Semiconducting compounds Сu₂CoGeS₄, Cu₂CoGeSe₄, Cu₂CoSnS₄, Cu₂CoSnSe₄: synthesis, structural, and thermoelectric properties

In this paper we report on the synthesis of polycrystalline Cu₂CoGeS₄, Cu₂CoGeSe₄, Cu₂CoSnS₄, and Cu₂CoSnSe₄ compounds and the investigation of their structural and thermoelectric properties. For each sample, the unit cell parameters and melting temperature were determined, and the sound velocity, X-ray density, and Debye temperature were calculated. Cu₂CoGeS₄, Cu₂CoSnS₄, Cu₂CoSnSe₄ crystallize in a tetragonal stannite structure (a/c ≈ 2), Cu₂CoGeSe₄ exhibits an orthorhombic structure. The thermoelectric properties of the compounds researched change in a predictable manner over the range 298–523 K. The Seebeck coefficient increases with temperature for all compounds; Cu₂CoSnSe₄ exhibits the lowest values in the series (135→195 µV/K), whilst Cu₂CoGeS₄ and Cu₂CoSnS₄ exhibit the highest (307→384 µV/K, respectively). The specific electrical resistivity decreases monotonically: from 0.92 (Cu₂CoSnSe₄) to 3.41 (Cu₂CoSnS₄) Ω · cm at 298 K to 0.13 (Cu₂CoSnSe₄) − 0.35 (Cu₂CoSnS₄) Ω · cm at 523 K. Thermal conductivity varies from 2.12 (Cu₂CoGeSe₄) to 3.50 (Cu₂CoSnS₄) W/(m · K) at 298 K and 1.80 (Cu₂CoGeSe₄) − 3.70 (Cu₂CoGeS₄) W/(m · K) at 523 K. The high Seebeck coefficient values combined with low electrical resistivity at elevated temperatures indicate the potential of these compounds for thermoelectric converter applications. The compounds Cu₂CoGeS₄, Cu₂CoGeSe₄, Cu₂CoSnS₄, and Cu₂CoSnSe₄ consist of non-toxic and earth-abundant elements, which meets modern eco-friendliness requirements for thermoelectric materials and opens up possibilities for their practical use in thermoelectric devices.

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