Crystalline materials with ultralow thermal conductivity are essential for thermal barrier coating and thermoelectric energy conversion. Nontoxic n-type bulk cubic AgBiS2 exhibits exceptionally low lattice thermal conductivity (κlat) of 0.68-0.48 W/mK in the temperature range 298-820 K, which is near to the theoretical minimum (κmin). The low κlat is attributed to soft vibrations of predominantly Ag atoms and significant lattice anharmonicity due to local structural distortions along the [011] direction, arising due to the stereochemical activity of the 6s2 lone pair of Bi, as suggested by pair distribution function (PDF) analysis of the synchrotron X-ray scattering data. Low temperature heat capacity of AgBiS2 shows a broad hump due to the Ag-induced low energy Einstein modes as also suggested from phonon dispersion calculated by first principle density functional theory (DFT). Low energy optical phonons contributed by Ag and Bi strongly scatter heat carrying acoustic phonons, thereby decreasing the κlat to a low value. A maximum thermoelectric figure of merit of ~0.7 is attained at 820 K for bulk spark plasma sintered n-type AgBiS2

Origin of ultra-low thermal conductivity in n-type cubic bulk AgBiS2: Soft Ag vibrations and local structural distortion induced by Bi 6s2 lone pair

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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