High-pressure behavior of the polymorphs of FeOOH

The high-pressure structural and electronic behavior of alpha -, beta -, and gamma -FeOOH were studied in situ using a combination of synchrotron X-ray diffraction (XRD) and X-ray emission spectroscopy (XES). We monitored alpha -FeOOH by XES as a function of pressure up to 85 GPa and observed an electronic spin transition that began at approximately 50 GPa, which is consistent with previous results. In the gamma -FeOOH sample, we see the initiation of a spin transition at 35 GPa that remains incomplete up to 65 GPa. beta -FeOOH does not show any indication of a spin transition up to 65 GPa. Analysis of the high-pressure XRD data shows that neither beta -FeOOH nor gamma -FeOOH transform to new crystal structures, and both amorphize above 20 GPa. Comparing our EOS results for the beta and gamma phases with recently published data on the alpha and epsilon phases, we found that beta -FeOOH exhibits distinct behavior from the other three polymorphs, as it is significantly less compressible and does not undergo a spin transition. A systematic examination of these iron hydroxide polymorphs as a function of pressure can provide insight into the relationship between electronic spin transitions and structural transitions in these OH- and Fe (super 3+) -bearing phases that may have implications on our understanding of the water content and oxidation state of the mantle.