Dynamic free energy surfaces for sodium diffusion in type II silicon clathrates

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Type II Si clathrates are promising photovoltaic materials due to their wide band gaps.
• Complete evacuation of guest species, like sodium (Na), is essential for realizing semiconducting properties.
• Previous experiments show Na can be reduced to less than 1 atom per unit cell.

Purpose of the Study:

• Investigate the energetics, kinetics, and mechanism of Na diffusion in type II Si clathrates.
• Determine the thermodynamic preferences and transition barriers for Na movement.
• Elucidate the pathway for complete Na removal from the clathrate structure.

Main Methods:

• Biased molecular dynamics simulations.
• Well-tempered metadynamics to calculate the potential of mean force.
• Kinetic Monte Carlo simulations to identify the diffusion mechanism.

Main Results:

• Identified a coupled diffusion mechanism for Na guests.
• Electrostatic guest-guest interactions drive Na movement between cages.
• A crossover point exists where large and small cages have equal Na occupancy.
• The process leads to a Na-free structure, consistent with experimental observations.

Conclusions:

• The coupled diffusion mechanism explains complete Na removal from type II Si clathrates.
• Understanding Na diffusion is key to optimizing Si clathrates for photovoltaic applications.
• Computational simulations provide insights into guest species dynamics in clathrate materials.