Tetragonal C 16 exhibits Vickers hardness and fracture toughness very close to those of diamond, and thus belongs to the family of ultra-hard phases . It should be noted that hardness of this phase is much higher than H V -values reported for all recently predicted carbon allotropes of C 16 stoichiometry. Predicted hardness of tet-B 8 N 8 and tet-C 6 N 8 are very close to corresponding values for cubic BN and cubic C 3 N 4 , i.e. both new phases are superhard.