The high-pressure behaviour of caesium hydroxide monohydrate

Although the anhydrous alkali hydroxides have been extensively studied, their hydrates have been much less explored despite the common nature of the combination of ionic interactions and hydrogen bonding. In this study, we characterised the compression behaviour of CsOH·H₂O, which contains a quasi-2D hydrogen-bonded network exhibiting high solid-state proton conductivity via the Grotthuss mechanism. No clear structural transition was observed up to 9 GPa, whilst Raman scattering exhibits slight changes in the spectrum at 3 GPa. X-ray and neutron diffraction revealed anisotropic compression of CsOH·H₂O with the largest compressibility along the c-axis, reducing the interlayer distances like the other layered materials. Accordingly, the quasi-2D hydrogen-bonded network becomes flattened. In Raman spectroscopy, a typical pressure response was found for hydrogen-bonded crystals, such as pressure-induced strengthening of hydrogen bonds characterised by the downshift of OH stretching modes as well as the upshift of other optical modes of translations and librational movements. On the other hand, some of the low-frequency bands became indistinguishable above 3 GPa, where the pressure dependence of the OH stretching mode has a kink as well. The anisotropic compression behaviour reduces the free volume, which will affect the peculiar phase behaviour and proton dynamics in the quasi-2D hydrogen-bonded network of CsOH·H₂O present at ambient pressure.