Vacancy-Ordered Double Perovskites Cs₂BI₆ (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials

Vacancy-ordered double perovskites (A₂BX₆), 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 Cs₂BI₆ (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 (G₀W₀) 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 ab initio molecular dynamics and the electron localization function. The maximum anharmonicity is observed in Cs₂PtI₆, followed by Cs₂PdI₆, Cs₂TeI₆, and Cs₂SnI₆. Also, the computed average thermoelectric figure of merit (zT) for Cs₂PtI₆, Cs₂PdI₆, Cs₂TeI₆, and Cs₂SnI₆ is 0.88, 0.85, 0.95, and 0.78, respectively, which reveals their promising renewable energy applications.