Enhanced thermoelectric properties of heavy-fermion compounds YbxCeySmzIr2Zn20 (x plus y plus z=1)

Thermoelectric materials hold tremendous promise for advances in fundamental science and practical applications, particularly for robust electricity generation in extreme and remote environments. Despite this, for most materials the energy conversion efficiency is limited by the proportionality between the electrical and thermal conductivities and small values of the Seebeck coefficient for metals. It was previously reported that the heavy-fermion compound YbIr2Zn20 exhibits large Seebeck coefficient [E. Mun et al., Phys. Rev. B 86, 115110 (2012)] and thermoelectric figure of merit ZT at 35 K [K. Wei et al., Sci. Adv. 5, eaaw6183 (2019)]. This behavior is primarily associated with strong hybridization between the f- and conduction electron states. Here, we seek to improve the thermoelectric properties through chemical substitution on the Yb site using Ce and Sm. By surveying different levels of substitution, we find that the thermoelectric properties vary strongly with the f-element ratio. This confirms that electronic hybridization dominates the thermoelectric properties and clarifies directions for optimizing these materials for applications. We also investigate the impact of the disorder on the thermal conductivity, where we find only weak variation with lanthanide content.