Octahedral Tilting and Cs‐Rattling Induce High Thermoelectric Performance in a Lead‐Free Vacancy‐Ordered Halide Perovskite

We investigated the structural, mechanical, and thermoelectric properties of lead‐free vacancy‐ordered double halide perovskites Cs 2 TeX 6 (X = Cl, Br, I) using first‐principles calculations combined with semiclassical Boltzmann transport theory. The electronic structure was calculated using the PBE functional including spin–orbit coupling (SOC), and the bandgaps were corrected using the HSE06 hybrid functional through a scissor operator approach. The calculated bandgaps are 3.27, 2.50, and 1.55 eV for Cs 2 TeCl 6 , Cs 2 TeBr 6 , and Cs 2 TeI 6 , respectively. The electronic band structure of Cs 2 TeI 6 exhibits a coexistence of light and heavy bands near the band edges, which favors a balance between a high Seebeck coefficient and enhanced electrical conductivity. As a result, Cs 2 TeI 6 shows superior thermoelectric performance with a maximum figure of merit ZT 1.97 at 800 K for an electron concentration of 3.35 10 . The high ZT primarily originates from its low lattice thermal conductivity ( 0.41 W at 300 K), arising from weak Te‐I bonding, localized TeX 6 octahedra, and enhanced phonon scattering associated with transverse acoustic and optical phonon modes. These findings highlight Cs 2 TeI 6 as a promising candidate for thermoelectric energy conversion.