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Scientists Identify Ice XXI, a Metastable High-Density Phase of Water

The phase shows a tetragonal lattice with an unusually large unit cell of 152 H2O molecules.

Credit: European XFEL
Image
ice cubes with red and blue background
A small crystal of ice XXI formed through nucleation and slow melting process. Image credit: Lee et al., doi: 10.1038/s41563-025-02364-x.

Overview

  • An international KRISS-led team reports the discovery in Nature Materials following time-resolved studies at European XFEL and DESY PETRA III.
  • Rapid compression in a dynamic diamond anvil cell drove liquid water to about 2 gigapascals in roughly 10 milliseconds before release over about one second.
  • Microsecond X-ray pulses recorded crystallization in real time during more than 1,000 compress–decompress cycles at room temperature.
  • Diffraction measurements at PETRA III established a distinct structure and revealed previously hidden pathways in the pressure range where ice VI usually forms.
  • The researchers classify ice XXI as metastable and plan to map related high-temperature phases to assess possible relevance for icy moons such as Titan and Ganymede.