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.