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.