Compression of ${\text{H}}_2\text{O}$ ice to 126 GPa and implications for hydrogen-bond symmetrization: Synchrotron x-ray diffraction measurements and density-functional calculations

We examined the volume compression and phase transformations of ${\text{H}}_2\text{O}$ ice by a combination of synchrotron x-ray diffraction measurements and density-functional calculations up to 126 GPa. The experimental data demonstrate that ice changes its compressibility at 40 and 60 GPa at room temperature, corresponding to the phase transitions from ice VII to dynamically disordered ice VII and subsequently to dynamically disordered ice X. The intermediate phase, dynamically disordered ice VII, is highly compressible, possibly due to quantum effects of protons. In contrast, dynamically disordered ice X and ice X show much smaller compressibility.

Figure 1

Difference in lattice parameters of bcc ice, determined separately from 110 and 200 diffraction lines. Inset: a typical XRD pattern obtained at 17 GPa and 300 K.

Figure 2

The $P\text{−}V$ data of ${\text{H}}_2\text{O}$ ice obtained from present experiments and theoretical calculations. The compression curves of ice VII and ice X/dynamically disordered ice X were determined by fitting calculated volumes to the Vinet equation of state. Note that the experimentally determined volume of ice matches the calculated volumes of ice VII and ice X/dynamically disordered X below 40 GPa and above 60 GPa, respectively.

Figure 3

$\text{ln}H\left(x\right)$ versus $1-x$ plot of ${\text{H}}_2\text{O}$ ice. Experimental data deviate from the linear plot above 40 GPa, suggesting a phase transition from ice VII to a highly compressible phase. Note that the anomalous volume reduction was observed repeatedly during both compression (run No. 1) and decompression cycles (runs No. 2 and No. 3) at the same pressure range.

Figure 4

The experimentally obtained volumes of ${\text{H}}_2\text{O}$ ice compared to the compression curves of ${\text{H}}_2\text{O}$ ice VII and ice X/dynamically disordered ice X calculated by density-functional theory. The predicted distances between the two neighboring oxygen atoms $\left(d_{\text{o-o}}\right)$ of 2.44, 2.36, and $2.27\text{Å}$ at the onset of tunneling, unimodal distribution, and single-well potential, respectively, are shown by bars (Ref. 8). Ice VII transforms to very compressible dynamically disordered ice VII (Ice ${\text{VII}}^{\prime }$) and subsequently to dynamically disordered ice X (Ice ${\text{X}}^{\prime }$) before symmetric ice X is formed. The previous experimental results by Hemley et al. (Ref. 1) are also plotted for comparison.