We report Raman and infrared absorption spectroscopy along with x-ray diffraction for brucite-type$\beta \text{−}\mathrm{Cd}{\left(\mathrm{O}\mathrm{H}\right)}_2$ to $28\mathrm{GPa}$ at $300\mathrm{K}$. The OH-stretching modes soften with pressure and disappear at $21\mathrm{GPa}$ with their widths increasing rapidly above $5\mathrm{GPa}$, consistent with a gradual disordering of the H sublattice at $5–20\mathrm{GPa}$ similar to that previously observed for $\mathrm{Co}{\left(\mathrm{O}\mathrm{H}\right)}_2$. Asymmetry in the peak shapes of the OH-stretching modes suggests the existence of diverse disordered sites for H atoms in $\mathrm{Cd}{\left(\mathrm{O}\mathrm{H}\right)}_2$ under pressure. Above $15\mathrm{GPa}$, the $A_{1g}\left(T\right)$ lattice mode shows nonlinear behavior and softens to $21\mathrm{GPa}$, 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 $21\mathrm{GPa}$ in $\mathrm{Cd}{\left(\mathrm{O}\mathrm{H}\right)}_2$, which has not been previously observed in the brucite-type hydroxides. The Raman spectra of the high-pressure phase of $\mathrm{Cd}{\left(\mathrm{O}\mathrm{H}\right)}_2$ is similar to those of the high-pressure phase of single-crystal $\mathrm{Ca}{\left(\mathrm{O}\mathrm{H}\right)}_2$ of which structure has been tentatively assigned to the $\mathrm{Sr}{\left(\mathrm{O}\mathrm{H}\right)}_2$ type.

• Received 17 March 2006

DOI:https://doi.org/10.1103/PhysRevB.74.024107