For the first time, scientists have obtained detailed experimental data about the structure of an exotic form of water called superionic ice, a feat some thought impossible (Nature 2019, DOI: 10.1038/s41586-019-1114-6).
Water ice has 18 possible crystalline arrangements, depending on temperature and pressure. Theoretical work predicted a particularly strange superionic structure would form at very high temperatures and pressures. In superionic ice, the oxygen atoms are closely packed and locked in place, while protons can move through the lattice, similar to atoms and electrons in a metal. Decades after it was first predicted, scientists have finally observed it in the lab.
“You always want to see validation by experiment, and in the case of ice systems, it is very difficult to do so,” says Princeton University’s Roberto Car, a theoretician who has studied superionic ice. In fact, many thought it would be impossible to experimentally create the necessary conditions for superionic ice.
To pull off the tricky experiment, Marius Millot, Federica Coppari, and their colleagues at Lawrence Livermore National Laboratory fired 6 laser beams at a thin layer of water in a 15-ns sequence, generating shockwaves that reverberated between diamond plates holding the samples. At temperatures near 2,000 K and 3,000 K and pressures between 160 and 420 GPa, X-ray diffraction (XRD) indicated formation of face-centered cubic crystals of superionic ice, which the researchers are calling ice XVIII.
The researchers say the extreme experimental conditions are the same ones found deep inside icy planets like Neptune and Uranus, raising the possibility that superionic ice may exist in nature.
Car is impressed with the experimental control that let the team not only reach those temperatures and pressures but also make XRD measurements. He says this setup could be used to study mixtures predicted to have superionic phases. Millot says they plan to study the water-ammonia system and to explore water ice under other pressures and temperatures.