Ultralow Thermal Conductivity in Chain-like TlSe Due to Inherent Tl⁺ Rattling

Abstract

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.

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n‐Type BiTe

“…We have derived lattice thermal conductivity of BiTe by subtracting the electrical thermal conductivity ( κ el , Figure S7(c)) from κ tot . BiTe exhibits intrinsically low κ lat along both the SPS ∥ and SPS ⊥ directions (Figure a), which is close to its theoretical minimum ( κ min ≈0.25 W m −1 K −1 ) calculated by Cahill's formula along the layer stacking direction (Γ–A direction) . However, BiTe exhibits lower κ lat (ca.…”

“…However, these extrinsic strategies also lead to charge carrier scattering, resulting in reduced charge carrier mobility ( μ ) . Hence, identification of crystalline solids with intrinsically low κ lat or development of novel strategies to decrease κ lat such as introduction of bonding hierarchy, and ferroelectric instability are important in designing high performance TE materials.…”

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n‐Type BiTe

“…This acoustic-optical phonon mode-coupling increases the scattering phase-space of phonons leading to reduced phonon lifetime and hence,alow k lat .T he anharmonicity of am aterial plays an important role in enhancing the phonon-scattering rates and thereby inducing al ow k lat in the material. [16,18,19,55] Thed egree of intrinsic anharmonicity in BiTeh as been assessed by evaluating the mode Grüneisen parameters (g qu )a te ach q point and for every phonon mode u as function of phonon frequency( w) (Figure 3e). It reveals very high g qu for the acoustic (maximum g qu % 11) and the low frequency optical (g qu % 5) phonon branches.…”

“…directions (Figure 3a), which is close to its theoretical minimum (k min % 0.25 Wm À1 K À1 )c alculated by Cahillsf ormula along the layer stacking direction (G-A direction). [19,49] However,B iTe exhibits lower k lat (ca. 0.47 Wm À1 K À1 at 300 K) along SPS k compared to that of SPS ?…”

“…[6][7][8][9] However,t hese extrinsic strategies also lead to charge carrier scattering, resulting in reduced charge carrier mobility (m). [10] Hence, identification of crystalline solids with intrinsically low k lat [10][11][12][13][14][15][16][17] or development of novel strategies to decrease k lat such as introduction of bonding hierarchy, [18,19] and ferroelectric instability [20] are important in designing high performance TE materials.…”