Exotic Quartic Anharmonicity Induced by Rattling Effect in Layered Isostructural Compounds

Anharmonicity of phonons correlates with less dispersive potential surfaces and usually governs the thermal transport of low-dimensional materials. Here, we demonstrate the significant role of the so-called "rattling" action in affecting lattice anharmonicity, originating from the ease of freedom of confined but loose atoms in two-dimensional space. Based on calculations of X2Si2Te6 (X = Sb and Bi) within the Peierls–Boltzmann framework, the degree of high-order four-phonon scattering differs strikingly despite their isostructural feature. Upon switching on four-phonon scattering, a significant drop of thermal conductivity (κph) occurs in Bi2Si2Te6 up to 43.15% (71.62%) at 300 K (1000 K), while a moderate reduction occurs for Sb2Si2Te6. This arises from a stronger quartic anharmonicity of Bi2Si2Te6 than Sb2Si2Te6, dominated by the redistribution four-phonon process (λ + λ′ → λ″ + λ‴). We show that the strong quartic anharmonicity is more likely to occur in systems with flat phonon bands, large atoms, and rattling atomic units. These new insights provide perspectives in the design of materials with low κph through introducing rattling units in layered materials or interfaces.