Understanding the nature of chemical bonding and lattice dynamics together with their influence on phonon-transport is essential to explore and design crystalline solids with ultralow thermal conductivity for various applications including thermoelectrics. TlInTe, with interlocked rigid and weakly bound substructures, exhibits lattice thermal conductivity as low as ca. 0.5 W/mK near room temperature, owing to rattling dynamics of weakly bound Tl cations. Large displacements of Tl cations along the c-axis, driven by electrostatic repulsion between localized electron clouds on Tl and Te ions, are akin to those of rattling guests in caged-systems. Heat capacity of TlInTe exhibits a broad peak at low-temperatures due to contribution from Tl-induced low-frequency Einstein modes as also evidenced from THz time domain spectroscopy. First-principles calculations reveal a strong coupling between large-amplitude coherent optic vibrations of Tl-rattlers along the c-axis, and acoustic phonons that likely causes the low lattice thermal conductivity in TlInTe.

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 synthesized thallium selenide (TlSe), which is comprised of intertwined stiff and weakly bonded substructures and exhibits intrinsically ultralow lattice thermal conductivity (κL) of 0.62–0.4 W/mK in the range 295–525 K. Ultralow κL of TlSe is a result of its low energy optical phonon modes which strongly interact with the heat carrying acoustic phonons. Low energy optical phonons of TlSe are associated with the intrinsic rattler-like vibration of Tl+ cations in the cage constructed by the chains of (TlSe2) n n–, as evident in low temperature heat capacity, terahertz time-domain spectroscopy, and temperature dependent Raman spectroscopy. Density functional theoretical analysis reveals the bonding hierarchy in TlSe which involves ionic interaction in Tl+–Se while Tl3+–Se bonds are covalent, which causes significant lattice anharmonicity and intrinsic rattler-like low energy vibrations of Tl+, resulting in ultralow κL.

Colloidal TlI and TlBr nanocrystals are prepared, which show violet-blue luminescence, high carrier mobility and long diffusion lengths thus suggesting the suppression of deep-trap states.