Compression and pressure-induced amorphization of Co(OH)2characterized by infrared vibrational spectroscopy

The infrared-active (${\mathit{A}}_{2\mathit{u}}$) O-H vibration of Co(OH${)}_2$ decreases in frequency under hydrostatic compression to 51 GPa at 290 K. Similarly, the bond anharmonicity, determined from the ${\nu }_1$→${\nu }_2$ absorption-band difference, increases by more than a factor of 2 between 0 and 20 GPa. Both changes are attributed to an increase in the O-H bond length due to enhanced hydrogen bonding under pressure. The full width at half maximum (FWHM) of the fundamental absorption band increases abruptly by ∼100 ${\mathrm{cm}}^{\mathrm{-}1}$ at 11.2 (±0.3) GPa, and continues to increase at a rate of ∼3.3 ${\mathrm{cm}}^{\mathrm{-}1}$/GPa up to 36 GPa. Above 36 (±2) GPa and below the onset of amorphization, the FWHM changes at a slower rate, 0.8 (±0.1) ${\mathrm{cm}}^{\mathrm{-}1}$/GPa. The abrupt change in FWHM is reversible on decompression, and is interpreted in terms of a pressure-induced crystal-to-glass transition exhibiting a small hysteresis compared to similar compounds. The rapid variation in FWHM above the transition pressure suggests that the amorphous structure is continuously modified between 11.3 and 36 GPa.