Infrared and Raman spectra of $\mathrm{Fe}{\left(\mathrm{OH}\right)}_2$, “white rust,” were measured between 7 and 21 GPa at ambient temperature. The frequency of the infrared-active $A_{2u}$ and of the Raman-active $A_{1g}$ stretching modes of the O-H group decrease linearly with increasing pressure with slopes of $-1.3\pm 0.1$ and $-4.9\pm 0.2{\mathrm{cm}}^{-1}∕\mathrm{GPa}$, respectively. The peak widths of both the infrared-active and Raman-active modes increase nonlinearly with pressure, with a discontinuous increase in broadening between 10 and 12.5 GPa. The overall broadening of the $A_{2u}$ and $A_{1g}$ stretching modes is approximately fourfold in the examined pressure range. The results of this spectroscopic study are compatible with the trends observed in recent neutron diffraction studies on the isostructural $\mathrm{Co}{\left(\mathrm{OH}\right)}_2$. Progressive pressure-induced H disordering could be a viable model to interpret both the broadening of the OH stretching mode and the changes in oxidation state of Fe recently observed by Mössbauer spectroscopy.

• Received 7 November 2004

DOI:https://doi.org/10.1103/PhysRevB.71.184106