Tunable Intrinsic Phonon Mode versus Anomalous Thermal Transport in Two-Dimensional Strongly Anharmonic Group IB Chalcogenides A2 IB Se1/2Te1/2 (A IB = Cu, Ag, or Au)

Group IB chalcogenides, as promising thermoelectric materials, have ultralow thermal transport. Here, we propose a peculiar intrinsic B 2 phonon mode that includes the in-plane rotational and stretching vibrations of metal atoms in two-dimensional A2 IB Se1/2Te1/2 (A IB = Cu, Ag, or Au). The B 2 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 B 2 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 B 2 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.