Thermal transport in thermoelectric materials with chemical bond hierarchy

Chemical bond hierarchy (CBH) depicts a unique structural type of solids, in which fractions of the materials are loosely bonded in a relatively rigid framework. The weak bonding in materials with CBH induces special atomic vibrational motions and has a significant influence on the thermoelectric transport properties. The architecture of compounds with CBH was first proposed by Slack in his 'phonon glass electron crystal' paradigm and has led to the discovery of numerous new thermoelectric compounds over the years. This review covers various types of thermoelectric materials with different levels of CBH, focusing on their lattice thermal conductivities (kappa(L)s). Caged compounds, with foreign impurities in the cages as the rattlers, are the first type of compounds stimulating the study of CBH. The fillers in both the clathrates and the filled skutterudites greatly reduce the kappa(L) accompanied by abnormal temperature dependence. As reviewed herein, the reduced kappa(L) is attributed to different mechanistic sources, i.e., the resonant scattering or the enhanced anharmonic phonon scatterings. Both may contribute to the kappa(L) reductions. In recent years, more materials with different types of CBH have been discovered, some containing complex atomic clusters as the rattlers, and others having flowing atoms that cause multiple equilibrium sites and even liquid-like behaviors. All CBHs strongly interfere with the heat transport of the corresponding materials. Future perspectives and possible research directions for thermal transport in thermoelectric materials with CBH have also been summarized herein.