Role of M-Site Cation Substitution on Phonon Rattling, Carrier Scattering, and Thermoelectric Performance in Vacancy-Ordered Double Perovskites Cs2MBr6 (M = Te, Pt)

Lead-free vacancy-ordered double perovskites Cs2MBr6 offer a compelling structural template for sustainable thermoelectric energy harvesting due to their intrinsically low lattice thermal conductivity, driven by rattling-like dynamics of under-coordinated Cs and Br sublattices. Using first-principles density functional theory coupled with semiclassical Boltzmann transport and electron–phonon Wannier (EPW) calculations, we demonstrate that M-site orbital engineering serves as an effective design strategy to dramatically boost thermoelectric performance. Comparative analysis reveals that replacing the main-group Te atom with transition-metal Pt introduces strong Pt-d–Br-p hybridization near the valence band edge, simultaneously increasing the density-of-states effective mass and suppressing electron–phonon coupling strength. Consequently, Cs2PtBr6 exhibits hole relaxation times approximately twice as long as Cs2TeBr6 across 300–900 K, alongside an ultralow lattice thermal conductivity of 0.47 W m–1 K–1 at 300 K. Driven by these synergistic electronic and thermal transport enhancements, p-type Cs2PtBr6 achieves an exceptional peak figure of merit (zT) of 2.02 significantly outperforming Cs2TeBr6 (zT = 0.67) at 900 K. These findings highlight the key role of d-orbital hybridization in mitigating carrier scattering, providing clear design principles for high-efficiency lead-free halide perovskite thermoelectrics.