Dynamical Structure, Bonding, and Thermodynamics of the Superionic Sublattice in alpha-AgI

TL;DR

This study uses first-principles molecular dynamics to analyze the phase transition, structure, and bonding in alpha-AgI, revealing unique dynamical and electronic features of its superionic phase.

Contribution

It provides new insights into the dynamical structure and electronic signature of silver ions during the superionic transition in alpha-AgI.

Findings

Superionicity is signaled by a phase transition of silver ions alone.

Distinct dynamical structure of the first silver shell around iodine in the superionic phase.

Unique electronic signature of the weakest-bound silver correlating with diffusion likelihood.

Abstract

Using extensive first-principles molecular dynamics calculations, we characterize the superionic phase transition and the lattice and electronic structures of the archetypal Type-I superionic conductor alpha-AgI. We find that superionicity is signalled by a phase transition of the silver ions alone. In the superionic phase, the first silver shell surrounding an iodine displays a distinct dynamical structure that would escape a time-averaged characterization, and we capture this structure in a set of ordering rules. The electronic structure of the system demonstrates a unique chemical signature of the weakest-bound silver in the first shell, which in turn is most likely to diffuse. Upon melting, the silver diffusion decreases, pointing to an unusual entropic contribution to the stability of the superionic phase.