Stable distorted T phase of MSi₂N₄ (M = Ru and Os) monolayers: First-principles insights into structural, vibrational, mechanical, electronic, and optical properties

Abstract

The recent synthesis of two-dimensional (2D) MoSi₂N₄ and WSi₂N₄ crystals has given rise to a new class of 2D materials with distinctive properties and significant potential for applications in advanced technologies. The transition metal (TM) elements can potentially be substituted with other TMs, adding the possibility of introducing new members to this emerging 2D family. In this regard, we propose three structural phases (1H, 1T, and 1T′) of MSi₂N₄ (M = Ru and Os) monolayers and examine their structural, vibrational, mechanical, electronic, and optical properties using ab initio methods. The results of cohesive energies (EC) indicate that the 1T′ structures are energetically more favorable than their 1H and 1T counterparts. The calculated phonon spectra reveal that the MSi₂N₄ nanosheets in distorted 1T′ phase are dynamically stable, while their 1H and 1T forms exhibit imaginary phonon modes, signifying vibrational instability of the systems. Ab initio molecular dynamics (AIMD) simulations also confirm that the 1T′−MSi₂N₄ structures remain thermally stable, even up to 600 K, without any notable structural deformations. The mechanical properties of 1T′ structures are assessed through the computation of in-plane stiffness (Y₂D), Poisson's ratio (ν), and ultimate tensile strain (UTS). The intrinsic geometrical anisotropy of the 1T′ nanosheets induces strong orientation-dependent elastic properties, and the calculated large values of Y₂D and high UTS indicate their rigidity and suitability for strain engineering of electronic and optical properties. The calculated electronic band structures reveal that, whereas unstable 1T−MSi₂N₄ structures exhibit ferromagnetic metal properties, the stable 1T′−RuSi₂N₄ and 1T′−OsSi₂N₄ monolayers possess a nonmagnetic semiconducting ground state with indirect band gaps of 1.40 and 1.47 eV, respectively, at the level of the HSE functional. The observed shifts from metallic characteristics in the 1T phase to semiconducting nature in the 1T′ structures can be attributed to the Peierls distortion. Additionally, in accordance with the direct electronic bandgap of the 1T′−MSi₂N₄ crystals, strong optical absorption within the visible parts of the optical spectrum is estimated. Our study not only expands the family of 2D M⁢A₂⁢Z₄ crystals, but also introduces members with distinctive 1T′ geometries that exhibit promising mechanical, electronic, and optical features for nanomechanical, optoelectronic, and green-energy applications.