Synthesis of paracrystalline diamond

Solids in nature are conventionally categorized into crystalline and non-crystalline (amorphous) states, based on the presence or absence of long-range lattice periodicity. However, the distinction between these two structural classes becomes increasingly ambiguous when the extent of long-range order within a crystalline material is substantially diminished. Here, we report the synthesis of a paracrystalline state of diamond, distinct from both crystalline and amorphous diamond forms. The paracrystalline diamond, synthesized under high-pressure and high-temperature (HPHT) conditions (for example, 30 GPa and 1,600 K) using face-centered cubic (fcc) C₆₀ as a precursor, comprises sub-nanometer-scale paracrystallites exhibiting well-defined medium-range crystalline order, extending over a few atomic shells. Comprehensive structural characterization was achieved through a combination of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and advanced molecular dynamics (MD) simulations. The formation mechanism of the paracrystalline diamond is attributed to the high density of nucleation sites generated within the compressed C₆₀ framework, along with the pronounced second-nearest-neighbor short-range order intrinsic to amorphous diamond, arising from robust sp³ covalent bonding. The discovery of this paracrystalline diamond phase not only enriches the known allotropes within the carbon family but also unveils a material exhibiting unique physical properties that could be harnessed for the development of advanced functional materials. Furthermore, this work provides critical insights into the continuum of structural order between amorphous and crystalline states, offering a deeper understanding of the complex structural evolution pathways in amorphous systems.

https://www.nature.com/articles/s41586-021-04122-w