Pressure of hundreds of millions of atmospheres turns diamond into a liquid. For nearly twenty years, laboratory measurements and computer calculations described exactly how this happens differently. Now physicists have compressed a tiny diamond plate with a powerful laser and tracked how the atoms rearrange at the moment of melting. The resulting data are relevant both to the interiors of ice giants and to nuclear fusion.
A Billionth of a Second
The experiments were carried out at the Omega facility at the University of Rochester’s Laboratory for Laser Energetics, while the study itself was conducted by physicists from Lawrence Livermore National Laboratory. The beam vaporized the outer layer of the sample, sending a shock wave through its interior. The pressure inside was three times higher than that in Earth’s core, while the temperature, according to one of the study’s authors, Marius Millot, exceeded the temperature of the Sun’s surface.
This state existed for only a billionth of a second. During that time, researchers had to record all the relevant measurements, including X-ray diffraction, which reveals the arrangement of atoms. Carbon consists of light atoms and scatters this radiation weakly, so the signal turned out to be barely detectable.
A Discrepancy of a Thousand Degrees
The first laboratory experiments on melting diamond were carried out about twenty years ago. The temperature measured at the time differed from computer models by roughly one-fifth. In absolute terms, that is more than a thousand kelvins. No improvements to the simulations were able to close the gap.
The new data matched calculations based on quantum mechanics almost exactly. The reason for the old discrepancy lay in the measurements themselves rather than in the theory. The work was published in the peer-reviewed journal Nature Physics, as Universe Today reports.
Another result concerns the structure of matter. Earlier data from experiments at Sandia National Laboratories suggested that before entering the liquid state, carbon might adopt a different type of crystal lattice. This time, nothing of the sort was observed: carbon retained the diamond structure all the way until melting. The authors explain this by noting that under a single shock, the material simply does not have enough time to rearrange.
Like Ice in a Glass of Water
Under these conditions, the solid phase of carbon turns out to be less dense than the liquid. Diamond floats on molten metallic carbon much like ice floats in a glass of water. The effect was already noticed in experiments twenty years ago, but precise values were lacking at the time.
Deep inside Neptune and Uranus, carbon is thought by some researchers to crystallize at depth and sink toward the core in the form of diamond rain. The experimental conditions exceeded the pressures inside these planets, so the new melting-temperature values provide a basis for more accurate calculations of the structure and evolution of the ice giants.
Both planets remain the least studied major worlds in the Solar System. The only spacecraft to fly past them was Voyager 2 in the late 1980s. No new missions have yet reached them, so laboratory experiments remain almost the only way to look into their interiors.
Benefits for Nuclear Fusion
In fusion facilities, fuel is placed inside a tiny diamond capsule and compressed with laser pulses. For the shell to deform uniformly, it must be completely melted by the first shock.
The new data show that a weaker pulse than the one currently used is sufficient to achieve this. According to the authors’ calculations, this could increase energy output threefold while using the same facility power.
