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

Area of Science:

• Geochemistry
• Crystallography
• Materials Science

Background:

• Carbonic acid (H2CO3) is a key molecule in various natural systems.
• Previous structural studies of carbonic acid were limited by lower pressures and different experimental techniques.
• Understanding carbonic acid's structure is crucial for geochemistry and materials science.

Purpose of the Study:

• To synthesize single crystals of water-free carbonic acid (H2CO3).
• To determine the crystal structure of H2CO3 at moderate pressures using single-crystal X-ray diffraction.
• To validate the structural model using computational and spectroscopic methods.

Main Methods:

• Synthesis of carbonic acid single crystals in a laser-heated diamond anvil cell (DAC).
• Synchrotron single-crystal X-ray diffraction for structural determination at ~8 GPa.
• Density functional theory (DFT) calculations and Raman spectroscopy for structural validation.

Main Results:

• Successfully synthesized single crystals of H2CO3 at 5-13 GPa.
• Determined the monoclinic crystal structure (P21/n, Z=4) of H2CO3 at ~8 GPa.
• Identified hydrogen atom positions and confirmed the structural model experimentally and computationally.

Conclusions:

• This work presents the first single-crystal structure solution for water-free carbonic acid.
• The determined structure at moderate pressures differs from previously reported models at lower pressures.
• The findings provide a more accurate structural understanding of carbonic acid under relevant geological conditions.