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.

水冰具有显著的结构多样性，迄今已发现多达17种晶型。此外，水冰还展现出诸多有趣且反常的特性：例如，我们在冰川与家用冰柜中常见的冰 Ih（ice Ih），其固态密度低于其液态相。鲜为人知的是，冰 Ih在约60开尔文以下会表现出负体积热膨胀效应（即晶体受热时体积收缩），同时还存在因同位素取代引发的反常体积差异——氘代水（D₂O）的摩尔体积大于普通水（H₂O），这与常规规律相悖：通常重同位素取代会使体积减小。在ISIS开展的一项近期研究中，我们发现：高压冰相冰 II（ice II）在恢复至常压后的样品中，其同位素效应表现为常规模式；本研究提案的目标在于，验证在冰 II稳定存在的高压环境下，这一现象是否依然成立。