Tunable Intrinsic Phonon Mode versus Anomalous Thermal Transport in Two-Dimensional Strongly Anharmonic Group IB Chalcogenides A₂^IBSe_1/2Te_1/2 (A^IB = Cu, Ag, or Au)

Group IB chalcogenides, as promising thermoelectric materials, have ultralow thermal transport. Here, we propose a peculiar intrinsic B₂ phonon mode that includes the in-plane rotational and stretching vibrations of metal atoms in two-dimensional A₂^IBSe_1/2Te_1/2 (A^IB = Cu, Ag, or Au). The B₂ mode is sensitive to the metal-atom mass, temperature, and strain for effectively tuning the lattice thermal conductivity. The in-plane stretching vibration leads to an unexpected increase in the lattice thermal conductivity from Cu to Ag and to Au systems, in contrast to Keyes’ theory. The s(I) phase can be stabilized by the temperature-hardened B₂ mode to reduce the lattice thermal conductivity, following the ∼T^-0.59 instead of the traditional ∼T^-1 trend. The s(II)-to-s(I) phase transition is driven by the strain-softened B₂ mode to greatly enhance thermal transport via weakening the anharmonicity. Our work establishes the relationship of tunable intrinsic phonon mode versus thermal transport in two-dimensional group IB chalcogenides.