Discovery of Paracrystalline Diamond
Speaker: Howard Sheng
Department of Physics and Astronomy, George Mason University
Materials are generally classified into crystalline and non-crystalline types. In this talk, I will introduce a paracrystalline state of materials, as exemplified by the synthesis of paracrystalline diamond [H. Tang et al, Nature 599, 605, 2021]. The paracrystalline state of diamond is structurally distinct from either crystalline or amorphous diamond, consisting of sub-nanometer-sized clusters with well-characterized crystalline medium-range order. The paracrystalline diamond was recently synthesized under high-pressure and high-temperature conditions with fullerene being its precursor and was characterized by means of high-resolution X-ray diffraction, transmission electron microscopy, and other spectroscopy techniques. The paracrystalline diamond exhibits different properties from the conventional diamond that we are familiar with. Theoretically, I will focus on how to identify the structural characteristics of the paracrystalline diamond as well as its formation mechanism through advanced computer simulation, including classical molecular dynamics incorporating a dynamics-acceleration technique, first-principles simulation, and the development of order parameters appropriate for characterizing its crystalline medium-range order. The discovery of paracrystalline diamond adds a new diamond form to the enriched carbon family but also unveils the missing link in the length-scale between amorphous and crystalline states across the structural landscape, having profound implications for understanding the structure of complex materials.
Time: Monday, January 31, at 3:00 – 4:00 pm