Non-Equilibrium Phase-Pure Synthesis and Thermal Transport Properties of Metallic Telluride Cu5Ge2Te7

Tellurides are of increasing interest owing to their emerging applications in energy and electronics, as well as their diverse structural chemistry. We introduce Cu 5 Ge 2 Te 7 , a novel ternary telluride, synthesized using traditional high-temperature synthesis and non-equilibrium methods. This is the first report on the Ge-based phase of the Cu 5 Sn 2 Ch 7 ( Ch : S, Se, Te) family, which has been extensively studied. The phase-pure Cu 5 Ge 2 Te 7 was synthesized by heating for 30-120 seconds using Direct Joule heating 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 traditional 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, and such a Ge–Te interaction has been rarely reported in inorganic systems so far. Electronic structure calculation shows that Cu 5 Ge 2 Te 7 is a poor metal, as a finite density of states is present at the Fermi level. Bonding analysis displays that the [Ge 2 Te 7 ]-unit in the structure exhibits Zintl-phase-like electronic features, though the overall electronic structure indicates that this 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, making it a suitable candidate for thermal insulation applications.