Phase Transition and Liquid-like Superionic Conduction in Ag₂S

Superionic conductivity in certain polymorphs of Ag₂S has inspired numerous concepts for materials applications, but the relationship between the structure and the mobility of silver ions remains poorly explored. Here, we report ab initio molecular dynamics simulations for low- (acanthite) and high-temperature (argentite) Ag₂S polymorphs that reveal the dynamical processes giving rise to the superionic behavior in the latter. Similarities between their sulfur sublattices enable simulations of silver ion diffusivities and pathways on essentially an equal footing. For the higher temperature polymorph, calculated temperature-dependent mean square displacements and activation energies by the nudged elastic band method show good correspondence with expectations from the experiment. In the superionic state, silver atoms diffuse in a liquid-like behavior with no preferred diffusion pathways, within the relatively stable body-centered cubic sulfur framework. In contrast, conduction in acanthite appears to depend more on the mobilities of electronic charge carriers.