Crystal chemistry rationale and ab initio investigation of ultra-hard dense rhombohedral carbon and boron nitride

Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout settings featuring extended lattices, calculation set elastic constants, provide results large bulk moduli i.e. B0(rh-C2) = 438 GPa close that diamond, B0(rh-BN) 369 cubic BN. The hardness assessment in framework three contemporary models enables both phases be considered as ultra-hard. From electronic band structures calculated Brillouin zones, is a small-gap semiconductor, oppositely characterized by gap 5 eV, well two BN phases.