Temperature-dependent structural stability, mechanical strength, and thermodynamics of pyrite-type silicon pernitride

As a family of prototypical high-temperature structural and multifunctional ceramics, silicon nitrides have garnered significant interest. Among them, the recently synthesized pyrite-type silicon pernitride (p-SiN₂), characterized by a structure of SiN₆ octahedra interconnected via N₂ dimers, has emerged with exceptional mechanical properties. To assess its suitability for extreme environments, we conducted a comprehensive investigation into its high-temperature structural stability, elastic behavior, mechanical strength, and thermodynamic behavior using ab initio molecular dynamics (AIMD) simulations. Our results reveal that p-SiN₂ maintains exceptional structural stability up to 3900 K, comparable to that of diamond (4100 K), as confirmed by both thermal and dynamic analyses. At 0 K, p-SiN₂ demonstrates enhanced resilience against tensile fracture and plastic shear deformation along major crystallographic directions compared to the benchmark γ-Si₃N₄, with all peak strengths consistently exceeding 40 GPa. Notably, an indentation-induced shear-strain-stiffening effect was observed, attributed to compression-driven shortening and strengthening of the N-N bonds within N₂ dimers. Despite only a moderate reduction (∼27%) in elastic moduli, the mechanical strengths of p-SiN₂ decrease significantly with increasing temperature, experiencing an approximately 50% reduction at 3000 K. Utilizing the quasi-harmonic approximation, we report for the first time the thermodynamic parameters of p-SiN₂ and compare them with those of γ-Si₃N₄. These findings clearly demonstrate the pronounced temperature dependence governing the structural stability, elastic/plastic response, and thermal properties of this novel silicon nitride phase, highlighting its potential for extremely high-temperature applications.