  Chalcogenide perovskites provide a promising avenue for non-toxic, stable
thermoelectric materials. Here, thermal transport and thermoelectric properties
of BaZrS$_3$ as a typical orthorhombic perovskite are investigated. An
extremely low lattice thermal conductivity $\kappa_L$ of 1.84 W/mK at 300 K is
revealed for BaZrS$_3$, 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 $\kappa_L$, arising from wave-like phonon
tunneling, leading to a 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$_3$ and ultimately
improves the particle-like thermal transport. Further, by replacing S atom with
Se and Ti-alloying strategy, $ZT$ value of BaZrS$_3$ is significantly increased
from 0.58 to 0.91 at 500 K, making it an important candidate for thermoelectric
applications.
