How Cubic Can Ice Be?

Area of Science:

• Physical Chemistry
• Materials Science
• Crystallography

Background:

• Understanding ice nucleation and crystal formation is crucial for atmospheric science and materials research.
• Homogeneous ice nucleation in supercooled water remains a complex phenomenon, especially at the nanoscale.
• Previous studies often focused on larger water samples or different nucleation conditions.

Purpose of the Study:

• To determine the crystal structure of ice formed via homogeneous nucleation in deeply supercooled water nanodrops.
• To investigate the influence of nanoscale confinement and rapid freezing on ice polymorph formation.
• To compare experimental findings with results from molecular dynamics simulations.

Main Methods:

• Utilized a supersonic nozzle to generate water nanodrops (radius ≈ 10 nm) at ~225 K.
• Employed femtosecond wide-angle X-ray scattering (FAXS) at a free-electron X-ray laser for probing.
• Analyzed X-ray diffraction spectra within 100 microseconds of freezing.

Main Results:

• Identified a metastable, predominantly cubic crystal structure in the nanodroplet ice.
• The diffraction peak shape indicated stacking-disordered ice with a cubicity value (χ) of 0.78 ± 0.05.
• Observed higher cubicity compared to micron-sized drops, aligning with molecular dynamics simulations.

Conclusions:

• The observed cubic ice structure in nanodrops is attributed to extremely low freezing temperatures and rapid, microsecond-timescale freezing.
• Nanoscale effects and rapid dynamics significantly influence the resulting ice crystal structure.
• Findings provide critical experimental data for validating models of ice nucleation and phase transitions.