Phonon dynamics in MoSi 2 N 4 : insights from DFT calculations.

We have reported the density functional theory investigations on the monolayered, 2 layered and bulk MoSi 2 N 4 in three structural modifications called α 1 [Y.-L. Hong, et al. , Chemical Vapor Deposition of Layered Two-Dimensional MoSi 2 N 4 Materials, Science , 2020, 369 (6504), 670-674, DOI: 10.1126/science.abb7023], α 2 and α 3 [Y. Yin, Q. Gong, M. Yi and W. Guo, Emerging Versatile Two-Dimensional MoSi 2 N 4 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 α 1 and α 3 phases, and less than 0.2 eV between α 1 and α 2 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 MoSi 2 N 4 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 MoSi 2 N 4 Materials, Science , 2020, 369 (6504), 670-674, DOI: 10.1126/science.abb7023].