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