(Left) Structure of crystalline diamond. (Right) Structure of paracrystalline diamond – units of carbon atoms arranged in a cube shape are marked in turquoise; units of carbon atoms arranged in a hexagonal shape are marked in yellow; irregular structures are marked in red. Image: Hu Tang.For the first time, researchers at the University of Bayreuth in Germany, together with collaborators in China and the US, have produced a carbon material that does not have the strictly ordered structures of a crystal but is not amorphous either. Rather, it is a paracrystalline diamond with unique optical, mechanical and thermophysical properties. The material, which is reported in a paper in Nature, offers important clues for understanding non-crystalline materials, as well as for the targeted synthesis of other new carbon materials.
Diamond is an extraordinarily hard material that forms naturally under extremely high pressures in the Earth's interior, and is composed of carbon atoms in a three-dimensional crystalline lattice structure. Within this structure, each carbon atom has four covalent bonds, and the four electrons involved in these bonds are distributed among the orbitals of the atom in a characteristic manner.
Diamond exists in many crystal forms, among which the most familiar are cubic diamond (CD) and hexagonal diamond (HD). But synthesis of non-crystalline diamond has proven technically difficult, limiting scientists' understanding of its structure, properties and synthesis mechanism.
A research group led by Tomo Katsura at the University of Bayreuth's Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) has been pursuing the goal of synthesizing millimeter-sized non-crystalline diamond using their recently developed ultrahigh-pressure technique in a large-volume multi-anvil press. By utilizing a pressure of 30 gigapascals and a temperature of more than 1300°C, they managed to get carbon atoms to form a large-scale non-crystalline structure in which regularly structured units can be identified.
"The new material can be described as a paracrystalline diamond, which differs from all previously known structural variations of diamond," says Hu Tang of the BGI, who was first author of the paper. "It has a non-amorphous structure in which the carbon atoms are arranged partly in cubes, partly in hexagons, and partly in irregular structures. The unusual physical properties of the new material are non-directional and expected to further advance the study of high-pressure materials."
"The material we synthesized is a hermaphrodite: for the first time, it forms a bridge between crystalline and amorphous, i.e. completely disordered, structures," adds Katsura, who is professor of high-pressure geoscience at BGI. "It will stimulate materials research to look specifically for other new materials in this intermediate range."
This story is adapted from material from the University of Bayreuth, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.