Ultralow Lattice Thermal Conductivity of A₀.₅RhO₂ (A = K, Rb, Cs) Induced by Interfacial Scattering and Resonant Scattering

Recently, superionic conductors featured with melted ionic sublattices have been proved to possess ultralow thermal conductivity, but the underlying physical mechanism is under hot debate. In this article, we report ultralow out-of-plane thermal conductivity (0.4 ∼ 0.6 W m–¹ K–¹ at 300 K) in A₀.₅RhO₂ (ARO, A = K, Rb, Cs) crystals with a layered structure. Analyzed by the empirical Debye–Callaway model, it reveals that both interfacial scattering and resonant scattering play dominant roles in ultralow thermal conductivity observed in RRO and CRO crystals, while in KRO crystals, resonant scattering is nearly negligible, and both interfacial scattering and three-phonon Umklapp scattering are contributed to the low thermal conductivity. Moreover, the frequency of resonant modes in ARO is dependent on ionic species, which makes it possible to modulate thermal conductivity by intercalation of different ions in the matrix. This work provides crucial information to the physical origin of ultralow thermal conductivity in superionic conductors.