Stable distorted T phase of MSi2 N4 (M= Ru and Os) monolayers: First-principles insights into structural, vibrational, mechanical, electronic, and optical properties

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 MA2Z4 crystals, but also introduces members with distinctive 1T′ geometries that exhibit promising mechanical, electronic, optical features for nanomechanical, optoelectronic, and green-energy applications.