Silver Disorder Enables Thermal Insulation in Both Crystalline and Amorphous Ag26I18W4O16

• • Thermal conductivity reaches the theoretical lower bound of 0.16 W/m-K in both crystalline and amorphous Ag26I18W4O16, with virtually no temperature dependence, enabling ultra-thin thermal barriers for microelectronics where conventional amorphous oxides (1–2 W/m-K) fail to meet sub-0.2 W/m-K targets. • • The material exhibits the lowest mean sound velocity ever recorded for a dense solid, directly suppressing phonon-mediated heat transport and providing a new metric for screening thermal insulators beyond porosity or alloying strategies. • • Short-range disorder (< 5 Å) governs thermal insulation, as revealed by PDF analysis of synchrotron X-ray total scattering, decoupling thermal performance from long-range crystallinity and enabling design of structurally stable crystals with glass-like thermal conductivity. • • Ag+ disorder stabilizes the thermal insulation against recrystallization and phase separation typical of amorphous materials, offering a pathway to overcome the thermodynamic instability that limits conventional amorphous thermal barriers in high-temperature or long-duration applications.