PRESSURE RESPONSE ON THE HYDROGEN BONDS IN METAL HYDROGEN CARBONATES

Potassium hydrogen carbonate (kalicinite) and sodium hydrogen carbonate (nahcolite) are materials having strong hydrogen bonding. Kalicinite possesses hydrogen bonds with a short distance of OcO distance of approximately 2.6 Å indicating a moderately strong hydrogen bond even at ambient pressure. Nahcolite is a similar hydrogen carbonate mineral with moderately strong hydrogen bonding. Despite their chemical similarity, the geometrical arrangements of hydrogen bonds are quite different; KHCO₃ consists of a stacking of dimmers formed by the fusion of the two HCO₃-ions through the formation of hydrogen bonds, whereas nahcolite has a structure consisting of infinite chains. These different structural arrangements offer the possibility of difference in pressure-response on the hydrogen bonds. Kalicinite undergoes pressure-induced structural change at 2.8 GPa and room temperature (Nagai et al., 2002, Solid State Communications), whereas no phase transition has been found for nahcolite up to 10 GPa.

In this study, we measured powder neutron diffractions of kalicinite and nahcolite at high pressure using a spallation neutron source at ISIS, Rutherford Appleton Laboratory. Kalicinite exhibits the decrease in O-HcO angle with increasing pressure and this change could be a trigger of the phase transition at 2.8 GPa (Kagi et al., 2003, American Mineralogist). On the contrary, our recent experimental results on nahcolite have shown that the hydrogen bonding is considerably stable up to 10 GPa. Details in the comparison of these two minerals at high pressure will be presented.