Pressure-induced changes in the bonding and orientation of hydrogen in FeOOH-goethite

Abstract

Infrared spectra of FeOOH-goethite are reported to pressures of 24 GPa at 300 K. The hydroxyl bending vibrations in this phase converge in frequency at pressures above 13 GPa, implying that the location of the hydrogen ion within this structure shifts markedly under pressure. At zero pressure, the hydroxyl bond in FeOOH is canted by 12° from a linear hydrogen bond; the pressure-induced degeneracy of the hydroxyl bending vibrations indicate that this angle increases to ∼28° at 13 GPa and above. This shift in hydrogen position is primarily produced by iron-hydrogen repulsion, and demonstrates that hydrogen can readily shift its position in oxides under compression. Thus, the bonding of hydrogen in this phase depends critically on not only oxygen-oxygen separation, but also on the local cation configuration. Our results on the pressure dependence of the hydroxyl stretching vibrations are in accord with this shift in H-position, with the strength of hydrogen bonding in this phase initially decreasing to 10 GPa, and then increasing, as the hydrogen approaches a position equidistant from three neighboring iron cations. These results emphasize that shifts in hydrogen location can play a crucial role in the bonding of hydroxyl units in oxides under compression.