XRD of water ice above 200 GPa under variable P-T conditions: revealing a structural complexity at planetary interiors pressures.

This proposal is a continuation of proposal HC-4677 which aimed at measuring the boundary line between bcc insulating and fcc-superionic (fcc-SI) phases of ice. A sample assembly with two boron-doped diamond absorbers has been optimized and it enabled a controlled laser heating of the ice sample up to 200 GPa. A negative slope for the bcc to fcc-SI transition and a metastable quenched fcc-SI phase at ambient temperature have been observed. That hints to the stability of the fcc-SI phase below 1000K by going to 300 GPa. Also, compact phases with greater packing density than bcc should be observed by compressing water ice up to 400 GPa at 300K. We plan to continue the exploration of the detailed structural properties of dense water ice above 200 GPa. These are very challenging measurements of great impact to model planetary interiors of Uranus, Neptune
and other ice giant exoplanets.

本提案为HC-4677号提案的延续，原提案旨在测定冰的体心立方（body-centered cubic, bcc）绝缘相与面心立方超离子相（fcc-superionic, fcc-SI）之间的相界。本研究优化了一款搭载两块硼掺杂金刚石吸收体的样品组装装置，实现了对冰样品的可控激光加热，最高压力可达200吉帕斯卡（GPa）。实验观测到体心立方相向面心立方超离子相转变的相变线具有负斜率，且在室温下存在经淬火保留的亚稳态面心立方超离子相。这表明当压力升至300吉帕时，面心立方超离子相在1000K以下仍具备稳定性。此外，在300K下将水冰压缩至400吉帕时，有望观测到堆积密度高于体心立方相的致密冰相。我们计划进一步探索压力高于200吉帕时致密水冰的精细结构性质。此类实验测量极具挑战性，其结果对天王星、海王星及其他冰巨系外行星的内部结构建模具有重大价值。