Solids in nature can be generally classified into crystalline and non-crystalline states [1][2][3][4][5][6][7] , 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 [8][9][10] . The paracrystalline diamond reported in this work, consisting of sub-nanometersized paracrystals that possess a well-defined crystalline medium-range order up to a few atomic shells 4,5,[11][12][13][14] , was synthesized in high-pressure high-temperature conditions (e.g., 30
GPa, 1600 K) employing fcc-C 60 as a precursor. The structural characteristics of paracrystalline diamond was identified through a combination of X-ray diffraction, highresolution transmission microscopy, and advanced molecular dynamics simulation. The formation of paracrystalline diamond is a result of densely distributed nucleation sites developed in compressed C 60 as well as pronounced second-nearest-neighbor short-range order in amorphous diamond due to strong sp 3 bonding. The discovery of paracrystalline diamond adds a new diamond form to the enriched carbon family 15-17 , 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.Amorphous solids refer to materials that do not possess long-range periodicity as exhibited in crystals [1][2][3][4][5][6][7][8][9][10][11] . Consequently, Bragg peaks associated with crystalline arrangements of atoms are absent or obscured in the diffraction signals of amorphous materials, which renders the recognition of their structural organizations notoriously difficult. Due to decades of research effort, it is now understood that structural ordering on the atomic level of amorphous solids is ubiquitous, as manifested by the short-to-medium-range ordering in metallic glasses [5][6][7] and the continuousrandom networks (CRN) of amorphous semiconductors 1-3 . Moving from the short range into the extended length-scale abutting the long-range scale, however, our understanding of the structural arrangements remains much more limited, and it is often complicated by capricious crystalline structural ordering encountered in amorphous materials 14,[18][19][20] . In an attempt to resolve this structural enigma, a paracrystalline structure model was proposed 11,20 , in which nanosized paracrystals, defined as severely distorted crystals, were introduced to the amorphous matrix to account for the crystalline medium range order (MRO). A crucial question to answer is, in the configurational space, are we able to identify a state of matter that is fully packed with...
