[Phonopy-users] Negative phonon frequencies
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From: Artem K. <ar...@ma...> - 2014-11-27 04:13:28
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Dear Forum, I am trying to calculate phonon dispersion for CuPbSbS3 (Pmn21, # 31 space group). In all my attempts, I always get negative frequencies. Several things were tried - different k-grids; different ISMEAR and SIGMA parameters in VASP; different ENCUT; different potentials (semi-core electrons were considered); inclusion of spin-orbit coupling. I also used Atom Displacements and DFPT approaches. Nevertheless, the phonon frequencies have the characteristic behavior of the attached figure (negative modes behaving strangely in the range of -1 THz). My questions are the following: - CuPbSbS3 (Pmn21) has been synthesized by my collaborator, which means the system is stable. However, the negative frequencies indicate the material is unstable. Why am I getting negative phonon frequencies for a material that exists? - How can I interpret these results providing the material exists? - The oxidation states for the atoms are Cu[+1]Pb[+2]Sb[+3]S[-2], Cu[+1] and Pb[+2]. The common oxidation state for Cu is +2, while for Pb is +3. Is it possible that unusual oxidation states are responsible for the negative frequencies? If yes, how can this be avoided? - Can we avoid these negative frequencies by making other improvements? I am attaching one figure showing the phonon dispersion for this material. I would be happy to share more details about the input atomic positions and other parameters if necessary. Thank you for your help in advance and regards, Artem K. -- ----------------------------------------------------------------- Artem R. Khabibullin, Advanced Material and Device Theory Group, Department of Physics, University of South Florida, Tampa, FL 33620. |