Rational Design of Multi-Polyhedral Units-Induced Hybrid Bonding and Quadruple-Valley Band Structure in Ag _0.5 CdBi _4.5 Se ₈ for Enhanced Thermoelectric Performance

We report a high-performance thermoelectric pavonite compound, Ag 0.5 CdBi 4.5 Se 8, featuring a unique quasi-superlattice structure assembled from five distinct polyhedral units ([AgSe6], [CdSe6], [(Bi1)Se5], [(Bi2)Se6], and [(Bi3)Se6]). Naturally arranged Bi–Se polyhedra with different distortions and crystallographic environments enable multidirectional orbital overlap, forming a quadruple-valley conduction band with a small energy separation of 0.07 eV. This electronic structure simultaneously enhances carrier transport and maintains a high Seebeck coefficient. Moreover, the inherently hybrid bonding network, which consists of alternating strong and weak bonds alongside coexisting ionic and covalent characters, combined with pronounced acoustic-optical phonon coupling originating from the [AgSe6]/[CdSe6] octahedra, results in an extremely low lattice thermal conductivity of 0.24 W m –1 K –1 at 823 K. While intrinsic Se vacancies render the pristine material an n-type degenerate semiconductor, Sb doping and Se-excess allow precise tuning of carrier concentration over a wide range (2.39 × 10 19 –4.81 × 10 20 cm –3 ), enabling further optimization of electrical transport. At an optimal carrier concentration of 2.52 × 10 20 cm –3, Ag 0.5 CdBi 4.5 Se 8 achieves a peak thermoelectric ZT of 0.96 at 823 K, outperforming most previously reported pavonite derivatives. This work validates the utilization of the compound’s intriguing multipolyhedral integration as a robust strategy to decouple electron–phonon transport, thereby providing insights for the rational design of high-performance thermoelectric materials.