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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.
Supplementary materials
Title
Supporting Information
Description
The supplementary material includes experimental sections, interatomic bond distances and bond angles from SCXRD data (Table S1), structural parameters obtained from Rietveld refinement of PXRD data (Table S2), comparison of transport properties of pristine Cu5TM2Ch7 (TM= Sn, Ge; Ch= S, Se. Te) (Table S3), Le Bail refinement plot for various sample annealed at different temperatures (Figures S1-S6), SEM-EDS analysis for selected single-crystal and polycrystalline samples (Figures S7-S8), TGA-DSC (Figure S9), PXRD pattern after exposure to 2 N HCl for 1 week (Figure S10), Crystal structure representations of various atomic connectivity (Figure S11), XPS survey (Figure S12), SPS pellets (Figure S13), Transport model plot (Figure S14), Temperature dependence of thermoelectric figure of merit (Figure S15).
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