The volume isotope effect in high-pressure ice II within its stability field

Water ice forms a remarkable diversity of structures, occurring in as many as 17 crystalline forms. Water ice also shows other interesting and unusual properties; e.g., solid ice Ih (as found in glaciers and in domestic freezers) is less dense than its own liquid. What is less well known is that ice Ih shows negative volume thermal expansion below ~60 K (i.e. the crystals shrink as they are heated) and an anomalous volume difference due to isotopic substitution - the molar volume of D2O is larger than that of H2O, rather than the more usual situation, whereby substitution of the heavier isotope decreases the volume. In a recent study at ISIS, we found that the isotope effect in the high-pressure form of ice called "ice II" is "normal" in samples recovered to room pressure; the aim of this proposal is to determine whether this is still true at the pressures at which ice II is stable.