Abstract Constraining the accommodation, distribution, and circulation of hydrogen in the Earth’s interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P21nm and Pnnm). These structures potentially form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO4H2) that may transport water (OH–) deep into the Earth’s lower mantle. Additionally, the pyrite-type polymorph (space group Pa3 of FeOOH), and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core (Nishi et al. 2017), the formation of ultralow-velocity zones (ULVZs) (Liu et al. 2017), and the Great Oxidation Event (Hu et al. 2016). In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X‑ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in Ɛ-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition; (2) Ɛ-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa; and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa).

中文翻译：

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ε-FeOOH 在高压和环境温度下的相变

摘要 由于氢对矿物材料和流变学性质的重要影响，限制地球内部氢的容纳、分布和循环对于我们更广泛地了解地球深部至关重要。最近，人们对 FeOOH 的高压多晶型物（空间群 P21nm 和 Pnnm）给予了极大的关注。这些结构可能与 AlOOH 和 H 相 (MgSiO4H2) 形成含氢固溶体，可以将水 (OH–) 输送到地球下地幔深处。此外，黄铁矿型多晶型物（FeOOH 的空间群 Pa3）及其潜在的脱水作用与将氢引入外核等多种现象有关（Nishi 等人，2017 年）、超低速区的形成(ULVZs) (Liu et al. 2017)，和大氧化事件（Hu et al. 2016）。在这项研究中，使用基于同步加速器的 X 射线衍射 (XRD)、傅里叶变换红外光谱 (FTIR) 和光学吸收光谱的组合重新评估了 FeOOH 的高压演化，最高可达 ~75 GPa。基于这些测量，我们报告了三个主要发现：（1）压力引起的氢键变化（质子无序或氢键对称化）在 Ɛ-FeOOH 中发生的压力比以前报道的要低得多，并且不太可能与高-自旋到低自旋过渡；(2) Ɛ-FeOOH 经历了 10% 的体积坍塌，同时在大约 45 GPa 处发生了同构 Pnnm → Pnnm 转变；