Raman spectroscopy of Co„OH...2 at high pressures: Implications for amorphization and hydrogen repulsion

Raman spectra of cobalt hydroxide Co(OH)2 were recorded to 31 GPa using a diamond anvil cell. In contrast to a previous infrared ~IR! spectroscopic study, our Raman data show no sharp change in the O-H stretch peak width near 11 GPa, ruling out the possibility of abrupt hydrogen sublattice amorphization at this pressure. This is consistent with neutron diffraction data for Co(OD)2 to 16 GPa which shows no evidence for such amorphization but instead indicates a possible change in local D ordering in response to D•••D repulsion. However, in agreement with the IR work, we find that the peak width of the OH stretch mode increases significantly over a broad pressure interval from 10 to 25 GPa and reaches ;200 cm at 25 GPa. An energy dispersive x-ray diffraction study carried out to 48 GPa indicates that Co(OH)2 remains crystalline to at least this pressure, although evidence for some anomalous broadening of x-ray peaks is observed at ;26 GPa. The peak widths (.200 cm) of both the Raman and infrared stretching vibration of Co(OH)2 are indicative of a high degree of disorder of the H positions while only minimal disordering of the Co-O substructure is allowed by the x-ray data. Thus, the possibility of substructure amorphization remains viable, but through a process of continuous disordering over a broad pressure interval rather than as a discrete discontinuity. Repulsion between neighboring H atoms is likely to be an important driving force for this disordering as suggested by previous neutron and theoretical studies, but it remains uncertain whether such a model can quantitatively describe the range of observed peak widths at high pressures.