Unravelling the Beneficial Influence of Ag insertion on the Thermoelectric Properties of the Cluster Compound K₂Mo₁₅Se₁₉

Mo-based cluster compounds are an emerging class of potential candidates for thermoelectric applications because of several key crystallographic characteristics that drive their ability to transport heat close to that of amorphous systems. Here, we report on a detailed investigation of the crystal structure and high-temperature transport properties of the cluster compound K₂Mo₁₅Se₁₉ and of its Ag-filled variant Ag₃K₂Mo₁₅Se₁₉. Single-crystal X-ray diffraction confirms that both compounds crystallize with a hexagonal crystal structure, successfully solved in the space group R3c, built by octahedral Mo₆Se₈ and bioctahedral Mo₉Se₁₁ clusters with K and Ag cations filling the large intercluster voids. In agreement with electronic band structure calculations, the electron transfer that occurs from the filling cations to the cluster subnetwork provides a simple guiding rule to predict the evolution of the transport properties upon filling with Ag. The metallic state observed in K₂Mo₁₅Se₁₉ is turned into a more heavily doped semiconducting character in the presence of Ag resulting in enhanced thermopower values in Ag₃K₂Mo₁₅Se₁₉. The beneficial influence of Ag on the thermoelectric properties is also reflected by lowered lattice thermal conductivity values that reach 0.45 W m^–1 K^–1 at 750 K. These results indicate that inserting an additional element in the ternaries M₂Mo₁₅Se₁₉ is an interesting route to improving their thermoelectric properties.