2-xVxO3+δ (0 ≤ x ≤ 2.00, and 0 ≤ δ ≤ 1) systems will be explored with a particular emphasis on synthesis and structure-reactivity relationships. Using in-situ methods, including powder diffraction (both X-ray and neutron), and thermogravimetric analysis, allows for a deeper understanding of reaction pathways, and provides insights into mechanistic details of reactions. Throughout this thesis, comments will be made on the importance of synthesis strategies and understanding structural details when designing functional materials.