Anharmonic and particlelike phonon propagation dominates ultralow lattice thermal conductivity in the binary quasi-2D InI phase

Understanding the ultra-low thermal conductivity and phonon transport in quasi-2D van der Waals (vdW) materials like SnSe is essential for advancing the field of thermoelectric functional materials. Our study delves into the anharmonicity and anisotropic phonon dynamics of quasi-2D InI using a comprehensive approach that combines the phonon Boltzmann equation and Wigner transport equation within a first principles framework. InI, sharing an orthorhombic crystal structure with SnSe, showcases a distinctive stacked quasi-2D layered arrangement, resulting in remarkably low averaged thermal conductivity (0.73–0.32 W/mK between 300 and 800 K) due to weak interlayer coupling and pronounced anisotropy in phonon behavior. To unravel the physical underpinnings of this anisotropic low thermal conductivity, we analyze the anisotropic particle-like κ_p and wave-like κ_c contributions. Our investigation reveals that the particle-like κ_p predominantly governs the thermal conductivity behavior. While the thermal conductivity values align reasonably well with experimental data (∼ 0.5 W/mK between 300 and 800 K), the non-linear dependency behavior, distinct from other perovskite or Zintl phase materials, cannot be solely attributed to the increasing significance of interbranch wavelike tunneling. By investigating and comprehending the dual-phonon channels, such as interbranch wavelike tunneling and particle contributions to thermal transport in ultralow thermal conductivity vdW materials like InI, we can gain deeper insights into the underlying phonon dynamics and optimize thermal properties for specific applications. This exploration opens up new avenues for discovering vdW compounds with low κ_L and tailored thermal transport characteristics.