Applied Physics Letters, Volume 118, Issue 14, April 2021. Quaternary compound Ag2BaSnSe4 has ultralow thermal conductivity. However, the physical reasons for it remain ambiguous. By using the first principles calculations combined with the Boltzmann phonon transport equation and inelastic neutron scattering theory, we find that the square-cylinder cage-like structure with rattling vibration is the most important objective reason. The vibrations of rattling Ag atoms cause hybridization and softening of phonon branches and, thus, lead to low specific heat and especially low group velocity, accounting for ultralow lattice thermal conductivity directly. We find that the thermal Grüneisen parameter seriously deviates from acoustic and elastic Grüneisen parameters. We consider that the lattice thermal conductivity is related to the acoustic/elastic Grüneisen parameter rather than the thermal Grüneisen parameter. Our research not only explains the physical origin for ultralow lattice thermal conductivity of compound Ag2BaSnSe4 but also will inspire much research on cage-like structures.

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