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

Abstract

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 alpha-B6O-like R3 over bar m B6N, whose observed unique metallicity originates from the electron deficiency caused by the sp2-like hybridization of the N atoms that leaves nonbonding lone pairs lying in the 2pz orbitals. We further unveil that the electron-deficient R3 over bar m B6N 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 B2-12 (N-N)2+. The strong N-N bond imparts R3 over bar m B6N the highest shear modulus among B12-based compounds (almost twice that of B6O) 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.