News Bureau

January 11, 2018

Transformation of graphite into diamond

Documented hexagonal diamond by WSU researchers

A new study by Washington State University researchers answers longstanding questions about the formation of a rare type of diamond during major meteorite strikes. Hexagonal diamond or lonsdaleite is harder than the type of diamond worn on an engagement ring and is thought to be naturally made when large, graphite-bearing meteorites slam into Earth.

Scientists have puzzled over the exact pressure and other conditions needed to make hexagonal diamond since its discovery in an Arizona meteorite fragment half a century ago. Now, a team of WSU researchers has for the first time observed and recorded the creation of hexagonal diamond in highly oriented pyrolytic graphite under shock compression, revealing crucial details about how it is formed.

The discovery could help planetary scientists use the presence of hexagonal diamond at meteorite craters to estimate the severity of impacts.

 “The transformation to hexagonal diamond occurs at a significantly lower stress than previously believed,” said WSU Regents Professor Yogendra Gupta, director of the Institute for Shock Physics and a co-author of the study. “This result has important implications regarding the estimates of thermodynamic conditions at the terrestrial sites of meteor impacts.”

WSU shock physicist Stefan Turneaure and a team of researchers found that the crystalline structure of a highly oriented form of graphite transforms to the uncommon hexagonal form of diamond at a pressure of 500K atmospheres, around four times lower than previous studies had indicated. Sector at Argonne National Labs Advanced Photon Source.

To obtain their results, the researchers shot a lithium fluoride impactor at 11K mph into a 2 mm thick graphite disk. They then used pulsed synchrotron x-rays to take snapshots every 150 billionths of a second, while the shockwave from the impact compressed the graphite sample.

Their work clearly showed the graphite sample transformed into the hexagonal form of diamond before being obliterated into dust. “Most past research relied on micro structural examination of samples after they were shock compressed to infer what might have happened,” Turneaure said. “Such late-time measurements do not tell the whole story of what happened to the material during shock compression.”

Turneaure and Gupta said the next step in the research will be to investigate under what conditions pure hexagonal diamond can be recovered after shock compression. “Diamond is a material that is very easy to get excited about and our work in this area is just beginning,” Gupta said. “Moving forward, we plan to investigate the persistence of this form of diamond under lower pressure.

Because it is thought to be 60 percent harder than the common cubic diamond, hexagonal diamond could have many potential uses in industry if it could be successfully recovered after shock compression.”