Diamonds may not be for ever: researchers think they have seen, for the first time, the molten form of what is arguably the world's hardest material.

Jianyu Huang of Sandia National Laboratories in New Mexico heated diamond, at high pressure, to more than 2,000 °C inside microscopic hollow shells of carbon and watched the diamond soften (J. Y. Huang Nano Lett. doi:10.1021/nl0709975; 2007).

Huang describes this as quasi-melting. It is not true melting, he says, because the diamond particles don't become liquid carbon, but instead flicker between crystal forms by continually melting and instantly refreezing in a different conformation.

This is the closest anyone has ever come to directly melting diamond. No one really knows what molten carbon looks like. One form, graphite, seems to break down into liquid-like blobs when heated to high temperatures, although they have not been examined while still molten. But until now, molten diamond had not been achieved.

A key problem is that when diamond is heated to very high temperatures, at atmospheric pressure, it turns into graphite rather than melting. To overcome this, Huang's team used structures known as 'carbon onions' to create very high pressures and temperatures.

Quick change: molten diamond flickers between crystal forms. Credit: PASIEKA/SPL

These roughly spherical concentric shells of graphite-like carbon act as high-pressure cells that shrink when blasted with a beam of electrons. As they contract, the material in their centres is squeezed to very high pressures. Previous studies by Florian Banhart of the Institute for Physical Chemistry in Mainz, Germany, and Pulickel Ajayan of the Rensselaer Polytechnic Institute, New York, showed that graphite-like carbon in the onion's centre can be converted to diamond in these chambers (F. Banhart and P. M. Ajayan Nature 382, 433–435; 1996).

Huang's team used carbon onions with a carbon nanotube attached to the outside. They wired up the nanotube and heated the tube–onion composite while irradiating it with electrons. Huang estimates that this created temperatures of more than 2,000 °C and pressures of around 400,000 atmospheres at the core.

As the onions shrank, the carbon at the centre was transformed from graphite to diamond. When this got hot enough, it adopted the fluctuating quasi-molten state, Huang explains. Ultimately, he hopes, it will be possible to make fully molten diamond, so that the onions have liquid carbon cores.

But not everyone is confident that the diamond is really melting. Although Banhart says the new work is interesting, he thinks that the carbon may instead be switching between diamond and graphite — each time it returns to diamond, the crystals will look different, he says.