Anharmonicity as a fundamental issue inspires numerous interesting phenomena in phase transition, electronic structure, thermal transport, and so on. Here, we find that the peculiar A 1 u phonon mode of in-plane rotational vibration of group-IB-atom ring introduces the anharmonicity into the s(I) and s(II) phases of two-dimensional group-IB chalcogenides. Compared to the high-symmetry s(I) phase, the A 1 u phonon mode is always active and the anharmonicity is stronger in the symmetry-breaking s(II) phase by releasing the strain energy. The temperature-hardened A 1 u mode stabilizes the s(I) phase and reduces the lattice thermal conductivity by strengthening the anharmonicity. The strain-softened A 1 u mode drives the s(II)-to-s(I) phase transition and enhances the lattice thermal conductivity by weakening the anharmonicity. We also establish the relationships of the anharmonicity vs the band structure and Poisson's ratio. As the anharmonicity is weakened during the strain-induced s(II)-to-s(I) phase transition, the bandgap significantly increases. Meanwhile, the weaker anharmonicity implies the lower Poisson's ratio, which further drops much faster with the strain. Our work realizes the tuning of anharmonicity by the peculiar phonon mode in 2D group-IB chalcogenides, which provides a useful guidance for further understanding the anharmonic effect.
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