Superionic UO₂: A model anharmonic crystalline material

Area of Science:

• Materials Science
• Solid-State Physics
• Chemical Engineering

Background:

• Crystalline materials at high temperatures and pressures can display liquid-like properties.
• Superionic crystalline materials possess liquid-like ionic conductivity (σ), making them vital for energy technologies like batteries and fuel cells.
• Uranium dioxide (UO2) is a commercially important reactor fuel and a relevant model system for studying superionic behavior.

Purpose of the Study:

• To investigate the thermodynamic and structural properties of UO2 under superionic conditions.
• To quantify structural relaxation, dynamic heterogeneity, and ion mobility in UO2.
• To elucidate the mechanisms governing ion transport in superionic crystalline materials.

Main Methods:

• Molecular dynamics simulations were employed to study UO2.
• Thermodynamic and structural properties were analyzed.
• Transport properties, including diffusion and structural relaxation times, were quantified.

Main Results:

• Non-Arrhenius diffusion and structural relaxation in UO2 were successfully modeled using a generalized activated transport ('string') model.
• The Adam-Gibbs model also effectively described the transport data, linking it to excess entropy and collective motion.
• Interfacial mobility showed distinct temperature dependence compared to nonionic materials due to cohesive interactions.

Conclusions:

• Cooperative motion plays a significant role in enhancing ion mobility within ionic materials.
• The findings provide insights into the behavior of superionic UO2 and offer principles for designing novel superionic materials for energy applications.