Polymorphism of carbonic acid (H2CO3) at low pressures.

Carbonic acid (H2CO3) has been detected on the surfaces of icy moons and has been suggested to occur in the interior of ice giants. Hence, understanding structure-property relations of phases in the H-C-O system is of significant interest to astrophysics and solid-state chemistry. Here we show, that at low pressures (≈9 GPa) two H2CO3 polymorphs can occur simultaneously at different temperatures. The two H2CO3 phases were obtained in different areas of a sample chamber of a diamond anvil cell after laser-heating a H2O + CO2 mixture. The orthorhombic crystal structure of H2CO3-Pnma was refined from synchrotron single crystal X-ray diffraction, including the position of the hydrogen atoms. The structure had previously been predicted, but had not been confirmed experimentally. Our density functional theory-based calculations reproduce the experimental Raman spectrum, confirming that the structural model of H2CO3-Pnma is appropriate. From the spatial phase distribution in the sample chamber we infer that H2CO3-Pnma is the low temperature polymorph of H2CO3-P21/n, implying that the p,T-diagram of carbonic acid is more complex at low pressures than previously assumed.