308(5) GPa marked a first-order transition in compressed water ice, where ice X shifted into a newly confirmed orthorhombic phase, ice XXII.
Diamond-anvil experiments with synchrotron X-ray diffraction and Raman spectroscopy showed ice X first distorting continuously under pressure, with Raman mode splitting appearing above 160 GPa before the full transition.
340(5) GPa was the minimum confirmed stability range for ice XXII, while a separate run indicated it persisted to at least 355(10) GPa; the phase also showed a small volume drop consistent with a first-order change.
Pbcm symmetry matched the diffraction data and long-standing density-functional predictions that had placed the transition around 250-350 GPa, resolving a decades-old experimental gap in dense ice physics.
The finding identifies the densest H2O polymorph yet observed and sharpens models of water inside giant planets, where ultrahigh-pressure ice phases and superionic states are expected.
If water forms bizarre orthorhombic structures at 308 gigapascals, what other undiscovered phases hide within the crushing depths of alien worlds?
After thirty years of theory, could the sudden discovery of ice XXII completely rewrite our understanding of extreme cosmic pressures?
Ice XXII Confirmed: The Experimental Discovery, Properties, and Planetary Impact of Water’s Densest Phase at 308–330 GPa
Overview
In 2026, scientists at the University of Edinburgh achieved a major breakthrough by compressing water to over 330 GPa, experimentally confirming the existence of Ice XXII—a new, ultradense phase of ice. This discovery not only validated decades of theoretical predictions but also provided a solid foundation for understanding the interiors of giant planets, where such extreme ice phases are believed to exist. Researchers are now exploring ways to create similar ice structures at lower pressures using nanoconfinement and chemical doping, while advanced machine learning models are enabling large-scale simulations of these complex phase transitions. These advances are reshaping planetary science and materials research alike.