Unraveling the Origins of Ultra-Low Thermal Conductivity in Ag₂S: A Synchrotron-Based Investigation into Low-Energy Phonon Modes and Crystal

Ag₂S, a superionic compound, exhibits ultra-low thermal conductivity (≤0.5 Wm⁻¹K⁻¹) and promising thermoelectric performance. Low-energy phonon modes, observed via inelastic neutron scattering, are speculated to arise from the silver sublattice, yet their origin remains unresolved. Inspired by synchrotron studies on argyrodites, which link anisotropic silver rattling to low-energy phonons, this proposal investigates Ag₂S’s structural dynamics. Using synchrotron X-ray diffraction, we will analyze local and global structural features in its rigid phase to uncover the mechanisms behind its thermal properties. High-resolution Bragg diffraction at lower temperatures will resolve its crystal structure, providing insights into atomic dynamics and phonon behavior. This work aims to advance understanding of superionic materials and guide the design of efficient thermoelectrics.