Non-Equilibrium Synthesis of Phase-Pure Cu5Ge2Te7

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Abstract
Tellurides attract enormous attention among the scientific community due to their emerging energy, electronic applications, and diverse crystal chemistry. We report a new ternary telluride, Cu5Ge2Te7, synthesized via both traditional high-temperature synthesis and direct Joule heating synthesis (DJS). The phase was discovered within the product mixture formed by the traditional high-temperature method. An ongoing challenge lies in synthesizing high-quality bulk tellurides, as traditional synthesis requires prolonged annealing at elevated temperatures, offers limited control over reactions, and frequently yields impurity phases. Herein, a phase-pure product was obtained in two minutes using DJS method. Moreover, DJS was used for the first time to develop an ex-situ “panoramic” reaction map that provides insight into the phase evaluation across different temperatures during the heating process. The structure of the new phase was characterized using X-ray diffraction techniques on the crystals grown by traditional high-temperature synthesis. The structure adopts the Cu5Sn2Se7-type and consists of corner-sharing distorted Te4-tetrahedral units surrounding copper and germanium atoms. Moreover, it is quite striking that the germanium atoms reside in tellurium tetrahedra, which are condensed into [Ge2Te7] dimers. Electronic structure calculation shows that the compound is a poor metal, as a finite density of states is present at the Fermi level. Bonding analysis displays that the [Ge2Te7]-unit in the structure exhibits Zintl-phase-like electronic features, though the overall electronic structure indicates that this Cu5Ge2Te7 is not a true Zintl phase. The transport measurements reveal that the compound exhibits a metal-like behaviour and has an ultralow lattice thermal conductivity of 0.4 W·m-1·K-1 at 683 K.

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