Pressure Response on Hydrogen Bonds in Potassium Hydrogen Carbonate and Sodium Hydrogen Carbonate

Pressure responses of hydrogen bonding interactions were compared between potassium hydrogen carbonate (KHCO₃) and sodium hydrogen carbonate (NaHCO₃). KHCO₃ undergoes a pressure-induced structural change at 2.8 GPa and room temperature whereas no phase transition has been found so far for NaHCO₃ up to 11 GPa. KHCO₃ exhibits a substantial decrease in the O—H···O angle with increasing pressure. This change could be a trigger for the phase transition at 2.8 GPa. By contrast, no significant change in O—H···O angle was observed for NaHCO₃ up to 7 GPa. Based on the observations using neutron diffraction, X-ray diffraction and vibrational spectroscopy at high pressure, a possible mechanism for the phase transition of KHCO₃ is suggested. In addition, the phase boundary of KHCO₃ caused by the pressure-induced transition was determined using in situ Raman spectroscopy at high pressure.