Improved Thermoelectric Performance of Cu12Sb4S13 through Gd-Substitution Induced Enhancement of Electronic Density of States and Phonon Scattering

Cu12Sb4S13 has aroused great interest because of its earth-abundant constituents and intrinsic low thermal conductivity. However, the applications of Cu12Sb4S13 are hindered by its poor thermoelectric performance. Herein, it is shown that Gd substitution not only causes a significant increase in both electrical conductivity sigma and thermopower S but also leads to dramatic drop in lattice thermal conductivity kappa(L). Consequently, large ZT reaches 0.94 at 749 K for Cu11.7Gd0.3Sb4S13, which is similar to 41% higher than the ZT value of undoped sample. Rietveld refinements of XRD results show that accompanying inhibition of impurity phase Cu3SbS4, the number of Cu vacancies increases substantially with substituted content x (x <= 0.3), which leads to reduced kappa(L) owing to intensive phonon scattering by the point defects and increased sigma arising from the charged defects (V-Cu'). Crucially, synchrotron radiation photoelectron spectroscopy reveals substantial increment of electronic density of states at Fermi level upon Gd substitution, which is proven, by our first-principle calculations, to originate from contribution of Gd 4f orbit, resulting in enhancement of S. Our study provides us with a new path to enhance thermoelectric performance of Cu12Sb4S13.