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

2002; American Physical Society; Volume: 66; Issue: 13 Linguagem: Inglês

10.1103/physrevb.66.134301

1095-3795

S. R. Shieh, T. S. Duffy,

Geological and Geochemical Analysis

Raman spectra of cobalt hydroxide $\mathrm{Co}(\mathrm{OH}{)}_{2}$ 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 $\mathrm{Co}(\mathrm{OD}{)}_{2}$ to 16 GPa which shows no evidence for such amorphization but instead indicates a possible change in local D ordering in response to $D\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}D$ 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 $\ensuremath{\sim}200{\mathrm{cm}}^{\ensuremath{-}1}$ at 25 GPa. An energy dispersive x-ray diffraction study carried out to 48 GPa indicates that $\mathrm{Co}(\mathrm{OH}{)}_{2}$ remains crystalline to at least this pressure, although evidence for some anomalous broadening of x-ray peaks is observed at $\ensuremath{\sim}26\mathrm{GPa}.$ The peak widths $(>200{\mathrm{cm}}^{\ensuremath{-}1})$ of both the Raman and infrared stretching vibration of $\mathrm{Co}(\mathrm{OH}{)}_{2}$ 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.