Phase-Pure Synthesis and Thermal Transport Properties of Metallic Telluride Cu ₅ Ge ₂ Te ₇

Tellurides are gaining attention due to their applications in energy and electronics, as well as their diverse structural chemistry. The telluride, Cu 5 Ge 2 Te 7, was synthesized using traditional high-temperature and nonequilibrium methods. Cu 5 Ge 2 Te 7 is a Ge-based phase of Cu 5 Sn 2 Ch 7 ( Ch: S, Se, Te), whose Sn-based systems have been extensively studied. The phase-pure Cu 5 Ge 2 Te 7 was synthesized by heating for 30–120 s using Direct Joule Synthesis (DJS), followed by quenching. DJS was used to develop an ex-situ “panoramic” reaction map that provides insight into phase formation at different DJS temperatures. The structure of Cu 5 Ge 2 Te 7 was characterized by X-ray diffraction techniques on the crystals grown by high-temperature synthesis. The structure consists of corner-sharing distorted Te 4 -tetrahedral units surrounding copper and germanium atoms. The germanium atoms reside in tellurium tetrahedra, which condense into [Ge 2 Te 7 ]-dimers. Electronic structure calculation shows that Cu 5 Ge 2 Te 7 is a poor metal, with a finite density of states at the Fermi level. Bonding analysis indicates that the [Ge 2 Te 7 ]-unit exhibits Zintl-phase-like electronic features, though the overall electronic structure of Cu 5 Ge 2 Te 7 is not a true Zintl phase. Transport measurements reveal an ultralow lattice thermal conductivity of 0.4 W·m –1 ·K –1 at 683 K.