On the Formation of the Rhombohedral Graphite Modification

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Abstract
The stability relations between the hexagonal and rhombohedral graphite modification have been investigated using predominantly “single crystals”. Neglecting small amounts of stacking disorder natural graphite crystals have been found to consist usually of the pure hexagonal modification. However, several examples were found to contain up to approximately 5% of rhombohedral modification. Rhombohedral modification could be produced in pure hexagonal graphite single crystals by unidirectional pressure associated with some shear. Several experimental procedures have been used in order to separate the influence of different factors that might be thought to be of importance for the formation of the rhombohedral modification. Studies were made on the influence of hydrostatic pressure, unidirectional pressure without gliding, gliding under application of as small a pressure as possible, mechanical twinning. In discussing the results of these experiments a process of “mechanical transformation” can be suggested that is closely related to processes of “mechanical twinning”. Relative movements of a √3/3 (equal to the carbon-carbon distance) between fixed carbon double nets in the direction of the carbon-carbon bonds are involved in the process suggested. Thus, s (amount of macroscopical shear) = 0.21. Heat treatment at and above 1300° C reverts the rhombohedral modification to the hexagonal one. Application of hydrostatic pressure up to 10.000 Atm. did not change the ratio of hexagonal to rhombohedral modification in the single crystals. X-ray powder diagrams taken at room temperature under approximately 8.000 atm. showed that the hexagonal modification did not invert to the rhombohedral modification under these conditions. Therefore, it is concluded that the rhombohedral modification is metastable against the hexagonal one in essential agreement with the results of B oehm and H ofmann (1955) who approached the problem independently in a different manner.

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