Direct atomic-scale observation of the Ag+ diffusion structure in the quasi-2D "liquid-like" state of superionic thermoelectric AgCrSe2

"Phonon-Liquid Electron-Crystal (PLEC)" materials are attractive thermoelectric systems due to their emergent liquid-like behavior, which potentially leads to ultralow lattice thermal conductivity and moderate thermoelectric performance at high temperature. In this paper, the "liquid-like" state's atomic structure of the quasi-2D PLEC AgCrSe2 is directly studied by in situ scanning transmission electron microscopy and ab initio molecular dynamics. It is revealed by experiments that the diffusing Ag atoms in the liquid-like state prefer to occupy lattice sites as in a disordered solid, a feature very distinct from that of real liquids, and theoretical calculations predict that the characteristic time for Ag+ diffusion is too slow to disrupt vibrations. A simple slow diffusion model is proposed to elucidate the "liquid-like" behavior, and the ultralow lattice thermal conductivity can be explained within the framework of disorder solids.