Ice XII in Its Second Regime of Metastability

We present neutron powder diffraction results which give unambiguous evidence for the formation of the recently identified new crystalline ice phase [[2]], labeled ice XII, in completely different conditions. Ice XII is produced here by compressing hexagonal ice ${\mathrm{I}}_h$ at $T=77\mathrm{}$, 100, 140, and 160 K up to 1.8 GPa. It can be maintained at ambient pressure in the temperature range $1.5<T<\mathrm{}135\mathrm{}\mathrm{K}$. Accurate structural properties are obtained from high-resolution diffraction carried out at $T=1.5\mathrm{}\mathrm{K}$. At $T=140\mathrm{}$ and 160 K additionally ice III/IX is formed. The increasing amount of ice III/IX with increasing temperature gives an upper limit of $T\approx 150\mathrm{K}$ for the successful formation of ice XII with the presented procedure.