Hexagonal Diamond—A New Form of Carbon

TLDR

Hexagonal diamond has recently been prepared in a laboratory from crystalline graphite using intense shock compression and strong thermal quenching. The synthesis begins with well‑crystallized graphite whose c‑axes are parallel to the compression direction, and the process is characterized by electrical and crystal studies under static pressure exceeding 130 kbar and temperature above 1000 °C. Hexagonal diamond was synthesized under >130 kbar and >1000 °C, starts transforming at room temperature but requires >1000 °C to recover, has a hexagonal lattice (a = 2.52 Å, c = 4.12 Å) with a theoretical density of.

Abstract

A new crystalline form of carbon—hexagonal diamond—has been synthesized in the laboratory under conditions of static pressure exceeding about 130 kbar and temperature greater than about 1000°C. It is necessary to start with well-crystallized graphite in which the c axes of the crystallites are parallel to each other and to the direction of compression. There is electrical evidence that the transformation starts at room temperature but hexagonal diamond is not retrieved unless a setting temperature exceeding about 1000°C is applied. The electrical and crystal characteristics have been studied. The crystal structure is hexagonal with a=2.52 Å and c=4.12 Å. The theoretical density is 3.51+g/cm3, same as cubic diamond. It has also been prepared recently in another laboratory from crystalline graphite by a method involving intense shock compression and strong thermal quenching. More recently it has been discovered to be present to the extent of over 30% in the Canyon Diablo meteorite diamonds.