Ice X

Ice XV

In detail, ice XV has a smaller density (larger unit-cell volume) than ice VI. This makes the VI-to-XV disorder-to-order transition much favoured at low pressures. Indeed, differential scanning calorimetry by Shephard and Salzmann revealed that reheating quench-recovered HCl-doped ice XV at ambient pressure even produces exotherms originating from transient ordering, i.e. more ordered ice XV is obtained at ambient pressure. Being consistent with this, the ice VI-XV transition is reversible at ambient pressure. It was also shown that HCl-doping is selectively effective in producing ice XV while other acids and bases (HF, LiOH, HClO₄, HBr) do not significantly enhance ice XV formations.