High Seebeck Coefficient and Unusually Low Thermal Conductivity Near Ambient Temperatures in Layered Compound Yb_2-xEu_xCdSb₂

Zintl phases are promising thermoelectric materials because they are composed of both ionic and covalent bonding, which can be independently tuned. An efficient thermoelectric material would have regions of the structure composed of a high-mobility compound semiconductor that provides the "electron-crystal" electronic structure, interwoven (on the atomic scale) with a phonon transport inhibiting structure to act as the "phonon-glass". The phonon-glass region would benefit from disorder and therefore would be ideal to house dopants without disrupting the electron-crystal region. The solid solution of the Zintl phase, Yb_2-xEu_xCdSb₂, presents such an optimal structure, and here we characterize its thermoelectric properties above room temperature. Thermoelectric property measurements from 348 to 523 K show high Seebeck values (maximum of ∼269 μV/K at 523 K) with exceptionally low thermal conductivity (minimum ∼0.26 W/m K at 473 K) measured via laser flash analysis. Speed of sound data provide additional support for the low thermal conductivity. Density functional theory (DFT) was employed to determine the electronic structure and transport properties of Yb₂CdSb₂ and YbEuCdSb₂. Lanthanide compounds display an f-band well below (∼2 eV) the gap. This energy separation implies that f-orbitals are a silent player in thermoelectric properties; however, we find that some hybridization extends to the bottom of the gap and somewhat renormalizes hole carrier properties. Changes in the carrier concentration related to the introduction of Eu lead to higher resistivity. A zT of ∼0.67 at 523 K is demonstrated for Yb_1.6Eu_0.4CdSb₂ due to its high Seebeck, moderate electrical resistivity, and very low thermal conductivity.