Coexistence of Two Sphalerite Frameworks: A Vector for the Enhancement of Thermoelectric Performances in Zn-Doped Cu5Sn2S7

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Sphalerite-derivative copper-based sulfides have emerged in the past decade as serious candidates for cost-efficient and environmentally friendly thermoelectric applications. In order to investigate pathways to adjust the charge carrier concentration and optimize the thermoelectric properties in Cu 5 Sn 2 S 7, a recently discovered degenerate semiconductor with an ordered monoclinic structure, we have synthesized a series of Zn-substituted Cu 5– x Zn x Sn 2 S 7 sphalerite derivatives. For 0 ≤ x ≤ 0.4, a monoclinic sphalerite derivative phase is mainly formed, while for 0.4 < x ≤ 1, two sphalerite derivatives with monoclinic and tetragonal symmetries coexist. By substituting Zn for Cu in Cu 5– x Zn x Sn 2 S 7, we observe a substantial increase of the thermoelectric figure of merit zT up to 0.36 at 673 K in the biphasic region around x = 0.8. This improvement mainly arises from the strong reduction of the lattice thermal conductivity, itself driven by the coexistence of two phases with distinct sphalerite S subnetworks and enhanced cationic disorder in the tetragonal phase with respect to the monoclinic phase. The role of percolation between the grains in the electrical conductivity in the biphasic samples is also considered. The relationships between this complex crystal chemistry and the electrical and thermal transport properties are addressed in detail by using a combination of synchrotron X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, first-principles calculations, and transport property measurements.

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