Symmetrization driven spin transition in ε-FeOOH at high pressure

- New XES and XRD data plus DFT on ε-FeOOH to Earthʼs mantle conditions.
- Our study unveils a connection between water (hydroxyl) content and the spin-transition pressure of Fe3+ in the Earthʼs mantle.
- Evidence that the spin collapse is assisted by symmetrization of hydrogen bonds.

Structural and electronic spin transitions in high-pressure ε-FeOOH are studied using a combination of high pressure X-ray emission spectroscopy (XES), X-ray diffraction (XRD) and density functional theory (DFT) calculations. Using XES, a high- to low-spin transition in trivalent iron is found in ε-FeOOH on compression between 40 and 60 GPa. This is accompanied by a sudden discontinuity in unit cell volume at 53(±2) GPa, obtained from XRD data collected over the same compression range. These results are consistent with DFT calculations using an on-site Coulomb repulsion term (GGA+U), which predict a spin transition in ε-FeOOH at 64.8 GPa. A second order phase transition from P21nm to Pnnm is predicted from DFT at ∼43 GPa and evidenced in the XRD data from the anisotropic stiffening of the lattice parameters around the spin transition. In addition, the DFT results give evidence that the spin collapse is assisted by symmetrization of hydrogen bonds during the transition from P21nm to Pnnm. As the presence of hydrogen, even in small quantities, can affect phase relations, melting temperature, rheology, and other key properties of the Earthʼs mantle, our study unveils a connection between water (hydroxyl) content and the spin-transition pressure of Fe3+ in the Earthʼs mantle.