In the phase diagram of water, superionic ices with highly mobile protons within the stable oxygen sublattice have been predicted at high pressures. However, the existence of superionic ices and the location of the melting line have been challenging to determine from both theory and experiments, yielding contradictory results depending on the employed techniques and the interpretation of the data. Here we report high-pressure and high-temperature synchrotron X-ray diffraction and optical spectroscopy measurements of water in a laser-heated diamond anvil cell and reveal first-order phase transitions to ices with body-centred and face-centred cubic oxygen lattices. Based on the distinct density, increased optical conductivity and the greatly decreased fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases. Our measurements determine the pressure–temperature stability fields of superionic ice phases and the melting line, suggesting the presence of face-centred cubic superionic ice in water-rich giant planets, such as Neptune and Uranus. The melting line determined here is at higher temperatures than previously determined in static compression experiments, but it is in agreement with theoretical calculations and data from shock-wave experiments.

在水的相图中，在高压下已经预测出在稳定的氧亚晶格内具有高度移动质子的超离子冰。然而，超离子冰的存在和融化线的位置一直难以从理论和实验中确定，根据所采用的技术和数据的解释产生矛盾的结果。在这里，我们报告了激光加热金刚石砧室中水的高压和高温同步加速器 X 射线衍射和光学光谱测量，并揭示了向具有体心和面心立方氧晶格的冰的一级相变。基于不同的密度，增加的光导率和大大降低的聚变焓，我们将这些观察到的结构分配给理论上预测的超离子冰相。我们的测量确定了超离子冰相和融化线的压力-温度稳定性场，表明在海王星和天王星等富含水的巨行星中存在面心立方超离子冰。这里确定的熔化线温度高于之前在静态压缩实验中确定的温度，但它与理论计算和冲击波实验的数据一致。