ANU Associate Professor Jodie Bradby said her team - including ANU PhD student Thomas Shiell and experts from RMIT, the University of Sydney and the United States - made nano-sized Lonsdaleite, which is a hexagonal diamond only found in nature at the site of meteorite impacts such as Canyon Diablo in the US.
"This new diamond is not going to be on any engagement rings. You'll more likely find it on a mining site - but I still think that diamonds are a scientist's best friend. Any time you need a super-hard material to cut something, this new diamond has the potential to do it more easily and more quickly," said Dr Bradby from the ANU Research School of Physics and Engineering.
Her research team made the Lonsdaleite in a diamond anvil at 400 degrees Celsius, halving the temperature at which it can be formed in a laboratory.
Lonsdaleite is simulated to be 58% harder than diamond on the < 100 > face and to resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa. This is yet exceeded by IIa diamond's < 111 > tip hardness of 162 GPa.
Nature Scientific Reports - Nanocrystalline hexagonal diamond formed from glassy carbon
"The hexagonal structure of this diamond's atoms makes it much harder than regular diamonds, which have a cubic structure. We've been able to make it at the nanoscale and this is exciting because often with these materials 'smaller is stronger'."
Lonsdaleite is named after the famous British pioneering female crystallographer Dame Kathleen Lonsdale, who was the first woman elected as a Fellow to the Royal Society.
Co-researcher Professor Dougal McCulloch from RMIT said the collaboration of world-leading experts in the field was essential to the project's success. "The discovery of the nano-crystalline hexagonal diamond was only made possible by close collaborative ties between leading physicists from Australia and overseas, and the team utilised state-of-the-art instrumentation such as electron microscopes," he said.
Corresponding author from the University of Sydney, Professor David McKenzie, said he was doing the night shift in the United States laboratory as part of the research when he noticed a little shoulder on the side of a peak. "And it didn't mean all that much until we examined it later on in Melbourne and in Canberra - and we realised that it was something very, very different."
Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.
SOURCES -ANU, Scientific Reports