Silver Disorder Enables Thermal Insulation in Both Crystalline and Amorphous Ag26I18W4O16
Superionic conductors exhibit high cation mobility arising from weak binding and continuous transport pathways, atomistically characterized by extensive structural disorder and partial occupancy akin to amorphization. This disorder, whether confined to a cation sublattice or extended to full amorphization, strongly impedes lattice thermal transport, rendering these materials intrinsically ideal thermal insulators. This work investigates Ag26I18W4O16, a superionic conductor tunable from fully amorphous to single-crystalline states, as a model system to probe the impact of disorder and amorphization on thermal transport. Extensive Ag+ disorder, in both crystalline and amorphous phases, reduces thermal conductivity to approximately the theoretical lower bound of 0.16 W/m-K with virtually no temperature dependence, while concurrently achieving the lowest mean sound velocity ever recorded for a dense solid. Pair distribution function (PDF) analysis of synchrotron X-ray total scattering data indicates that short-range disorder (< 5 Å), rather than long-range periodicity, governs thermal insulation performance in both phases. These findings suggest a design strategy reconciling structural stability with glass-like thermal insulation.