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Strong Chemical Bond Hierarchy Leading to Exceptionally High Thermoelectric Figure of Merit in Oxychalcogenide AgBiTeO.

The chemical bond hierarchy (CBH) in prototype cage structures has been considered important for achieving high thermoelectric performance. By performing first principles calculations and lattice dynamics, we demonstrate CBH hosted distinct rattlers in a non-caged oxychalcogenide AgBiTeO, cause ultralow κl of 0.9 W/m-K at room temperature. The CBH in this compound leads to a unique structural bonding, where Ag and Te are loosely bonded to the rigid framework of the lattice and form distorted four-centered Ag-Te tetrahedra. These clusters exhibit large atomic vibrational motions in a very shallow potential energy surface resulting into rattling motion. The presence of multiple avoided crossing points of low lying optical mode with longitudinal acoustic mode in phonon dispersion further confirms the rattling induced thermal damping. Additionally, unique in-plane off phase collective vibrations of Ag- Te tetrahedra introduce localized flat phonon dispersions that lower the group velocity and significantly reduce the lattice thermal conductivity. Most importantly, it prevents carrier-phonon scattering leading to a high electrical conductivity in AgBiTeO. Combination of intrinsic low lattice thermal conductivity and excellent electronic transport properties give an unprecedented range of ZT from 1.00 to 1.99 in the large temperature range of 700-1200K, for n-type charge carriers.