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Melting diamond produces super diamonds

30/3/2024

Translation: machine translated

Under high pressure, diamond should transform into an even more stable structure. But it stubbornly refuses to do so: until now, no one has been able to produce the mysterious BC8 carbon. Now we know the reason - and may be able to circumvent it with a trick.

Diamond is one of the hardest materials there is - but it can be even harder. According to theoretical calculations, if diamond is subjected to extreme pressure, it should transform into an even more stable material, BC8 carbon. This designation refers to its crystal structure, in which the atoms are bound in a similar way to diamond, but even more efficiently packed. Experts suspect that the exotic material occurs in the core of certain exoplanets. In principle, it should be possible to produce it from diamond in the laboratory - there is just one problem: diamond does not adhere to the prediction. It retains its structure even at a pressure at which, according to the rules of thermodynamics, it should have long since transformed into BC8 carbon.

This is why no one has ever seen this supposedly easy-to-obtain material. A research group led by Ivan I. Oleynik from the University of South Florida has now found the probable reason for this in high-resolution simulations of carbon at high pressure. And they suggest a way in which BC8 carbon could actually be produced. As the team reports in "The Journal of Physical Chemistry Letters", a high energy barrier exists between diamond and BC8 carbon. This prevents diamond from rearranging into the new structure, even if it has long been energetically more favourable.

In fact, this is similar to the situation at normal pressure: graphite is lower in energy, which is why diamond should actually transform into it immediately. But here too, an energy barrier prevents the spontaneous and tragic loss of value of countless engagement rings. Such a state is referred to as metastable: it should actually decay, but it never does. And that is precisely why you can press diamond as much as you like, it simply does not produce BC8 carbon. However, very similar to the comparably exotic hexagonal diamond, it is possible to trick your way around the problem. The working group's simulations show that diamond can melt into BC8 carbon.

The funny thing is that BC8 carbon is the most stable structure even above the melting point of diamond. This is why small crystals of BC8 carbon immediately form when the diamond melts. According to the simulation, this happens at pressures above 1200 gigapascals - twelve million times the atmospheric pressure - and temperatures from 3700 Kelvin, which is cooler than the surface of the sun. The experts suggest exposing diamond to two strong shock waves in high-pressure experiments in order to obtain this range. This possibility is currently being investigated, they report.

Spectrum of Science

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Original article on Spektrum.de

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