A thermodynamically @[email protected] allotropic form of @[email protected] with an ABCABC stacking @[email protected] of the layers. The exact crystallographic description of this allotropic form is given by the space group \(D_{3\text{d}}^{5}- R\overline{3}m\), (@[email protected] constants: \(a=256.6\ \mathrm{pm}\), \(c=1006.2\ \mathrm{pm}\)).
The structure of rhombohedral @[email protected] can be best considered as an extended stacking fault in @[email protected] Rhombohedral @[email protected] can not be isolated in pure form (@[email protected] and laboratory preparations contain less than 40% of rhombohedral @[email protected] in combination with @[email protected]). It is produced by shear deformation of @[email protected] and transforms progressively to the hexagonal (ABAB) modification on heating above \(1600\ \text{K}\).
PAC, 1995, 67, 473. (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 504 [Terms] [Paper]