Ultralow Lattice Thermal Transport and Considerable Wave-like Phonon Tunneling in Chalcogenide Perovskite BaZrS.

Chalcogenide perovskites provide a promising avenue for nontoxic, stable thermoelectric materials. Here, the thermal transport and thermoelectric properties of BaZrS as a typical orthorhombic perovskite are investigated. An extremely low lattice thermal conductivity κ of 1.84 W/mK at 300 K is revealed for BaZrS, due to the softening effect of Ba atoms on the lattice and the strong anharmonicity caused by the twisted structure. We demonstrate that coherence contributions to κ, arising from wave-like phonon tunneling, lead to an 18% thermal transport contribution at 300 K. The increasing temperature softens the phonons, thus reducing the group velocity of materials and increasing the scattering phase space. However, it simultaneously reduces the anharmonicity, which is dominant in BaZrS and ultimately improves the particle-like thermal transport. In addition, via replacement of the S atom with Se- and Ti-alloying strategy, the value of BaZrS is significantly increased from 0.58 to 0.91 at 500 K, making it an important candidate for thermoelectric applications.