Exploring High-Pressure Polymorphism in Carbonic Acid through Direct Synthesis from Carbon Dioxide Clathrate Hydrate.

Carbon dioxide (CO 2 ) is widespread in astrochemically relevant environments, often coexisting with water (H 2 O) ices and thus triggering a great interest regarding the possible formation of their adducts under various thermodynamic conditions. Amongst them, solid carbonic acid (H 2 CO 3 ) remains elusive, yet being widely studied. Synthetic routes followed for its production have always been characterised by drastic irradiation on solid ice mixtures or complex procedures on fluid samples (such as laser heating at moderate to high pressures). Here we report about a simpler yet effective synthetic route to obtain two diverse carbonic acid crystal structures from the fast, cold compression of pristine clathrate hydrate samples. The two distinct polymorphs we obtained, differing in the water content, have been deeply characterised via spectroscopic and structural techniques to assess their composition and their astonishing pressure stability, checked up to half a megabar, also highlighting the complex correlations between them so to compile a detailed phase diagram of this system. These results may have a profound impact on the prediction and modelisation of the complex chemistry which characterises many icy bodies of our Solar System.