Anharmonic nuclear motion and the relative stability of hexagonal and cubic ice

TL;DR

This study uses quantum mechanical calculations to demonstrate that anharmonic vibrational effects stabilize hexagonal ice over cubic ice, primarily due to differences in molecular vibrational modes and layer stacking.

Contribution

It reveals the significant role of anharmonic vibrational energies in determining the relative stability of ice polymorphs, a factor previously underestimated.

Findings

Hexagonal ice is stabilized by at least 1.4 meV/H2O due to anharmonic effects.

Anharmonic vibrational energy differences mainly originate from O-H bond stretching modes.

Layer stacking differences influence the anharmonic stabilization of ice forms.

Abstract

We use extensive first-principles quantum mechanical calculations to show that, although the static lattice and harmonic vibrational energies are almost identical, the anharmonic vibrational energy of hexagonal ice is significantly lower than that of cubic ice. This difference in anharmonicity is crucial, stabilising hexagonal ice compared with cubic ice by at least 1.4 meV/H2O, in agreement with experimental estimates. The difference in anharmonicity arises predominantly from molecular O-H bond stretching vibrational modes and is related to the different stacking of atomic layers.