High-pressure Raman and FTIR studies of solid potassium, rubidium and caesium hydroxides

Effects of compression on the Raman and IR modes of KOH(D), RbOH(D) and CsOH(D) have been measured at pressures up to 20 GPa and temperatures down to 100 K using the diamond anvil technique. For all three hydroxides an ordering transition into the respective hydrogen-bonded low-temperature phases was found below 1 GPa at room temperature. In addition, the spectra of KOH(D) and RbOH(D) show significant changes at pressures above 6.5 GPa, which point to the existence of a new phase. The decrease in frequency of the IR- and Raman-active O-H(D) stretching vibrations with increasing compression indicates that hydrogen bonds exist also in the new high-pressure phase VI. The variation of the 0-0 distance at the phase transition IV implies VI is estimated by using a double-minimum potential.