Fluorite-pyrochlore dual-phase compositionally complex ceramics

The thermodynamic equilibrium between fluorite and pyrochlore phases in compositionally complex ceramics (CCCs) was systematically investigated. Twelve fluorite–pyrochlore dual-phase CCCs were synthesized to examine rare-earth cation partitioning between the two compositionally complex phases. The rare-earth cation distribution was found to correlate with ionic radius, revealing a critical radius of approximately 1.065 Å for compositionally complex rare-earth zirconates. Rare-earth cations with radii larger than this critical value preferentially dissolve in the pyrochlore CCC, whereas smaller cations favor the fluorite CCC. A thermodynamic model was developed to describe the dual-phase equilibrium, showing that the differential enthalpy of solution of rare-earth cations between fluorite and pyrochlore phases correlates linearly with the disorder energy of the corresponding ternary oxides. These findings provide a thermodynamic framework for understanding fluorite–pyrochlore dual-phase equilibria in CCCs and offer guidance for the design of dual-phase CCCs to tailor microstructures and optimize functional properties. • 12 fluorite-pyrochlore dual-phase compositionally complex ceramics are synthesized. • The partition of cations in the two phases correlates with cation radius. • For compositionally complex zirconates, the critical cation radius is ∼1.065 Å . • The disorder energy directs the cations partition in the two phases. • A thermodynamic model is developed for the dual-phase equilibrium.