Crystal Structure of Carbonic Acid (H₂CO₃) at Elevated Pressures from Single-Crystal Diffraction

Single crystals of carbonic acid (H₂CO₃) were synthesized in a laser-heated diamond anvil cell at moderate pressures between 5 and 13 GPa by reacting H₂O with CO₂. Its monoclinic crystal structure (P2₁/n with Z = 4) has been obtained from synchrotron single-crystal X-ray diffraction experiments at ≈8 GPa. The positions of the hydrogen atoms have been determined from the experimental data. Density functional theory-based calculations in combination with experimental Raman spectroscopy confirmed the structural model derived from the diffraction data. This is the first single-crystal structure solution of water-free carbonic acid, H₂CO₃. The structural model provided here differs from structural models reported earlier for lower pressures derived from neutron powder diffraction data.