Coinage-Metal-Stuffed Eu ₉ Cd ₄ Sb ₉: Metallic Compounds with Anomalous Low Thermal Conductivities

The synthesis and transport properties of the family of coinage metal-stuffed Zintl compounds, Eu ₉ Cd _4-x CM _2y □ _y Sb ₉ (CM = coinage metal, □ = vacancies), is presented as a function of coinage metal substitution. Eu ₉ Cd _4-x CM _2y □ _y Sb ₉ compounds are shown to be rare examples of metallic Zintl phases with low thermal conductivities. While the lattice thermal conductivity is low, which is attributed to the complex structure and presence of interstitials, the electronic contribution to thermal conductivity is also low. In these p-type compounds, the carriers transmit less heat than expected, based on the Wiedemann-Franz law and metallic conduction, κ _e = L ₀ T/φ. Density functional theory (DFT) calculations indicate that the Fermi level resides in a pseudo-gap, which is consistent with the metallic description of the properties. While the contribution from the interstitial CM states to the Fermi level is small, the interstitial CMs are required to tune the position of the Fermi level. Analysis of the topology of electron localization function (ELF) basins reveals the multicenter Eu-Cd(CM)-Sb interactions, as the Eu and Sb states have the largest contribution at the top of the valence band. Regardless of the success of the Zintl concept in the rationalization of the properties, the representation of the CM-stuffed Eu ₉ Cd ₄ Sb ₉ structure as Eu cations encapsulated into a polyanionic (Cd/Cu)Sb network is oversimplified and underestimates the importance of the Eu-Sb bonding interactions. These results provide motivation to search for more efficient thermoelectric materials among complex metallic structures that can offer less electronic thermal conductivity without deteriorating the electrical conductivity.