Vacancy-Ordered Double Perovskites Cs 2 BI 6 (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials

Vacancy-ordered double perovskites (A BX ), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much, despite their potential applications. Here, we explore Cs BI (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using state-of-the-art first-principles-based methodologies, viz., density functional theory combined with many-body perturbation theory (G W ) and spin-orbit coupling. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom, leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well-captured by the Fröhlich mesoscopic model. The lattice anharmonicity is further studied using molecular dynamics and the electron localization function. The maximum anharmonicity is observed in Cs PtI , followed by Cs PdI , Cs TeI , and Cs SnI . Also, the computed average thermoelectric figure of merit ( ) for Cs PtI , Cs PdI , Cs TeI , and Cs SnI is 0.88, 0.85, 0.95, and 0.78, respectively, which reveals their promising renewable energy applications.