First principles study of volume isotope effects in ices VIII and X

The volume isotope effect (VIE) in ice Ih at ambient pressure has been known to be anomalous, i.e., the volume of D ₂ O is surprisingly larger than that of H ₂ O. In contrast, the VIE in ice VIII at 0 GPa was predicted to be normal, as is expected in common materials. To resolve this complicated nature of the VIE in ice, several ab initio calculations have been carried out. This article reviews recent progress toward a unified view of the VIE in several phases of ice. These calculations, based on the quasi-harmonic approximation, invoke zero-point motion effects to explain the diverse behavior of the VIE. Hydrogen-bond length plays a crucial role in determining the nature of VIE. Therefore, a change in nature of VIE can be induced by the application of pressure. This predicted behavior was confirmed experimentally and can also be applied to ice X, a regular ionic phase, at Mbar pressures.