Ultralow cross-plane lattice thermal conductivity caused by Bi–O/Bi–O interfaces in natural superlattice-like single crystals
Misfit-layered compounds constitute a significant component in the development of thermoelectric materials. Prominent thermoelectric properties arise from their low lattice thermal conductivity, which was supposed to be a consequence of intense phonon scattering at the misfit interfaces. Nevertheless, in most misfit compounds, the coexistence of van der Waals interfaces makes it hard to reveal the effect of misfit interfaces individually. Here, we investigate the impact of misfit interfaces and Bi–O/Bi–O interfaces on the thermal transport of a series of single crystals, including layered KxCoO2 (KCO, reference), misfit-layered compounds Ca3Co4O9 (CCO, with misfit interfaces) and Bi2Ca2Co2Oy (BCCO, with misfit interfaces and Bi–O/Bi–O interfaces) and layered compound Bi2Sr2CaCu2Oy (BSCCO, with Bi–O/Bi–O interfaces). Ultralow thermal conductivity approaching the theoretically predicted thermal conductivity minimum (∼0.28 W m−1 K−1 at 300 K) is achieved only with the existence of Bi–O/Bi–O interfaces with van der Waals interactions. Further analyses of the phonon scattering mechanisms verify that the strength of phonon Umklapp scattering and phonon–boundary (interface) scattering is comparable in KCO and CCO, while in BCCO and BSCCO phonon–boundary scattering dominates. Furthermore, the characteristic lengths for phonon–boundary scattering in BCCO and BSCCO are one magnitude lower than those of KCO and CCO, which proves the remarkable effect of the Bi–O/Bi–O interfaces in reducing thermal conductivity. Our work demonstrates the vital impact of interfaces with strong van der Waals interactions on the thermal conductivity reduction, and provides inspiration for the design of advanced thermoelectric materials.