Pressure-induced phase transformation of kalicinite (KHCO₃) at 2.8 GPa and local structural changes around hydrogen atoms

The pressure-induced structural phase transition in kalicinite, KHCO₃, has been studied by neutron powder diffraction, and infrared (IR) and Raman spectroscopy at high pressure and room temperature. The neutron diffraction study of deuterated kalicinite (KDCO₃) revealed that for the one site for hydrogen (deuterium) found in the low-pressure phase, the O-D⋯O angle decreases from 176 to 161° and the distance between donor and acceptor O atoms of the O-D⋯O group decreases from 2.66 to 2.59 Å in the pressure range from 0 to 2.5 GPa. The crystal structure of the high-pressure polymorph was not determined. Infrared spectra were obtained at pressures up to 6.3 GPa using a diamond anvil cell. At ambient pressure, the O-H stretching, O-H⋯O in-plane bending, and O-H⋯O out-of-plane bending modes occur at 2620, 1405, and 988 cm^-1, respectively. The frequency of the O-H stretch mode was nearly constant in the pressure range from 0 to 2.8 GPa, while that of O-H⋯O in-plane bending and out-of-plane modes increased with increasing pressure up to 2.8 GPa and remained constant above the phase transition pressure. The Raman spectra showed a clear phase transition at 2.8 GPa. The three Raman modes observed are assigned to internal vibrational modes of HCO₃ and this suggests that the surrounding environment did change dramatically at the phase transition. These results suggest that the phase transition in kalicinite is triggered by the distortion of C-O-H bond at high pressure.