A thermodynamically unstable allotropic form of graphite with an ABCABC stacking sequence of the layers. The exact crystallographic description of this allotropic form is given by the space group D3d5 - R3(bar)m, (unit cell constants: a = 256.6 pm, c = 1006.2 pm).
Note:
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 K.
Source:
PAC, 1995, 67, 473. 'Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)' on page 504 (https://doi.org/10.1351/pac199567030473)