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Liquid diamond and carbon spaghetti

What happens if you melt diamond? No-one knows, because no one has been able to do it. Despite carbon?s status as the ?element of life?, its liquid state is still a mystery. Now scientists from the Lawrence Livermore National Laboratory in California predict that carbon has not one, but two liquid states.

Well, perhaps we should not be too surprised. The two solid states of carbon are well known, and utterly dissimilar. One is diamond - hardest of all materials and valued for its crystalline brilliance. The other is graphite - dull and black, and soft enough to rub off on your fingers. The difference between the two is all down to the kind of chemical bonds that the carbon atoms form, and it seems that this is what distinguishes the two putative liquid carbons too.

In diamond, each carbon atom is linked to four others, and the bonds form the struts of a vast network extending through space like a climbing frame. In graphite, on the other hand, each atom forms only three unions with others, so that they are joined into sheets of hexagons, like chicken wire. This motif is also found in other forms of carbon, such as soot (where the particles are largely disorderly clusters of small fragments of graphite-like carbon). Because there are no strong bonds between the sheets, they can slide over one another, making graphite soft and fragile. The different bonding pattern also makes graphite electrically conducting, whereas diamond is an insulator.

But the problem in seeing what happens to these bonding patterns when carbon melts is that its melting point is so ridiculously high - around four and a half thousand degrees Centigrade. Making any sort of measurements at this temperature is extremely tricky.

All the same, Motohiro Togaya at Osaka University in Japan managed it in 1997 by discharging huge amounts of electrical charge into graphite rods. The electrical ?flash-heating? raised the temperature to thousands of degrees in an instant, and Togaya was able to glimpse the molten carbon briefly before it cooled again. He was also able to measure the electrical conductivity of the liquid, and found that at high pressure - around fifty thousand atmospheres - it changed abruptly, suggesting that the structure of the liquid might change here.

Nearly all liquids have just one structure - a pretty disorderly one, like a surging crowd in a busy railway station. But a few liquids seem to be special, offering the strange prospect of two different types of disorder: two forms of the same liquid, with different densities. This has been suggested mostly for rather exotic substances: liquid sulphur, gallium, tellurium and caesium. But it is also thought to be possible for water, and Togaya?s work raised the possibility for carbon too.

Now this idea is given further support by computer simulations of liquid carbon reported in the 7 June issue of_ Physical Review Letters _by James Glosli and Francis Ree at Lawrence Livermore. If you can?t easily generate a sample of real liquid carbon, the next best thing is to look at a computer model of it. Glosli and Ree found that their simulations predicted two types of liquid carbon - one with a higher density than the other - from around 5,000 degrees at 20,000 atmospheres to nearly 9,000 degrees at higher pressures.

The high-density form of liquid carbon is like a jumbled-up diamond - most of the atoms have four neighbours, but they are not arranged regularly as they are in diamond. The low-density form, meanwhile, is not like graphite at all, but contains mostly carbon atoms linked to just two others in chain-like structures: a kind of carbon spaghetti.

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Ball, P. Liquid diamond and carbon spaghetti. Nature (1999). https://doi.org/10.1038/news990617-10

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