Accessing ultralow thermal conductivity in chalcogen perovskite CaSnS₃: Harnessing wave-like phonon tunnelling and significant anharmonicity

There has been an endless stream of possible candidates developed by the quest for high-performance thermoelectric materials; however, materials with intrinsically low lattice thermal conductivity are especially desirable [1]. In this study, we looked at the new emerging perovskite family: chalcogen perovskite CaSnS₃ in orthorhombic phase for intrinsically low lattice thermal conductivity (κ_l) [2]. The structural and mechanical properties suggest the ionic-covalent bond and ductile nature. The direct semiconducting band gap of 1.7 eV is noticed and could avoid the bipolar effect. The interatomic interactions in CaS₈ bicapped trigonal prism geometry are noticed weak, leading to the bond heterogeneity and thus an effective anharmonicity that facilitates the low κ_l. Phonon transport and lattice thermal conductivity are significantly impacted by the strong anharmonicity caused by the bond heterogeneity in the crystal structure. Combining the quartic anharmonicity-based self-consistent phonon theory and the unified theory of lattice thermal transport employing particle and wavelike propagation of phonons, a lower value of average lattice thermal conductivity at 300 K is found at ~ 1.18 W m^-1K^-1, while at 800 K it further reduces to a lower value of ~ 1.0 W m^-1K^-1. Our findings emphasize the relevance of lattice anharmonicity and wave-like tunnelling of phonons in thermal transport in chalcogen perovskite.