Vibrational spectroscopy and x-ray diffraction of Cd (OH)2 to 28 GPa at 300 K

We report Raman and infrared absorption spectroscopy along with x-ray diffraction for brucite-type β-Cd (OH)2 to 28 GPa at 300 K. The OH-stretching modes soften with pressure and disappear at 21 GPa with their widths increasing rapidly above 5 GPa, consistent with a gradual disordering of the H sublattice at 5-20 GPa similar to that previously observed for Co (OH)2. Asymmetry in the peak shapes of the OH-stretching modes suggests the existence of diverse disordered sites for H atoms in Cd (OH)2 under pressure. Above 15 GPa, the A1g (T) lattice mode shows nonlinear behavior and softens to 21 GPa, at which pressure significant changes are observed: some Raman modes appear, two Raman-active lattice modes and the OH-stretching modes of the low-pressure phase disappear, and the positions of some x-ray diffraction lines change abruptly with the appearance of weak diffraction features. These observations suggest that amorphization of the H sublattice is accompanied by a crystalline-to- crystalline transition at 21 GPa in Cd (OH)2, which has not been previously observed in the brucite-type hydroxides. The Raman spectra of the high-pressure phase of Cd (OH)2 is similar to those of the high-pressure phase of single-crystal Ca (OH)2 of which structure has been tentatively assigned to the Sr (OH)2 type.