Intrinsically Low Lattice Thermal Conductivity Derived from Rattler Cations in an AMM′Q₃ Family of Chalcogenides

Crystalline semiconductors exhibiting innate low lattice thermal conductivity (κ_l) are technologically very important for the development of thermal barrier coatings, thermal data-storage devices, and high-performance thermoelectrics. Here, using first-principles calculations based on density functional theory and anharmonic lattice dynamics, we predict intrinsically low κ_l (<1 W/m K along the stacking direction for T ≥ 400 K) in many known layered semiconductors, AMM′Q₃ (A = Na, K, Cs, Tl; M = Cu; M′ = Zr, Hf; Q = S, Se), that possess chemical bonding heterogeneity. We show that low κ_l in this class of materials arises from (a) the rattling vibrations of the weakly bonded A atoms, characterized by low-frequency localized phonon branches with very small dispersion that give rise to numerous additional scattering channels and (b) strong lattice anharmonicity which is manifested in the large-mode Gruneisen parameters that increase the phonon scattering rates. Our work uncovers inherent low κ_l in this previously unexplored class of metal chalcogenides which should open up opportunities for applications of these compounds in various thermal energy management devices.