Dinitrogen bonding induced metal-semiconductor transition leading to ultrastiffening in boron subnitride

Extreme environments enable the discovery of atypical phases with enhanced properties and their transition mechanisms. Boron subnitride is a promising alternative to boron carbide/suboxide; however, its development is severely hampered by its long-standing unresolved crystal structure. Herein, by analyzing the pressure-dependent stabilities of the B-N system and scrutinizing several candidates against the available experimental results, the experimentally synthesized boron subnitride is identified as $\ensuremath{\alpha}{\text{-B}}₆\mathrm{O}$-like $R\overline{3}m\phantom{\rule{0.28em}{0ex}}{\mathrm{B}}₆\mathrm{N}$, whose observed unique metallicity originates from the electron deficiency caused by the $s{p}²$-like hybridization of the N atoms that leaves nonbonding lone pairs lying in the $2{p}_z$ orbitals. We further unveil that the electron-deficient R$\overline{3}m$ ${\mathrm{B}}₆\mathrm{N}$ undergoes an isopointal metal-semiconductor transition at 120 GPa, dominated by the unique local orbital population coupling between different homonuclear bonds. Here, breakage of the lone pairs resulting from the dinitrogen bonding compensates for the unoccupied bonding states of intericosahedra B-B bonds, thus converting it into the electron-precise ${\mathrm{B}}₁₂^{2\ensuremath{-}}{(\text{N-N})}²⁺$. The strong N-N bond imparts R$\overline{3}m\phantom{\rule{0.28em}{0ex}}{\mathrm{B}}₆\mathrm{N}$ the highest shear modulus among ${\mathrm{B}}₁₂$-based compounds (almost twice that of ${\mathrm{B}}₆\mathrm{O}$) and serves as the main load-bearing unit resisting large plastic strain to produce superior strength. These findings substantially deepen our fundamental understanding of icosahedral boron-rich solids, and the underlying effect may contribute to fully grasping the changes in oxidation state and bonding pattern under high pressures.