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

The existence of polymeric carbonic acid (H 2 CO 3 ) at elevated pressures has been predicted, but has not been investigated experimentally. Here, polymeric carbonic acid containing s p 3 -hybridized carbon was synthesized and characterized at ≈40 GPa. H 2 CO 3 single crystals were obtained by laser-heating a H 2 O + CO 2 mixture in a diamond anvil cell. The orthorhombic crystal structure ( C m c 2 1 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 H 2 CO 3 - C m c 2 1 is characterized by [CO₄]⁴⁻ [ CO 4 ] 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 H 2 CO 3 - C m c 2 1 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.