Phosphide–Tetrahedrite Ag₆Ge₁₀P₁₂: Thermoelectric Performance of a Long-Forgotten Silver-Cluster Compound

The air-stable phosphide, Ag₆Ge₁₀P₁₂, was synthesized from its elements in gram amounts. As its structure is closely related to high-performance thermoelectric tetrahedrites (Ag₆□Ge₄Ge₆P₁₂ ≡ Cu₆SSb₄Cu₆S₁₂), we studied temperature dependent single-crystal X-ray diffraction experiments, quantum chemical calculations, and thermoelectric transport properties of spark plasma sintered and pristine, single crystalline samples, in order to give a comprehensive picture of its thermoelectric performance and its origin. The semiconducting character of this material is reflected in band structure calculations. Measurements of the thermal diffusivity exhibit a very low thermal conductivity, κ < 1 W m^–1 K^–1, which is close to a phonon glass-like behavior, and has its origin in a strong local bonding asymmetry, induced by strong bonding of the phosphorus–germanium (Ge⁴⁺) covalent framework and weak bonding of lone-pair electrons (Ge²⁺). This chemical bond hierarchy creates a pronounced anisotropic behavior of the silver atoms leading to low-frequency vibrations and thermal damping. Combining this with a moderate electrical resistivity (ρ ∼ 15 mΩ cm) and a high Seebeck coefficient (S ∼ 380 μV K^–1) results in a remarkably high figure of merit (zT) of about 0.6 at 700 K. These results demonstrate that Ag₆Ge₁₀P₁₂ is one of the best thermoelectric phosphides and a promising new platform for the future development of thermoelectrics.