X‑ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag₂Tl₂Mo₉Se₁₁

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag₂Tl₂Mo₉Se₁₁. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo₉Se₁₁ units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m^–1·K^–1. The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.