Glassy Ultralow Thermal Conductivity in Cs2TiX6 (X = Cl, Br, I) Arises from Bond Weakening

Abstract

Fundamental understanding of the interplay between chemical bonding and lattice dynamics is essential for manipulating phonon transport in crystalline solids. Vacancy-ordered halide perovskites, characterized by their intrinsically glass-like lattice dynamics and ultra-low lattice thermal conductivity (κL ), offer a compelling platform to tailor phonon transport through deliberate halogen substitution. In this work, we employed first-principles calculations that explicitly incorporate four-phonon scattering and wave-like phonon tunneling to investigate the lattice dynamics and phonon transport properties in the Cs2TiX6 (X = Cl, Br, I) halide perovskites. Our research shows that heavier halogen atoms, although possessing a stronger covalent character, paradoxically weaken the bonding strength of the TiX6 octahedra. The weakened bonding in Cs2TiX6 concurrently suppresses particle-like heat conduction and enhances the wavelike tunneling contribution. Consequently, Cs2TiI6 demonstrates an ultralow κL of 0.27 W/mK at room temperature and the weakest temperature dependence, characterized by T-0.23 . Visualization of the vibrational modes in Cs2TiI6 reveals that dynamic rotations of the halogen framework introduce pronounced anharmonicity, which suppresses particle-like heat transport. At the same time, weakly bound atoms exhibit large mean square displacement at equilibrium and generate strong local rattling vibrations, thereby enhancing the wave-like contribution to total κL. Our findings elucidate how chemical bonding governs the glassy ultra-low thermal conductivity in vacancy-ordered halide perovskites and provide strategic guidance for designing novel perovskite materials for energy-conversion applications.