Distorted Polyhedral Architecture Enabled High Thermoelectric Performance of Columnar Double Halide Perovskites Cs ₂ AgPdCl ₅ and Cs ₂ AgPtCl ₅

We investigate the thermoelectric properties of two newly synthesized columnar double halide perovskites Cs 2 AgPdCl 5 and Cs 2 AgPtCl 5 . These materials accommodate a distorted local polyhedral architecture with tetrahedral symmetry compared to traditional double halide perovskites. By employing density functional theory along with the semiclassical transport model, we have analyzed the electronic and transport properties of these materials. Our results show that at 800 K, the largest figure of merit (zT) is 1.30(0.86) for p-type (n-type) Cs 2 AgPdCl 5 and 0.87 for n-type Cs 2 AgPtCl 5 at doping concentrations of 1.94 × 10 20 (3.76 × 10 19 ) cm –3 and 3.52 × 10 19 cm –3, respectively. Remarkably, a very low doping concentration is required to achieve a high zT, setting these materials apart from others in this field. Our calculations demonstrate that Cs 2 AgPdCl 5 benefits from the presence of conduction and valence band valleys near the band edges; however, the flat bands present in the valence band of Cs 2 AgPtCl 5 do not improve its thermoelectric performance. Among these systems, the hole-doped Cs 2 AgPdCl 5 has shown remarkable thermoelectric performance. Interestingly, the local octahedral distortions present in these perovskites contribute to a marked reduction in the lattice thermal conductivity to be 0.27 W/mK in Cs 2 AgPdCl 5 and 0.20 W/mK in Cs 2 AgPtCl 5 by causing enhanced scattering of phonons, further improving the thermoelectric figure of merit. This drop in thermal conductivity, combined with the favorable electronic properties, underscores the potential use of these materials for applications in highly efficient thermoelectric devices.