Synergistic Lattice Softening and Polaron Engineering in Cu ²⁺ -Doped Cs ₂ SnCl ₆ : Decoupling Thermal and Electrical Transport for Enhanced Thermoelectric Performance

Understanding the local lattice distortion in response to charge carriers is pivotal for understanding the mechanisms of electron-phonon coupling in halide perovskites. In this work, we present synergistic lattice softening and polaron engineering in Cu²⁺-doped Cs₂SnCl₆, revealing mechanisms through which dopant-induced structural perturbations suppress lattice thermal conductivity (κlat) while simultaneously enhancing electrical conductivity (σ). Systematic analysis via temperature-dependent Raman spectroscopy and thermal conductivity studies elucidated the mechanism of phonon scattering. The Cu²⁺ doping induces chloride vacancies that modulate lattice anharmonicity and acoustic phonon contributions (κa) through tensile-stress-induced reduced bond stiffness. Conversely, the charge transport via the small polaron hopping (SPH) mechanism governs that Cu²⁺ doping reduces charge carrier activation (Ehop) from 0.57 to 0.15 eV, facilitating superior electrical conductivity. Thus, rational dopant engineering in halide perovskites enhances the charge carrier, which offers a critical strategy for maximizing the thermoelectric figure of merit (zT).