Vacancy-Ordered Double Perovskites Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials

Vacancy-ordered double perovskites (A$_2$BX$_6$), 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$_2$BI$_6$ (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using the state-of-the-art first-principles based methodologies, viz., density functional theory combined with many-body perturbation theory (G$_0$W$_0$) and spin-orbit coupling. %The phonon dispersion plots and Poisson's and Pugh's ratios show the dynamical and mechanical stability of this class of perovskites. 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 Fr\"{o}hlich mesoscopic model. % to investigate the interaction of longitudinal optical phonon modes with the carriers that strongly influence the carrier mobility. The lattice anharmonicity is further studied using {\it ab initio} molecular dynamics and electron localization function. The maximum anharmonicity is observed in Cs$_2$PtI$_6$, followed by Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$. Also, the computed average thermoelectric figure of merit ($zT$) for Cs$_2$PtI$_6$, Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$ are 0.88, 0.85, 0.95 and 0.78, respectively, which reveals their promising renewable energy applications.