Extraordinary heat insulation in RbAg₄I₅

Superionic conductors are characterized by highly mobile charged ions, reflecting the presence of loosely bonded and partially occupied species within their crystal structures. These structural features are also essential for achieving high-performance thermal insulation. However, conventional skeleton materials used for thermal insulation, such as SiO₂, lack such crystallographic attributes. As a result, room-temperature thermal conductivity (κ) values for fully dense and 3%-dense SiO₂ remain around 1400 and 15 mW m^-1 K^-1, respectively. This work demonstrates the use of superionic conductors as a new paradigm for thermal insulation. Single-crystalline RbAg₄I₅ is shown to exhibit an exceptionally low κ of 130 mW m^-1 K^-1 at room temperature. Furthermore, a 3.6%-dense porous form of RbAg₄I₅ achieves a record-low κ of only 6 mW m^-1 K^-1, substantially outperforming existing porous thermal insulators. When applied as filler in a conventional Bi₂Te₃ thermoelectric device, this porous insulator suppresses gas-mediated heat transfer, leading to a 10% improvement in device performance. In addition, this material effectively isolates heat from a CPU to enable a temperature drop of 5.3 K for the adjacent flash memory, thus achieving a 16% improvement in its startup speed. The design strategy introduced here is expected to open new pathways for advancing thermal management technologies.