Phonon dynamics in MoSi₂N₄: insights from DFT calculations

We have reported the density functional theory investigations on the monolayered, 2 layered and bulk MoSi₂N₄ in three structural modifications called α1 [Y.-L. Hong, et al., Chemical Vapor Deposition of Layered Two-Dimensional MoSi₂N₄ Materials, Science, 2020, 369(6504), 670-674, DOI: 10.1126/science.abb7023], α2 and α3 [Y. Yin, Q. Gong, M. Yi and W. Guo, Emerging Versatile Two-Dimensional MoSi₂N₄ Family, Adv. Funct. Mater., 2023, 2214050, DOI: 10.1002/adfm.202214050]. We showed that in the case of monolayers the difference in total energies is less than 0.1 eV between α1 and α3 phases, and less than 0.2 eV between α1 and α2 geometries. The most energetically favorable layer stacking for the bulk structures of each phase was investigated. All considered modifications are dynamically stable from a single layer to a bulk structure in energetically favorable stacking. Raman spectra for the monolayered, 2 layered and bulk structures were simulated and the vibrational analysis was performed. The main difference in the obtained spectra is associated with the position of the strongest band which depends on the Mo-N bond length. According to the obtained data, we can conclude that the Raman line at 348 cm^-1 in the experimental spectra of MoSi₂N₄ can have more complex explanation than just Γ-point Raman-active vibration as was discussed before in [Y.-L. Hong, et al., Chemical Vapor Deposition of Layered Two-Dimensional MoSi₂N₄ Materials, Science, 2020, 369(6504), 670-674, DOI: 10.1126/science.abb7023].