Structural, Mechanical, and Electronic Properties of High-Hardness Silicon Tetranitride.

Materials with high hardness are critical for industrial and aerospace applications, prompting the search for novel compounds with robust covalent networks. Using a first-principles structure prediction method, we systematically explored the phase stability of Si-N compounds under high pressure. We identified two thermodynamically stable phases: Si6N with P-1 symmetry and SiN4 with space group R-3c. Phonon spectra and ab initio molecular dynamics simulations confirm the dynamical and thermal stability of R-3c SiN4 at ambient pressure and up to 2000 K. Notably, R-3c SiN4 exhibits exceptional mechanical properties with a Vickers hardness of 31 GPa, a bulk modulus of 259.53 GPa, and a Young's modulus of 485.38 GPa. Furthermore, SiN4 possesses a high energy density (1.1 kJ·g-1) and outstanding detonation pressure and velocity (228 kbar, 7.11 km·s-1), both exceeding those of TNT, making it a potential high-energy-density materials. In addition, electronic structure analysis reveals SiN4 has a band gap of 2.5 eV, confirming its nonmetallic characteristics and strongly covalent nature. These findings provide theoretical guidance for the future synthesis of Si-N phases and establish a foundation for designing novel materials that combine high hardness with high-energy density performance.