Ultralow Thermal Conductivity Approaching the Disordered Limit in Crystalline TlCuZrSe₃

A comprehensive understanding of thermal transport is crucial for many applications, including heat dissipation systems, thermal barrier coatings, and harnessing potentials of thermoelectric materials. Here, we report an ultralow thermal conductivity, κ, in a p-type layered chalcogenide, TlCuZrSe₃. Our investigation reveals that the anisotropic values of κ in two perpendicular directions in this compound vary between 0.88 and 0.41 Wm^–1K^–1 and 1.15–0.62 Wm^–1K^–1, respectively, over the temperature range of 295–600 K. The low-temperature specific heat data could only be explained by considering Einstein oscillator terms in addition to the conventional Debye model-based contributions, consistent with the presence of localized Tl¹⁺ rattlers. The unique anisotropic crystal structure of TlCuZrSe₃ and the rattling of Tl¹⁺ ions lead to the generation of low-frequency phonons. These relatively flat optical phonon modes hybridize with acoustic phonons, giving rise to strong anharmonicity and phonon scattering channels. Raman spectroscopy confirms that these low-frequency phonon modes have extremely short lifetimes (∼1 ps), explaining the ultralow κ values, approaching the disordered limit, in this highly crystalline material.