Diffusion within α-CuI studied using ab initio molecular dynamics simulations.

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Superionic conductors like copper iodide (CuI) are crucial for energy storage applications.
• Understanding the atomic-level structure and dynamics of CuI is key to optimizing its properties.
• Previous studies suggest complex cation behavior in the superionic phase.

Purpose of the Study:

• To elucidate the detailed structure and dynamics of superionic alpha-copper iodide (α-CuI).
• To investigate the origins of cation disorder and diffusion mechanisms.
• To compare simulation results with experimental data.

Main Methods:

• Ab initio Born-Oppenheimer molecular dynamics (MD) simulations.
• Analysis of atomic density profiles, pair distribution functions, and bond angle distributions.
• Lattice static calculations for comparison.

Main Results:

• Extreme cation disorder and a soft, immobile face-centered cubic sublattice were observed.
• Pronounced short-range copper-copper correlations were identified, indicated by lower than average Cu-Cu distances and Cu-I-Cu bond angles.
• Copper ions exhibit rapid jumps between tetrahedral cavities, often via curved trajectories and short-lived interstitial positions.
• Local iodine coordination varies, with a mixture of threefold, fourfold, and fivefold coordination.
• Correlated diffusion is attributed to unsuccessful jumps and cooperative motion of copper pairs.

Conclusions:

• The study reveals significant short-range order in the disordered cation sublattice of α-CuI.
• The diffusion mechanism involves rapid, correlated jumps of copper ions.
• Simulated diffusion coefficients at 750 K agree well with experimental values, validating the simulation approach.