Dear Dr. Wei-Bing Zhang, The curves show unstable phonon at and near q = 0.19 . You can compute phonon explicitly at q = 0.19 using QE by giving one q point specifically instead of computing nq = NxNxN . This will avoid any artifact which may arise from the interpolation method used. After all careful checking, if you find that the phonon is indeed imaginary, then the comment of Prof. Paolo Giannozzi is correct. No one can be sure that silicene is mechanically stable, unless there is any experimental proof.
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