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Two Types of Diamonds

A team of scientists from Australia and the United States has created two types of diamond — regular diamond and a diamond-like phase called lonsdaleite, which is found in nature at the sites of meteorite impacts — in minutes in a laboratory at room temperature, a process that normally takes billions of years, huge amounts of pressure and super-hot temperatures.

McCulloch et al. created ‘rivers’ of regular diamond and lonsdaleite, named after the crystallographer Dame Kathleen Lonsdale, the first woman elected as a Fellow to the Royal Society. Lonsdaleite has a different crystal structure to regular diamond, and is predicted to be 58% harder. Image credit: McCulloch et al., doi: 10.1002/smll.202004695.

McCulloch et al. created ‘rivers’ of regular diamond and lonsdaleite, named after the crystallographer Dame Kathleen Lonsdale, the first woman elected as a Fellow to the Royal Society. Lonsdaleite has a different crystal structure to regular diamond, and is predicted to be 58% harder. Image credit: McCulloch et al., doi: 10.1002/smll.202004695.

Diamond is an attractive material due to its extreme hardness, high thermal conductivity, quantum optical, and biomedical applications.

There is still much that is not understood about how diamonds form, particularly at room temperature and without catalysts.

“Natural diamonds are usually formed over billions of years, about 150 km deep in the Earth where there are high pressures and temperatures above 1,000 degrees Celsius,” said senior author Professor Jodie Bradby, a researcher in the Research School of Physics at the Australian National University.

In the new study, Professor Bradby, RMIT Professor Dougal McCulloch and their colleagues used advanced electron microscopy techniques to capture solid and intact slices from the experimental samples to create snapshots of how nanocrystalline diamond and lonsdaleite formed.

“Our pictures showed that the regular diamonds only form in the middle of lonsdaleite veins under this new method developed by our team,” Professor McCulloch said.

“Seeing these little ‘rivers’ of lonsdaleite and regular diamond for the first time was just amazing and really helps us understand how they might form.”

The team previously created lonsdaleite, also called hexagonal diamond, in the lab only at high temperatures.

But their new results show both lonsdaleite and regular diamond can also form at normal room temperatures by just applying high pressures of 100 GPa.

“The twist in the story is how we apply the pressure,” Professor Bradby said.

“As well as very high pressures, we allow the carbon to also experience something called ‘shear’ — which is like a twisting or sliding force.”

“We think this allows the carbon atoms to move into place and form lonsdaleite and regular diamond.”

“Lonsdaleite has the potential to be used for cutting through ultra-solid materials on mining sites,” she said.

“Creating more of this rare but super useful diamond is the long-term aim of this work.”

“Being able to make two types of diamonds at room temperature was exciting to achieve for the first time in our lab,” said co-author Xingshuo Huang, a PhD student in the Research School of Physics at the Australian National University.

The research is described in a paper in the journal Small.

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Dougal G. McCulloch et al. Investigation of Room Temperature Formation of the Ultra-Hard Nanocarbons Diamond and Lonsdaleite. Small, published online November 4, 2020; doi: 10.1002/smll.202004695

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