Thermoelectric Properties of Thiospinel-Type CuCo₂S₄

Eco-friendly thiospinel-type CuCo₂S₄ material has been investigated as a potential thermoelectric material. The temperature, T, dependence of electrical resistivity, ρ, of CuCo₂S₄ shows a metallic conductivity (∂ρ/∂T > 0) and a strong degenerate state, in the range of 323–723 K. Besides a high carrier concentration consistent with the metallic nature, its Hall mobility is still unexpectedly estimated to be 8.5 cm² V⁻¹ s⁻¹ at room temperature. The positive Seebeck coefficient S confirms a p-type carrier conduction. Similar to most of the transition-metal spinel chalcogenides, the S value is very low, 12–36 μV K⁻¹ at 323–723 K. As a result, a relatively low power factor PF&!thinsp;∼ 0.35 mW m⁻¹ K⁻² was obtained at 723 K. Due to the dominant role of electronic thermal conductivity, the total thermal conductivity к was high and increases with a linear dependence on T. However, the intrinsic lattice conductivity к_l was relatively low, ranging from 1.48 W m⁻¹ K⁻¹ at 323 K to 0.57 W m⁻¹ K⁻¹ at 723 K. It follows there is a T⁻¹ dependence indicative of Umklapp type phonon–phonon interaction. Importantly, the intrinsically low к_l in CuCo₂S₄ is attributed to multiple mechanisms, mainly including the large unit cell with primarily octahedral coordination, the high distortion and complexity of the structure, and additional interfacial thermal resistance.