Symmetry Breaking in the Superionic Phase of Silver-Iodide

TL;DR

This study investigates the symmetry-breaking phenomena in the superionic phase of silver iodide, revealing competing structures, ion distribution dependencies, and dynamic fluctuations that explain experimental anomalies and memory effects.

Contribution

It uncovers the existence of a distorted tetragonal phase with symmetry breaking, competing with the bcc phase, and demonstrates the persistence of structural memory in superionic silver iodide.

Findings

Distorted tetragonal structure characterized by symmetry breaking.

Small energy difference (~few meV/atom) between phases.

Persistence of memory effects on nanosecond timescales.

Abstract

In the superionic phase of silver iodide, we observe a distorted tetragonal structure characterized by symmetry breaking in the cation distribution. This phase competes with the well known bcc phase, with a symmetric cation distribution at an energetic cost of only a few meV/atom. The small energy difference suggests that these competing structures may both be thermally accessible near the superionic transition temperature. We also find that the distribution of silver ions depends on the low-temperature parent polymorph, with memory persisting in the superionic phase on the nanosecond time scales accessible in our simulations. Furthermore, simulations on the order 100ns reveal that even at temperatures where the bcc phase is stable, significant fluctuations toward the tetragonal lattice structure remain. Our results are consistent with many "anomalous" experimental observations and offer a molecular mechanism for the "memory effect" in silver iodide.