The High Pressure Behavior of the FeOOH Polymorphs

Understanding the high-pressure behavior of the polymorphs of FeOOH; α, β, γ and ε, represents an important step to quantifying the more complex hydrogen-bearing compounds relevant to the Earth’s interior. Monitoring the spin state of these iron hydroxides as a function of pressure can provide insight into the relationship between electronic spin transitions and structural transitions. We studied α-FeOOH as a function of pressure up to 75 GPa using X-ray emission spectroscopy (XES) and observed that it undergoes a spin transition beginning at approximately 50 GPa. In γ-FeOOH, we see a spin transition beginning around 35 GPa. β- FeOOH does not show any indication of a spin transition for pressures up to 65 GPa. Analysis of our X-ray diffraction (XRD) data shows that neither β-FeOOH nor γ-FeOOH undergoes any obvious phase structural transition with both appearing to amorphize above ~20 GPa. We present the derived EOS of β and γ and compare them with recently published data on α and ε. We note that β –FeOOH displayed distinct behavior when compared to the other three polymorphs; it was significantly less compressible and did not undergo a spin transition over the pressure range studied.