High-pressure responses of alkali metal hydrogen carbonates, RbHCO3 and CsHCO3: Findings of new phases and unique compressional behavior

Abstract High-pressure responses of RbHCO3 and CsHCO3 were characterized by in situ Raman spectroscopy and X-ray and neutron diffraction observations. RbHCO3 exhibited a monoclinic (phase IV) and a triclinic (phase V) high-pressure phase at ~0.5 ​GPa and room temperature. Increasing compression induced unique behavior in a specific cell parameter (a in phase IV or c in phase V), which first increased, and then decreased at ~1 ​GPa, likely owing to the rearrangement of Rb+ and reconfiguration of the ordered (HCO3−)2 dimers. Deuterium positions in phase IV of RbDCO3 were determined. The hydrogen bonding remained moderately strong, and possibly did not affect the phase transition despite the accompanying disordering and ordering of the dimers. CsHCO3 showed no structural change up to 5 ​GPa, suggesting that its ambient phase (isostructural to phase IV of KHCO3) was already stable at high pressure. The structural stability appeared to be systematically related to the cation size.