Carbonic Acid (H2CO3) Decomposes to Its Monohydrate (H2CO3·H2O) and CO2 on Decompression

Abstract The presence of H2O and CO2 on the icy moons in our solar system and in potentially habitable water worlds makes the understanding of phase stabilities and structure–property relations of phases in the H2O–CO2 system highly relevant for astrophysical research. Here we show that single crystals of carbonic acid monohydrate can be synthesized in a laser-heated diamond anvil cell at low pressures ( ≈3 GPa) either by reacting H2O with CO2 or by decompressing H2CO3 to this pressure. The crystal structure of H2CO3·H2O was obtained using synchrotron single crystal X-ray diffraction. At 3 GPa, H2CO3·H2O crystallizes in the orthorhombic space group Pca21 with Z = 8 formula units per unit cell. Density functional theory (DFT)-based calculations satisfactorily reproduce the experimental Raman spectrum, thus corroborating the structural model derived from the diffraction data. DFT enthalpy calculations are also consistent with the formation of H2CO3·H2O by decompression of H2CO3, with a concomitant release of CO2. Carbonic acid and its monohydrate store far more CO2 than filled-ices or clathrates, and hence they provide a mechanism for sequestering large amounts of carbon in icy moons in our solar system or in the icy mantles of water-rich exoplanets.