Vibrational spectroscopy of Fe(OH)2 at high pressure:: Behavior of the O-H bond -: art. no. 184106

Infrared and Raman spectra of Fe(OH)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,g stretching modes of the O-H group decrease linearly with increasing pressure with slopes of -1.3 +/- 0.1 and -4.9 +/- 0.2 cm(-1)/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 Co(OH)(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 Mossbauer spectroscopy.