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.