Amorphous Ice Is Partly Crystalline
When water freezes rapidly well below 0 °C, you get a disordered glass-like solid called low-density amorphous (LDA) ice. Abundant in the cores of comets and on icy moons, LDA ice can be prepared via vapor deposition in the lab. But since its discovery 90 years ago, its atomic-scale structure has remained contested. Michael Davies of University College London and Cambridge University, UK, and his colleagues now show that the structure is not a true glass but instead is partly crystalline [1]. The finding, which was derived from numerical simulations and lab experiments, highlights the need for caution when identifying glassy materials and raises questions about our theories on the fundamental nature of liquid water.
In their simulations the researchers started with liquid water at 300 K and cooled it to 125 K at different cooling rates. The resulting “ice cubes” ranged from fully crystallized structures to crystalline grains embedded in an amorphous bulk. To determine which of these best represented LDA ice samples created in various laboratory approaches, Davies and colleagues then computed x-ray diffraction patterns for their simulated structures. Comparing the synthetic patterns with published ones revealed that ice cubes with 16%–19% crystallinity provided the best match.
The researchers also prepared their own LDA ice samples. After creating many different samples with a variety of techniques, they warmed the samples from the low LDA formation temperature up to 0 °C, where a transition to crystalline ice was expected. Remarkably, the samples adopted crystalline structures that were not completely homogenous—suggesting memory of their parent phase. This memory effect cannot arise from a fully amorphous progenitor and indicates that LDA ice harbors crystalline regions. Next, the researchers say, is to clarify whether it is possible to create a truly amorphous ice.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- M. B. Davies et al., “Low-density amorphous ice contains crystalline ice grains,” Phys. Rev. B 112, 024203 (2025).