A thermodynamically @U06569@ allotropic form of @G02684@ with an ABCABC stacking @ST06775@ of the layers. The exact crystallographic description of this allotropic form is given by the space group \(D_{3\text{d}}^{5}- R\overline{3}m\), (@U06562@ constants: \(a=256.6\ \mathrm{pm}\), \(c=1006.2\ \mathrm{pm}\)).
The structure of rhombohedral @G02684@ can be best considered as an extended stacking fault in @H02818@. Rhombohedral @G02684@ can not be isolated in pure form (@N04088@ and laboratory preparations contain less than 40% of rhombohedral @G02684@ in combination with @H02818@). It is produced by shear deformation of @H02818@ 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]