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 HCO3and 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.

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