Uniaxiality-Induced Reduced-Pressure Synthesis of Ultrahard Paracrystalline Diamond

Synthesizing fully sp³-bonded non-crystalline carbon remains a long-standing challenge due to the intrinsic instability of the sp³ bond at ambient pressure. Recently, paracrystalline diamond, a new-form sp³-bonded non-crystalline carbon consisting of sub-nanometer-sized paracrystallites, has been synthesized from face-centered cubic C₆₀ at 30 GPa, which has attracted attention due to its unique structural features and excellent physical properties. However, the ultrahigh synthesis pressure of paracrystalline diamond poses an obstacle to its large-scale production and applications. In this study, paracrystalline diamond is synthesized at an exceptionally low pressure (16 GPa) via inducing uniaxiality at high-pressure and high-temperature conditions, thereby breaking through the temperature-pressure phase diagram of C₆₀. By combining structural characteristics and advanced molecular dynamics simulation, the remarkable reduction of synthesis pressure is attributed to the fact that the symmetry of the C₆₀ cage is broken due to the uniaxiality, which further allows the C₆₀ cage to collapse at much lower pressures. This work reveals the critical role of uniaxiality in the reduced-pressure synthesis of paracrystalline diamond, which may provide a potent methodological strategy for the development of novel low-cost high-pressure materials.