Novel superhard phases in the B-C-N system from first principles: tetragonal C16, B8N8 and C6N8

In the B-C-N system, novel extended lattices of dense carbon C16, boron nitride B8N8 and carbon nitride C6N8 were proposed based on crystal chemistry rationale and geometry optimization onto ground state structures within the density functional theory (DFT). New tetragonal (space group P-4m2) C16 allotrope was identified as ultra-hard (HV ~ 97 GPa, close to that of diamond) with corner sharing C4 tetrahedra illustrated by charge density projections exhibiting sp3 like carbon hybridization. New binary compounds B8N8 and C6N8 which belong to the same space group P-4m2 were then derived from C16 through the same protocol and found to be superhard. All three new phases are mechanically (elastic constants) and dynamically (phonons) stable, and their electronic band structures are characteristic for insulators like diamond with large band gaps of about 5 eV.