Under normal conditions, pure carbon exhibits vastly different physical properties depending on its structure. For example, graphite is soft, but diamond is one of the hardest materials known. Graphite conducts electricity, but diamond is an insulator.
In the middle is the form of carbon confirmed by the Yale-led team, dubbed M-carbon and predicted by theoretical methods initially in 2006. M-carbon is made when graphite is compressed to pressures approximately 200,000 times room pressure, at room temperature.
Although changes were first observed in graphite under high pressure and room temperature conditions 50 years ago, it is only now that the crystal structure has been confirmed by experiment, using long duration x-ray diffraction, Raman spectroscopy and optical techniques to verify these predictions.
“Besides the unique mechanical properties discovered in M-carbon, we find that the transformation of graphite to M-carbon is extremely sluggish and requires a long time to reach equilibrium, which may be the additional reason why this puzzle remained unsolved for the past half century,” said Yuejian Wang, the study’s first author and former postdoctoral researcher at Yale, who is now assistant professor of physics at Oakland University.