Ultrahard bulk amorphous carbon from collapsed fullerene

Amorphous materials inherit short- and medium-range order from the corresponding crystal and thus preserve some of its properties while still exhibiting novel properties^1,2. Due to its important applications in technology, amorphous carbon with sp² or mixed sp²-sp³ hybridization has been explored and prepared^3,4, but synthesis of bulk amorphous carbon with sp³ concentration close to 100% remains a challenge. Such materials inherit the short-/medium-range order of diamond and should also inherit its superior properties⁵. Here, we successfully synthesized millimetre-sized samples-with volumes 10³-10⁴ times as large as produced in earlier studies-of transparent, nearly pure sp³ amorphous carbon by heating fullerenes at pressures close to the cage collapse boundary. The material synthesized consists of many randomly oriented clusters with diamond-like short-/medium-range order and possesses the highest hardness (101.9 ± 2.3 GPa), elastic modulus (1,182 ± 40 GPa) and thermal conductivity (26.0 ± 1.3 W m^-1 K^-1) observed in any known amorphous material. It also exhibits optical bandgaps tunable from 1.85 eV to 2.79 eV. These discoveries contribute to our knowledge about advanced amorphous materials and the synthesis of bulk amorphous materials by high-pressure and high-temperature techniques and may enable new applications for amorphous solids.