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<p>High-pressure spectroscopic measurements of ε-FeOOH were conducted up to ~ 65 GPa at room temperature in diamond anvil cells. The pressure evolution of the Raman vibrational modes confirms that a hydrogen-bond-symmetrization-induced phase transition from <i>P</i>2<sub>1</sub><i>nm</i> to <i>Pnnm</i> occurs at ~ 18 GPa. Infrared (IR) spectroscopic measurements suggest that the <i>Pnnm</i> phase has a disordered hydrogen state, and no spectroscopic evidence for fully centered hydrogen bonds is observed within the investigated pressure range. Above ~ 45 GPa, Fe<sup>3+</sup> in ε-FeOOH undergoes a high-spin to low-spin transition as indicated by a reduction of the unit cell volume, together with reductions in IR transmitted and Raman signals. These results demonstrate that ε‑FeOOH preserves a disordered hydrogen‑bond configuration up to at least 45 GPa, whereas δ-AlOOH transforms to a centered hydrogen-bond configuration at ~ 18 GPa. This compositional contrast suggests that Fe‑bearing oxyhydroxides follow a distinct evolution of hydrogen bonding under compression, providing insight into hydrogen behavior in deep Earth materials.</p>

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High-pressure spectroscopic investigation of ε-FeOOH: toward a better understanding of pressure-induced hydrogen-bond symmetrization

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月24日

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  • Izumi Mashino,
  • Shigeru Yamashita,
  • Takashi Yoshino

摘要

High-pressure spectroscopic measurements of ε-FeOOH were conducted up to ~ 65 GPa at room temperature in diamond anvil cells. The pressure evolution of the Raman vibrational modes confirms that a hydrogen-bond-symmetrization-induced phase transition from P21nm to Pnnm occurs at ~ 18 GPa. Infrared (IR) spectroscopic measurements suggest that the Pnnm phase has a disordered hydrogen state, and no spectroscopic evidence for fully centered hydrogen bonds is observed within the investigated pressure range. Above ~ 45 GPa, Fe3+ in ε-FeOOH undergoes a high-spin to low-spin transition as indicated by a reduction of the unit cell volume, together with reductions in IR transmitted and Raman signals. These results demonstrate that ε‑FeOOH preserves a disordered hydrogen‑bond configuration up to at least 45 GPa, whereas δ-AlOOH transforms to a centered hydrogen-bond configuration at ~ 18 GPa. This compositional contrast suggests that Fe‑bearing oxyhydroxides follow a distinct evolution of hydrogen bonding under compression, providing insight into hydrogen behavior in deep Earth materials.