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 -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 OHstretching 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 OHstretching 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.