Published: December 11, 2017 6:20 pm
Scientists have for the first time turned diamond into graphite using ultra-short flashes of an X-ray laser. It is a decisive step forward in understanding the fundamental behaviour of solids when they absorb energetic radiation, researchers said. For the first time, the researchers including Franz Tavella from SLAC National Accelerator Laboratory in the US, were able to follow the graphitisation in a time-resolved manner.
“In addition to these fundamental aspects, understanding the graphitisation process is important for diamond-based technologies, since diamond is increasingly used for practical applications,” said Tavella, first author of the study published in the journal High Energy Density Physics. Diamond and graphite are different forms of carbon that differ in their inner crystal structure. Diamond is the high-pressure phase that forms deep in the earth.
Under normal conditions, diamond is metastable, meaning that it converts back to graphite when the process is initiated with sufficient energy. There are different ways to trigger the conversion of diamond to graphite, for instance by simply heating the diamond under exclusion of oxygen or even with an aimed mechanical stroke, researchers said. With heat and high pressure, graphite can be converted into synthetic diamonds that already have quite a market worldwide, they said.
The team used the Italian soft X-ray free-electron-laser FERMI to shoot ultra-short flashes at tiny diamond slices with a thickness of just 0.3 millimetres. “Usually, if you shoot such intense laser pulses at solid matter, it becomes unordered, or amorphous. Diamond is a different example,” researchers said. It can switch its internal structure to a different order, thereby turning into graphite.
“In principle, it was known that if you send enough energy into diamond, it should graphitise. But it was not known exactly how this happens,” said Sven Toleikis from German national research centre DESY. There are two possible paths: the common so-called thermal transition during which the absorbed energy is transferred to the internal crystal lattice of the diamond until it re-organises itself into the graphite structure.
Another is a non-thermal mode, where the energy absorbed by just a small fraction of the electrons in the diamond changes the internal potential energy surface, triggering a re-arrangement of the crystal lattice, researchers said. “Non-thermal transition is much faster than thermal, the latter occurring on picosecond timescales,” said Beata Ziaja from DESY. A picosecond is a trillionth of a second.
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