Exceptional Thermoelectric Performance of Naturally High‐Entropy Pavonite Ag ₃ Pb ₄ Bi ₁₁ Se ₂₂ with Unique Quasi‐Superlattice Structure and Sextuple Conduction Valleys

ABSTRACT Developing high‐performance thermoelectric materials with intrinsically low lattice thermal conductivity and tunable electronic properties is critical for advancing efficient energy conversion technologies toward global sustainability. Herein, we report a pavonite compound Ag ₃ Pb ₄ Bi ₁₁ Se ₂₂ integrating a unique quasi‐superlattice architecture and naturally occurring quasi‐high‐entropy alloy characteristics. These structural features synergistically induce intense phonon scattering yielding an ultra‐low lattice thermal conductivity of 0.32‐0.57 W m ⁻¹ K ⁻¹ over the temperature range of 300–773 K. First‐principles calculations reveal that Ag ₃ Pb ₄ Bi ₁₁ Se ₂₂ is a narrow‐bandgap semiconductor with a multi‐valley conduction band featuring up to six closely spaced energy valleys. By tailoring the carrier concentration through controlled Se deficiency, the Fermi level is optimized to activate multi‐valley cooperative transport, significantly enhancing the band degeneracy from 2 to 10 and the density of states effective mass from 0.6 m ₀ to 1.8 m ₀ . This optimization mitigates the degradation of the Seebeck coefficient at high carrier concentrations while simultaneously improving electrical conductivity. Ultimately, the Ag ₃ Pb ₄ Bi ₁₁ Se _21.9 sample attains a maximum thermoelectric figure of merit ( ZT ) of 0.9 at 773 K. This work establishes Ag ₃ Pb ₄ Bi ₁₁ Se ₂₂ as a benchmark pavonite‐based thermoelectric material and offers a viable paradigm for optimizing complex chalcogenides via integrated structural engineering and band structure modulation.