Anomalous Nuclear Quantum Effects in Ice

TL;DR

This paper explains unusual isotope effects in ice, showing that replacing hydrogen with deuterium causes expansion contrary to normal expectations, and predicts a normal contraction when oxygen isotopes are substituted, all supported by ab initio density functional theory.

Contribution

The study provides a theoretical explanation for anomalous isotope effects in ice and predicts a new normal isotope effect with oxygen substitution, confirmed experimentally.

Findings

Deuterium substitution causes ice expansion, contrary to normal isotope effects.

Oxygen isotope substitution results in lattice contraction, as predicted and confirmed experimentally.

Ab initio density functional theory accurately models these anomalies.

Abstract

One striking anomaly of water ice has been largely neglected and never explained. Replacing hydrogen ($^1$H) by deuterium ($^2$H) causes ice to expand, whereas the "normal" isotope effect is volume contraction with increased mass. Furthermore, the anomaly increases with temperature $T$, even though a normal isotope shift should decrease with $T$ and vanish when $T$ is high enough to use classical nuclear motions. In this study, we show that these effects are very well described by {\it ab initio} density functional theory. Our theoretical modeling explains these anomalies, and allows us to predict and to experimentally confirm a counter effect, namely that replacement of $^{16}$O by $^{18}$O causes a normal lattice contraction.