Theoretical Evaluation of the Persistence of Transverse Phonons across a Liquid-like Transition in Superionic Conductor KAg3Se2

Abstract

A fundamental understanding of unusual atomic dynamics and collective modes in superionic conductors has been of great importance in improving renewable energy technologies such as thermoelectric energy conversion and electrolytes for recharge-able batteries. In this work, we have performed an in-depth theoretical study of the evolution of lattice dynamics across the superionic transition of KAg3Se2 by analyzing the thermally driven current fluctuations. We show that the diffuson-like phonons induced by the strong anharmonicity of the Ag sublattice dominate thermal transport below the superionic transition, resulting in nearly temperature-independent ultralow thermal conductivity in the normal ordered state. We find that the contributions of convection and conduction-convection interactions to thermal conductivity increase significantly due to the liquid-like flow of Ag atoms. We further demonstrate that the dynamic disorder is too slow to completely suppress the propagation of long-wavelength transverse phonons in the superionic state of KAg3Se2, while the propagation of Ag-dominated short-wavelength transverse phonons near the Brillouin zone boundary breaks down. These results provide new physical insights into the complex atomic dynamics of superionic conductors.