Effects of Sb Deviation from Its Stoichiometric Ratio on the Micro- and Electronic Structures and Thermoelectric Properties of Cu₁₂Sb₄S₁₃

Thermoelectric material tetrahedrite Cu₁₂Sb₄S₁₃ has attracted much attention because of its intrinsic low lattice thermal conductivity, excellent electrical transport property, and environment-friendly constituents. However, its thermoelectric figure merit, ZT, is limited because of the low Seebeck coefficient (S) and power factor (PF). Hence, it is indispensable to enhance its S and PF to increase its ZT. Here, we show that when Sb deviation from its stoichiometric ratio in the Cu₁₂Sb₄S₁₃ band structure is modulated, it gives rise to increased density of states and enhancement of the Seebeck coefficient. Moreover, carrier concentration is tuned by changing sulfur and copper vacancies through controlling the Cu₃SbS₄ phase with an atomic ratio of Sb, leading to increased electrical conductivity. In addition, as large as ∼60% reduction of lattice thermal conductivity is obtained by intensified phonon scattering using an impurity phase/element and vacancy-like defects induced by different Sb contents. As a result, a high ZT = 0.86 is achieved at 723 K for the Cu₁₂Sb_4+δS₁₃ sample with δ = 0.2, which is ∼50% larger than that of stoichiometric Cu₁₂Sb₄S₁₃ studied here, indicating that ZT of Cu₁₂Sb₄S₁₃ can be improved through simple modulation of the Sb stoichiometric ratio.