UNLV physicists compress water beyond 222 GPa and see the ice-X structure become less symmetrical

Researchers in the US compressed water to pressures expected on planets such as Uranus and Neptune and found that its atomic structure becomes less symmetrical, ScienceAlert reports on a study in Communications Chemistry. sciencealert.comScienceAlertsciencealert.comOpen source ↗ The team includes physicist Ashkan Salamat of the University of Nevada, Las Vegas. This appears to be the first time such a transformation has been observed in the lab; earlier it had been predicted in simulations. sciencealert.comScienceAlertsciencealert.comOpen source ↗

How the experiment worked

The researchers placed a tiny water droplet between two diamond tips and raised the pressure beyond 222 GPa. That is about 2.2 million times the pressure felt on Earth's surface. sciencealert.comScienceAlertsciencealert.comOpen source ↗ X-ray diffraction, which relies on how X-rays scatter as they pass through the sample, was used to identify changes at the smallest scales. sciencealert.comScienceAlertsciencealert.comOpen source ↗

Compression would normally be expected to make atoms more symmetrical as they pack more tightly. Here the opposite happened: the crystal structure of the ice became more lopsided and stretched. sciencealert.comScienceAlertsciencealert.comOpen source ↗ Only the pressure matched conditions on some other planets, not the temperature. sciencealert.comScienceAlertsciencealert.comOpen source ↗

Salamat noted that the interiors of these planets cannot be reached, but data from missions such as Voyager give an idea of their compositions, pressures and temperatures. Laboratories can mimic the chemistry and experiment with what could be occurring inside. sciencealert.comScienceAlertsciencealert.comOpen source ↗

Ice-X and its upper limit

The work concerns ice-X, one of more than 20 types of ice known to scientists. Ice-X forms gradually across a broad range of pressures and shifts into a highly symmetrical arrangement of oxygen and hydrogen atoms at around 50 GPa. sciencealert.comScienceAlertsciencealert.comOpen source ↗ The team pushed it beyond its known limits and obtained a structure that looks new, although further study is needed to confirm this. sciencealert.comScienceAlertsciencealert.comOpen source ↗

In the paper, the authors write that the transition marks the onset of a new regime of structural complexity in high-pressure H2O and redefines the upper stability limit of ice-X. sciencealert.comScienceAlertsciencealert.comOpen source ↗

According to the researchers, the change in structure has implications for how water conducts heat, influences magnetic fields and affects the density of a planet. These properties matter for interpreting telescope observations. sciencealert.comScienceAlertsciencealert.comOpen source ↗ They add that if the structure of water differs, the chemical reactions it causes may differ too, possibly involving rock and metal compounds not found on Earth. sciencealert.comScienceAlertsciencealert.comOpen source ↗