Phonons and Anomalous Thermal Expansion Behaviour of H2O and D2O ice Ih

TL;DR

This study combines neutron scattering experiments and density functional theory calculations to analyze phonon anharmonicity and explain the anomalous thermal expansion behavior in ice Ih, revealing the microscopic origins of negative thermal expansion.

Contribution

It provides a quantitative analysis of phonon anharmonicity related to thermal expansion in ice Ih using combined experimental and computational methods, which is novel.

Findings

Negative thermal expansion below 60 K is due to anharmonic librational motion.

Anharmonic phonons are visualized across the Brillouin zone.

The anomalous expansion is quantitatively reproduced by experiments and calculations.

Abstract

In order to identify and quantitatively analyze the anharmonicity of phonons relevant to the anomalous thermal expansion in the Ih phase of ice, we performed neutron inelastic scattering measurements of the phonon spectrum as a function of pressure up to 1 kbar at 225 K in deuterated ice (D2O), and as a function of temperature over 10-225 K at ambient pressure in both H2O and D2O ice. We also performed density functional theory calculations of the lattice dynamics. The anomalous expansion is quantitatively reproduced from the analysis of the neutron data as well as from the ab-initio calculations. Further, the ab-initio calculations are used to visualize the nature of anharmonic phonons across a large part of the Brillouin zone. We find that the negative thermal expansion below 60 K in the hexagonal plane is due to anharmonic librational motion of the hexagonal rings of the ice molecules, and that along the hexagonal axis originates from the transverse vibrations of the hexagonal layers.