The microscopic origin of anomalous properties of ice relies on the strong quantum anharmonic regime of atomic vibrations

TL;DR

This study uses advanced quantum simulations to uncover how nuclear quantum effects cause anomalous properties in ice, revealing nonlinear volume changes and phonon frequency renormalizations.

Contribution

It demonstrates the significant nonlinear quantum effects on hydrogen in ice and provides high-accuracy phonon dispersion comparisons with experimental data.

Findings

Quantum effects on hydrogen cause nonlinear volume expansion and contraction.

Anharmonic phonon frequency renormalization of about 10%.

First accurate comparison of phonon dispersion with experiments.

Abstract

Water ice is a unique material presenting intriguing physical properties, like negative thermal expansion and anomalous volume isotope effect (VIE). They arise from the interplay between weak hydrogen bonds and nuclear quantum fluctuations, making theoretical calculations challenging. Here, we employ the stochastic self-consistent harmonic approximation (SSCHA) to investigate how thermal and quantum fluctuations affect the physical properties of ice XI ab initio. Regarding the anomalous VIE, our work reveals that quantum effects on hydrogen are so strong to be in a nonlinear regime: when progressively increasing the mass of hydrogen from protium to infinity (classical limit), the volume firstly expands and then contracts, with a maximum slightly above the mass of tritium. We observe an anharmonic renormalization of about 10% in the bending and stretching phonon frequencies probed in IR and Raman experiments. For the first time, we report an accurate comparison of the low energy phonon dispersion with the experimental data, possible only thanks to high-level accuracy in the electronic correlation and nuclear quantum and thermal fluctuations, paving the way for the study of thermal transport in ice from first principles and the simulation of ice under pressure.