High-Pressure Neutron Diffraction Study on ε-FeOOH: The Spin-Reorientation Transition and Hydrogen-Bond Symmetrization.

The compression behavior of iron oxyhydroxide ε-FeOOH is complex, with variations in its magnetic property and bonding character. In this study, in situ powder neutron diffraction experiments were conducted on ε-FeOOH and ε-FeOOD up to pressures exceeding 20 GPa to investigate a spin-reorientation (spin-flop) transition, hydrogen-bond (H-bond) symmetrization, and their correlation. The magnetic transition was observed at 8 GPa in both ε-FeOOH and ε-FeOOD. The Fe-spins aligned in an A-type antiferromagnetic order and reoriented from the c-direction to the b-direction above 8 GPa. The crystal symmetry and H-bonds of ε-FeOOH transitioned from P21nm with asymmetric H-bonds to Pnnm with disordered H-bonds at 17.90 GPa, while ε-FeOOD remained asymmetric. The pressure evolution of the interatomic angles and distances indicates that the coordinate geometry of Fe transitioned toward H-bond symmetrization and would have affected the magnetic anisotropy. This study provides insights into the pressure effect of the magnetic transition on compounds containing hydrogen bonds.