From lone-pair electrons to dual phonon channels: unraveling Te-dominated transport in monolayer Sb 2 Te 3

Abstract Conventional Peierls theory is inadequate for describing thermal transport in strongly anharmonic compounds, such as two-dimensional layered chalcogenides containing lone-pair electrons; the underlying physics of their ultralow thermal conductivity remains to be explored. Here, using monolayer Sb 2 Te 3 as a model system, we elucidate the mechanism behind its ultralow lattice thermal conductivity by evaluating the competing contributions of particle-like propagation and wave-like tunneling. The particle-like transport channel is suppressed by the dominant quartic anharmonicity induced by the stereochemically active lone-pair electrons of the tetrahedrally coordinated Te ( 1 ) - 5 p z orbital, primarily via enhancement of four-phonon scattering. In contrast, the wave-like transport channel is enhanced by the flattening of the Te ( 2 ) -derived phonon branches, a result of the relatively weak covalent bonding in the octahedral coordination, thereby promoting phonon coherence. Consequently, the combined model of the Te ( 1 ) -dominated particle-like and Te ( 2 ) -dominated wave-like channels accurately accounts for the total thermal conductivity, achieving excellent agreement with experimental measurements in both magnitude and temperature dependence. For example, the calculated T − 0.73 scaling exponent for the temperature dependence closely matches the experimental value of T − 0.76 . Our work identifies the interplay of lone-pair electrons and covalent bonding configuration as a key governing mechanism for thermal transport in two-dimensional layered chalcogenides, providing a novel perspective on their thermal properties.