Synthesis of paracrystalline diamond

Solids in nature can be generally classified into crystalline and non-crystalline states, depending on whether long-range lattice periodicity is present in the material. The differentiation of the two states, however, could face fundamental challenges if the degree of long-range order in crystals is significantly reduced. Here we report a unique paracrystalline state of diamond that is distinct from either crystalline or amorphous diamond. The paracrystalline diamond reported in this work, consisting of sub-nanometer-sized paracrystals that possess a well-defined crystalline medium-range order up to a few atomic shells, was synthesized in high-pressure high-temperature conditions (e.g., 30 GPa, 1600 K) employing fcc-C60 as a precursor. The structural characteristics of paracrystalline diamond was identified through a combination of X-ray diffraction, high-resolution transmission microscopy, and advanced molecular dynamics simulation. The formation of paracrystalline diamond is a result of densely distributed nucleation sites developed in compressed C60 as well as pronounced second-nearest-neighbor short-range order in amorphous diamond due to strong sp3 bonding. The discovery of paracrystalline diamond adds a new diamond form to the enriched carbon family, which exhibits distinguishing physical properties and can be furthered exploited to develop new materials. Furthermore, this work reveals the missing link in the length-scale between amorphous and crystalline states across the structural landscape, which has profound implications for recognizing complex structures arising from amorphous materials.