Diamond (both cubic and hexagonal) is still considered to have superior atomic density, elastic moduli and hardness . Recently, however, several superdense (ρ > 3.635 g/cm 3 ) carbon allotropes have been predicted from the first-principles studies , and hexagonal (space group P 6 222) C 3 with assigned quartz topology (SACADA qtz #11) has even been claimed to have a hardness of 113 GPa , i.e., 15% harder than diamond. Also, very recently superdense (ρ = 3.666 g/cm 3 ) ultrahard (H V ≈ 102 GPa) hexagonal C 6 (space group P6 5 22) allotrope with qtz topology was proposed by us .