Lone pair electrons and undersized atoms mediate anomalous thermal conductivity in TlSe-type ABX2 compounds

Crystalline materials with intrinsically low lattice thermal conductivity (κ_L) are crucial for thermoelectric and thermal insulating applications. In recent years, TlSe-type ABX₂ compounds have attracted considerable attention owing to their ultralow κ_L, typically attributed to rattling cations with lone pair electrons. However, a systematic understanding of the microscopic mechanisms governing thermal transport trends in these structures remains elusive. Combining first-principles calculations and analysis, we reveal distinct and synergistic roles of lone pair electrons and atomic size effects in suppressing κ_L. Counterintuitively, introducing lone pair electrons (A = Ga, In, Tl) reverses conventional mass-dependent trends, increasing κ_L with average atomic mass due to enhanced anharmonicity related to lone pair activity evidenced by large Grüneisen parameters and reduced phonon lifetimes. Conversely, in systems lacking lone pairs, atomic size effects dominate; undersized atoms (e.g., Na⁺ in NaInTe₂) induce strong anharmonicity, yielding lower κ_L (∼1.33 W m⁻¹ K⁻¹) than heavier analogs (RbInTe₂, ∼1.45 W m⁻¹ K⁻¹). Strikingly, the synergy of lone pair electrons and undersized atoms in GaInTe₂ amplifies phonon scattering, achieving lower κ_L (∼0.58 W m⁻¹ K⁻¹) than that of InTe (∼0.64 W m⁻¹ K⁻¹). This work elucidates the competing mechanisms in ABX₂ systems and establishes a dual design strategy—leveraging lone pair electrons and atomic size mismatch—for engineering materials with intrinsically ultralow thermal conductivity.