Interplay between high-entropy effects and the superionic transition in fluorite oxides: A liquid-like perspective

Minor actinide (MA)-bearing mixed oxide (MOX), a prominent high-entropy ceramic (HEC), is a promising candidate for advanced fast reactor fuels. Using a hybrid Monte Carlo/Molecular Dynamics (MC/MD) approach, we reveal that Np and Am segregation establishes a distinct short-range order (SRO) that induces severe lattice distortion. We uncover a competitive mechanism between localized high-entropy blocking effects and collective superionic dynamics, where the anionic kinetic and structural evolution within the 2000-2800 K range is partitioned and governed by the Tammann crossover temperature and superionic transition temperature. In the low-temperature regime, severe lattice distortions induced by the SRO structures create localized electrostatic and strain-coupled potential wells, which spatially confine the O-ions and lead to their sluggish diffusion. The system exhibits increased dynamical heterogeneity as thermal activation alleviates these structural constraints. Crucially, these localized traps spontaneously transform into a highly interconnected, string-like polyhedral topological network. Driven by the maximization of configurational entropy, this structural change creates low-viscosity transport pathways that trigger confinement-enhanced diffusion. Consequently, a dramatic kinetic inversion occurs, where the O-ion mobility of SRO model surpasses that of random solid solution (RSS) model via an entropy-driven compensation mechanism. Finally, through Gibbs free energy minimization and defect analysis, structural and thermodynamic origins of the superionic transition in HEC are elucidated. This study unveils how local chemical ordering regulates ion transport under extreme conditions, providing theoretical support for the design of high-performance nuclear fuels with both superior radiation tolerance and optimized mass transfer capabilities.