New carbon material, labelled Paracrystalline diamond, has been created

Bayreuth University researchers successfully created a carbon material with a unique structure, developed under an ultrahigh-pressure technique.

Researchers at the University of Bayreuth have collaborated with partners in China and the US, to successfully produce a carbon material that does not have the strictly ordered structures of a crystal but is also not amorphous. This is a first-time achievement.

They created a paracrystalline diamond with unique optical, mechanical and thermophysical properties. The material offers significant clues for understanding non-crystalline materials as well as for the targeted synthesis of other new carbon materials. The international team presents its discovery in Nature.

Diamond properties

Diamond is an extraordinarily hard material that naturally forms under extremely high pressures within the Earth’s interior. It is composed of carbon atoms that form a three-dimensional crystalline lattice structure. Within this structure, each individual carbon atom has four covalent bonds.

The four electrons implicated in these bonds are distributed among the orbitals of the atom in a characteristic manner. Consequently, the state in which the carbon atoms of a diamond are located is also called “sp3 hybridization”.

Diamond is present in many crystal forms, among which the most familiar are cubic diamond (CD) and hexagonal diamond (HD). Although, the synthesis of non-crystalline diamond has proven technically complex, limiting our understanding of its structure, properties, and its synthesis mechanism.

In response to this, a research group led by Professor Dr Tomo Katsura at the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) has been pursuing the goal of synthesising a millimetre-sized non-crystalline diamond.

They have conducted this using their recently developed ultrahigh-pressure technique in a large-volume multi-anvil press (MAP). At a pressure of 30 gigapascals and a temperature of more than 1,300 degrees Celsius, they succeeded. In the state of sp3 hybridisation, the carbon atoms successfully formed a large-scale non-crystalline structure in which regularly structured units can be detected.

Paracrystalline diamond

First author, Dr Hu Tang of the BGI, stated that “the new material can be described as a paracrystalline diamond, which varies from all previously established structural variations of diamond.” He went on to explain that “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.”

Professor Dr Tomo Katsura, professor of high-pressure geoscience at BGI added that “the material we synthesised is a hermaphrodite. Meaning for the first time, it forms a bridge between crystalline and amorphous, i.e., completely disordered, structures. It will stimulate materials research to look specifically for other new materials in this intermediate range.”

The paracrystalline diamond was synthesised on a high-pressure press at BGI. The assessment of its structures and properties incorporated experiments under both high pressures and temperatures and elaborate computer simulations. The Bayreuth scientists closely collaborated with research partners in China and the US. In particular, they worked with Dr Huiyang Gou at the Centre for High Pressure Science and Technology Advanced Research in Beijing, Professor Dr Ming-Sheng Wang at Xiamen University, China, and Professor Howard Sheng at George Mason University in Fairfax.

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