Ultralow Thermal Conductivity in Chain Like TlSe due to Inherent Tl+ Rattling

Understanding the mechanism that correlates phonon transport with chemical bonding and solid-state structure is the key to envisage and develop materials with ultralow thermal conductivity, which are essential for efficient thermoelectrics and thermal barrier coatings. We synthesised thallium selenide (TlSe), which comprised of intertwined stiff and weakly bonded substructures, exhibits intrinsically ultralow lattice thermal conductivity (κL) of 0.62 - 0.4 W/mK in the range 300 to 573 K. Ultralow κL of TlSe is a result of its low energy optical phonon modes which strongly interacts with the heat carrying acoustic phonons. Low energy optical phonons of TlSe are associated with the intrinsic ratter-like vibration of Tl+ cations in the cage constructed by the chains of ((TlSe2)n)n-, as evident in low temperature heat capacity, terahertz (THz) time-domain spectroscopy and temperature dependent Raman spectroscopy. Density functional theoretical (DFT) analysis reveals the bonding hierarchy in TlSe which involve...