Novel superhard boron nitrides, B2N3 and B3N3: Crystal chemistry and first principles studies

Tetragonal and hexagonal hybrid sp3/sp2 carbon allotropes C5, proposed on the basis of crystal chemistry, were subsequently used as template structures to identify new binary phases of the B N system, specifically tetragonal and hexagonal boron nitrides, B2N3 and B3N3. The ground structures and energy-dependent quantities of new phases were computed within framework of the quantum density functional theory (DFT). All four new boron nitrides were found to be cohesive and mechanically (elastic constants) stable. Vickers hardness (Hv) evaluated by various models qualifies all new phases as superhard (Hv > 40 GPa). Dynamically, all new boron nitrides were found to be stable from positive phonon frequencies. The electronic band structures reveal their conductive behavior due to the presence of pi electrons of sp2-like hybrid atoms.