High-performance thermoelectric materials provide an environmentally friendly route for future large-scale electricity production from waste heat recovery. In this work, we present an in-depth analysis of electronic, phononic, and thermoelectric properties of vacancy-ordered double perovskite Cs2SnX6 (X = Br, I) compounds based on generalized gradient approximations (GGA) and post-density functional theory treatments. We find that the optical phonon scattering mechanism plays a more critical role than the acoustical phonon and impurity scattering in both materials’ transport properties. These materials also exhibit a relatively large Seebeck coefficient and ultra-low lattice thermal conductivity about 0.123/0.062 W/mK for Cs2SnBr6/Cs2SnI6 at room temperature calculated by single relaxation time model (SMRT). Such ultra-low lattice thermal conductivity arises from significant phonon-phonon scattering due to highly overlapped acoustic and optical phonon branches around ~1 THz. This occurrence introduces the Cs atom’s rattling motion inside [SnX6]2- (X = Br, I) cages. The most considerable ZT values with the SMRT method at 300/500 K are about 0.32/0.82 for Cs2SnBr6 and 0.9/1.96 for Cs2SnI6. Our theoretical results suggest that both compounds have a great potential to be good candidates for thermoelectric materials and can be used to guide experimental investigation to obtain the optimal ZT performance.
