diamond

https://doi.org/10.1351/goldbook.D01671
An allotropic form of the element @C00821@ with cubic structure (space group \(O_{h}^{7}- Fd3m\)) which is thermodynamically @S05900@ at pressures above \(6\ \text{GPa}\) at room temperature and @M03872@ at atmospheric pressure. At low pressures diamond converts rapidly to @G02684@ at temperatures above \(1900\ \text{K}\) in an @I03026@ atmosphere. The chemical bonding between the carbon atoms is covalent with \(\mathrm{sp}^{3}\) @H02874@.
Note:
There is also a hexagonal diamond-like structure of the element carbon (lonsdaleite).
Source:
PAC, 1995, 67, 473. (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 487 [Terms] [Paper]