Synthesis, Crystal Structure, and Transport Properties of the Hexagonal Mo₉ Cluster Compound Ag₃RbMo₉Se₁₁

Mo-based cluster compounds are promising candidates for thermoelectric applications at high temperatures due to their very low lattice thermal conductivity values. Here, we report on a detailed investigation of the crystal structure and transport properties measured in a wide range of temperatures (2-800 K) of polycrystalline Ag₃RbMo₉Se₁₁. Single-crystal X-ray diffraction shows that this compound crystallizes in the hexagonal space group P6₃/m. The crystal structure is formed by stacked Mo₉Se₁₁ units leaving channels that are randomly filled by Rb⁺ cations, while Ag⁺ cations are located between the Mo₉Se₁₁ units. The high disorder in the unit cell induced by these atoms and their large anisotropic thermal displacement parameters are two key characteristics that lead to very low lattice thermal conductivity as low as 0.6 W m^-1 K^-1 at 800 K. The combination of semiconducting-like electrical properties and low ability to transport heat leads to a maximum dimensionless thermoelectric figure of merit ZT of 0.4 at 800 K.