The role of excess Sn in Cu4Sn7S16 for modification of the band structure and a reduction in lattice thermal conductivity

Abstract

In this work, we have investigated the band structures of ternary Cu₄Sn_7+xS₁₆ (x = 0–1.0) compounds with an excess of Sn, and examined their thermoelectric (TE) properties. First principles calculations reveal that the excess Sn, which exists as Sn²⁺ and is preferentially located at the intrinsic Cu vacancies, unpins the Fermi level (F_r) and allows F_r to enter the conduction band (CB) at x = 0.5. Accordingly, the Hall carrier concentration (n_H) is enhanced by about two orders of magnitude when the x value increases from x = 0 to x = 0.5. Meanwhile, the lattice thermal conductivity (κ_L) is reduced significantly to 0.39 W K⁻¹ m⁻¹ at 893 K, which is in reasonably good agreement with the estimation using the Callaway model. As a consequence, the dimensionless TE figure of merit (ZT) of the compound Cu₄Sn_7+xS₁₆ with x = 0.5 reaches 0.41 at 863 K. This value is double that of the stoichiometric Cu₄Sn₇S₁₆, proving that excess Sn in Cu₄Sn₇S₁₆ is beneficial for improving the TE performance.