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.