A carbon allotrope that can scratch diamond

Applying high temperatures and pressure to buckminsterfullerene C₆₀ produces amorphous carbon materials with a compressive strength similar to diamond, scientists have found.

Under ambient conditions, C₆₀ remains stable at pressures up to 25GPa. But at temperatures above 800˚C it compresses at 5GPa to form a disordered carbon material as the extreme conditions cause the fullerene’s bonding character to change from sp² to sp³. The resulting material’s physical properties depend on the ratio of sp² to sp³ carbons. Raising the temperature during synthesis increases the sp³ fraction, producing a harder material.

The hardest among these amorphous materials made by researchers was created at 1200˚C. Its tetragonally-arranged network of sp³ carbons is so hard it scratches diamond and is of a similar compressive strength, which describes the material’s ability to withstand being squashed under a load. But unlike insulating diamond, the material has a small bandgap, making it a semiconductor. This could be useful in solar cells and photoelectric applications as the bandgap is similar to silicon’s.