Vibrational spectroscopy and x-ray diffraction ofCd(OH)2to28GPaat300K

We report Raman and infrared absorption spectroscopy along with x-ray diffraction for brucite-type$\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Cd}{(\mathrm{O}\mathrm{H})}_{2}$ to $28\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The OH-stretching modes soften with pressure and disappear at $21\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ with their widths increasing rapidly above $5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, consistent with a gradual disordering of the H sublattice at $5--20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ similar to that previously observed for $\mathrm{Co}{(\mathrm{O}\mathrm{H})}_{2}$. Asymmetry in the peak shapes of the OH-stretching modes suggests the existence of diverse disordered sites for H atoms in $\mathrm{Cd}{(\mathrm{O}\mathrm{H})}_{2}$ under pressure. Above $15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the ${A}_{1g}(T)$ lattice mode shows nonlinear behavior and softens to $21\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, 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 $21\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in $\mathrm{Cd}{(\mathrm{O}\mathrm{H})}_{2}$, which has not been previously observed in the brucite-type hydroxides. The Raman spectra of the high-pressure phase of $\mathrm{Cd}{(\mathrm{O}\mathrm{H})}_{2}$ is similar to those of the high-pressure phase of single-crystal $\mathrm{Ca}{(\mathrm{O}\mathrm{H})}_{2}$ of which structure has been tentatively assigned to the $\mathrm{Sr}{(\mathrm{O}\mathrm{H})}_{2}$ type.