Crystallisation competition between cubic and hexagonal ice structures: molecular-dynamics insight

ABSTRACT

Under ambient pressure, two different ice polytypes of ice, cubic (I_c) and hexagonal (I_h) can be crystallised. Here, we elucidate the mechanism and kinetics of the phase transition of cubic ice with no stacking disorder to a perfect hexagonal ice at various temperatures, via molecular simulation. The study was performed through non-isothermal and isothermal molecular-dynamics simulations; these conditions were enforced on an equilibrated I_c/LDA/I_h system. Different results have been drawn from analysis in terms of local density, annealing kinetics, and phase transition. The evolution from cubic to hexagonal ice becomes prevalent at higher temperatures during the performed non-isothermal studies; to this end, a model-free kinetic study was proposed to address this phenomenon’s kinetic properties over a characteristic temperature range. Ice crystallisation is also studied in solid/liquid systems, where cubic or hexagonal ice was in contact with liquid water at sub-zero temperatures. It was revealed that water molecules have a propensity to follow and crystalise in the existing ice structure rather than forming a new interface.

KEYWORDS:

• Molecular dynamics
• annealing
• non-isothermal kinetics
• cubic ice
• hexagonal ice

Acknowledgements

M.R.G. thanks the Irish Research Council for a Government-of-Ireland postdoctoral fellowship (GOIPD/2016/365). All authors thank Christian Burnham for technical advice, assistance, and many interesting discussions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Irish Research Council: [grant number GOIPD/2016/365].