Observation of an Ultrahard Phase of Graphite Quenched from High-pressure

Report No. ARL-TR-5466
Authors: Jennifer A. Ciezak-Jenkins
Date/Pages: February 2011; 22 pages
Abstract: In-situ isothermal high-pressure synchrotron x-ray diffraction, and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of highly ordered pyrolytic graphite. The x-ray measurements show that the pressure-volume relations remain smooth to near 5.9 GPa, where there is an abrupt discontinuity. This discontinuity is correlated to a hexagonal-to-monoclinic phase transition, which was previously predicted via ab initio crystal structure algorithms. However, discontinuities in the Raman vibrational modes do not appear until near 19 GPa. The discrepancy in the onset pressure of the phase transition is correlated to edge effects. The high-pressure monoclinic phase was recovered to ambient pressure and remains stable for long periods.
Distribution: Approved for public release
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Last Update / Reviewed: February 1, 2011