Synthesis and characterization of crystalline polymeric carbonic acid (H₂CO₃) with sp³-hybridized carbon at elevated pressures

The existence of polymeric carbonic acid (H₂CO₃) at elevated pressures has been predicted, but has not been investigated experimentally. Here, polymeric carbonic acid containing sp³-hybridized carbon was synthesized and characterized at  ≈40 GPa. H₂CO₃ single crystals were obtained by laser-heating a H₂O + CO₂ mixture in a diamond anvil cell. The orthorhombic crystal structure (Cmc2₁ with Z = 4) was refined from synchrotron single crystal X-ray diffraction data and is in agreement with a structural model predicted earlier. The crystal structure of H₂CO₃-Cmc2₁ is characterized by ${\left[{\mathrm{CO}}_4\right]}^{4-}$ building blocks which are connected via corner sharing, forming chains along the c-axis. The combination of single crystal X-ray data with experimental Raman spectroscopy and DFT-calculations confirms that the structural model of H₂CO₃-Cmc2₁ is appropriate. The synthesis condition of polymeric carbonic acid points towards its potential existence in ice giants including the ones present in our solar system.