Carbon: A New Crystalline Phase

Abstract

The electrical resistance of single crystal graphite shows a very sharp increase at above 150 kilobars, accompanied by a drifting upward with time. The behavior is typical of a first-order phase transition, and is irreversible. X-rays on the material after removal from the cell show lines of a new material with a structure which can be indexed as a cubic lattice with a unit cell edge of 5.545 angstroms. The density of the new phase is estimated at 2.80 grams per cubic centimeter.

From soft to superhard: Fifty years of experiments on cold-compressed graphite

“…In these studies, Bundy and Kasper [13] determined that chemical purity of the graphite samples did not matter, but crystalline order did: the transition occurred in well-ordered samples, but did not occur in poorly-ordered graphite, consistent with earlier measurements [28]. They also reported a similar hysteresis as observed by Aust and Drickamer [28], but ultimately observed a decrease in resistance upon depressurization below approximately 7 GPa.…”

“…1, c), was investigated by Bundy and Kasper [13] while exploring the transition between graphite and diamond (see X-ray Diffraction section). Hexagonal diamond forms at temperatures greater than 1000 °C and pressures higher than 13 GPa, implying that the phase found by Aust and Drickamer [28] was not lonsdaleite, and instead the phase partially synthesized by Aust and Drickamer has since been referred to as "cubic" graphite. In these studies, Bundy and Kasper [13] determined that chemical purity of the graphite samples did not matter, but crystalline order did: the transition occurred in well-ordered samples, but did not occur in poorly-ordered graphite, consistent with earlier measurements [28].…”

“…Hexagonal diamond forms at temperatures greater than 1000 °C and pressures higher than 13 GPa, implying that the phase found by Aust and Drickamer [28] was not lonsdaleite, and instead the phase partially synthesized by Aust and Drickamer has since been referred to as "cubic" graphite. In these studies, Bundy and Kasper [13] determined that chemical purity of the graphite samples did not matter, but crystalline order did: the transition occurred in well-ordered samples, but did not occur in poorly-ordered graphite, consistent with earlier measurements [28]. They also reported a similar hysteresis as observed by Aust and Drickamer [28], but ultimately observed a decrease in resistance upon depressurization below approximately 7 GPa.…”

“…Using a specifically-designed high-pressure electrical resistance cell [27] Aust and Drickamer [28] measured a large increase in resistance above ~ 17 GPa, measuring samples of single-crystal graphite oriented in both the a and c directions, respectively. At the transition, the a-axis resistance increased by more than two orders of magnitude, while the c-axis resistance increased by a factor of 4 (Fig.…”

“…As such, there are several XRD studies of graphite under pressure that show the highly anisotropic nature of hexagonal graphite [42,[48][49][50]: the caxis is approximately 35 times more compressible than the a-axis (Table 1). The first XRD measurement attempts of the post-graphite phase yielded an interpretation of a cubic structure [28] for a sample synthesized at high pressure and room temperature and quenched to room temperatures. Since then, the cubic structure has not been reproduced and the initial identification was likely spurious [13].…”