Hakite: solid-state synthesis and thermoelectric performance

Hakite is a promising thermoelectric material owing to its low theoretically calculated lattice thermal conductivity. However, experimental studies and data on the thermoelectric properties of hakite have not yet been reported. In this study, the solid-state synthesis of pristine hakite (Cu₁₂Sb₄Se₁₃) and charge-compensated hakite (Zn₂Cu₁₀Sb₄Se₁₃) was attempted using mechanical alloying (MA) and hot pressing (HP). Pure hakite could not be synthesized; instead, a large amount of permingeatite (Cu₃SbSe₄) and a small amount of bytizite (Cu₃SbSe₃) were produced. However, when Zn²⁺ compensates the charge at the Cu⁺ site, Zn-hakite (Zn₂Cu₁₀Sb₄Se₁₃) can be synthesized. The changes in the thermoelectric properties with respect to the MA–HP process conditions were investigated. As the measurement temperature increased, non-degenerate semiconductor behavior was observed, and the electrical conductivity increased. As the HP temperature increased, the electrical conductivity decreased but became saturated at high temperatures. The Seebeck coefficient exhibited positive values in the measured temperature range, indicating that Zn-hakite is a p-type semiconductor. When the measurement temperature was increased, an intrinsic transition occurred; thus, the Seebeck coefficient reached a peak value at a certain temperature. When the HP temperature increased, the onset temperature of the intrinsic transition decreased. The thermal conductivity had minor temperature dependence in the measured temperature range, mainly owing to the lattice thermal conductivity; however, the electronic thermal conductivity was not significant owing to the low carrier concentration. A maximum power factor of 0.06 mWm⁻¹ K⁻² and the highest dimensionless figure of merit (ZT) of 0.10 were obtained at 623 K.