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6CCVD Research

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

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-06-25
JournalChemistry - A European Journal
AuthorsDominik Spahr, Elena Bykova, Lkhamsuren Bayarjargal, Maxim Bykov, Lukas Brüning
InstitutionsDassault Systèmes (United Kingdom), Goethe University Frankfurt
Citations2
AnalysisFull AI Review Included

Crystal Structure of Carbonic Acid (H2CO3) at Elevated Pressures

Section titled “Crystal Structure of Carbonic Acid (H2CO3) at Elevated Pressures”

Executive Summary

Section titled “Executive Summary”
  • Novel Structure Determination: The first single-crystal structure solution for water-free carbonic acid (H2CO3) was successfully obtained, resolving long-standing structural ambiguities.
  • Synthesis Conditions: H2CO3 was synthesized by reacting H2O and CO2 in a Laser-Heated Diamond Anvil Cell (LH-DAC) at moderate pressures (5-13 GPa) and temperatures up to ~800 K.
  • Structural Identity: The new polymorph, H2CO3-P21/n, is monoclinic (space group P21/n, Z = 4) and features H2CO3 molecules in a cis-cis conformation.
  • Validation: The structural model derived from synchrotron X-ray diffraction at 8 GPa was rigorously confirmed by Density Functional Theory (DFT) calculations, showing excellent agreement with experimental Raman spectroscopy data.
  • Distinction from Prior Work: This P21/n structure is distinct from previously proposed models (Pnma and P21/c) derived from neutron powder diffraction data at lower pressures.
  • Engineering Impact: The established structure provides the necessary fundamental data (lattice parameters, bulk modulus, vibrational spectra) for lattice dynamical calculations and for identifying solid H2CO3 in complex environments, such as planetary ices.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Synthesis Pressure Range5 to 13GPaLH-DAC reaction conditions
Synthesis Temperature (Max)≤800KEstimated maximum temperature during heating
Diffraction Pressure (P)8(1)GPaCondition for SCXRD structure solution
Crystal SystemMonoclinic-H2CO3-P21/n polymorph
Space GroupP21/n-Z = 4
Lattice Parameter a4.428(1)AngstromAt 8 GPa
Lattice Parameter b4.498(1)AngstromAt 8 GPa
Lattice Parameter c9.034(4)AngstromAt 8 GPa
Beta Angle (β)100.82(4)degreesAt 8 GPa
Unit Cell Volume (V)176.7(1)Angstrom3At 8 GPa
Bulk Modulus (K0)14.2(4)GPaDerived from DFT p,V relation
Pressure Derivative (K0’)6.1(1)-Derived from DFT p,V relation
C-O Bond Length (Isolated O)1.237(3)AngstromShortest C-O bond in the molecule
Characteristic Raman Mode~1095cm-1Strong C-O stretching vibration
R1-value (Refinement)5.7%Indicating reliable structure refinement

Key Methodologies

Section titled “Key Methodologies”

  1. High-Pressure Synthesis:

    • Synthesis was conducted in a Laser-Heated Diamond Anvil Cell (LH-DAC).
    • Starting materials (H2O and CO2) were loaded cryogenically into the DAC chamber.
    • The mixture was laser-heated from both sides for approximately 30 minutes, reaching temperatures up to the onset of optically detectable thermal radiation (~800 K).

  2. Phase Mapping (Raman Spectroscopy):

    • Spatially resolved Raman spectroscopy was used before and after heating to monitor the distribution and consumption of starting phases (CO2-I, H2O-VII) and the formation of the new H2CO3 phase.
    • The characteristic Raman modes of the product (e.g., ~1095 cm-1) were used to confirm the synthesis.

  3. Structural Determination (SCXRD):

    • Synchrotron X-ray Single-Crystal Diffraction (SCXRD) was performed at PETRA III, beamline P02.2, using a focused ~2 x 2 µm2 beam.
    • The structure was solved at 8(1) GPa, yielding the monoclinic P21/n space group.

  4. Refinement and Validation:

    • Hydrogen atom positions were initially located in the difference Fourier map.
    • A restraint was applied to the O-H bond distance, fixing it to the value derived from DFT calculations (~1 Angstrom), as is standard practice for high-pressure light-element refinement.
    • The final structural model was validated using Density Functional Perturbation Theory (DFPT) calculations, confirming the structural stability (no negative frequencies) and matching the calculated phonon density of states (PDOS) and Raman spectra to the experimental data.

Commercial Applications

Section titled “Commercial Applications”
Industry/SectorRelevance to H2CO3 Structure Research
Planetary Science & AstrophysicsThe definitive structural and vibrational data (Raman modes) are essential for creating spectral libraries used in remote sensing to identify solid carbonic acid on icy planetary bodies (e.g., moons of Jupiter and Saturn) and in meteorites.
Geochemistry & Carbon CyclingThe accurate ground-state structure and calculated bulk modulus (K0 = 14.2 GPa) are critical inputs for thermodynamic models of deep-earth carbon storage and the stability of related hydrous pyrocarbonates under high-pressure conditions.
Computational Materials ScienceThe experimentally validated DFT model of H2CO3-P21/n serves as a high-fidelity reference for developing and benchmarking computational methods used to predict the behavior and stability of molecular solids under extreme pressure/temperature regimes.
High-Pressure Materials SynthesisThe demonstrated LH-DAC methodology provides a reliable “recipe” for synthesizing novel, chemically simple compounds (like carbonates and hydrides) that are kinetically or thermodynamically unstable at ambient pressure, opening pathways for new material discovery.
View Original Abstract

Abstract Single crystals of carbonic acid (H 2 CO 3 ) were synthesized in a laser‐heated diamond anvil cell at moderate pressures between 5 and 13 GPa by reacting H 2 O with CO 2 . Its monoclinic crystal structure ( P 2 1 / 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, H 2 CO 3 . The structural model provided here differs from structural models reported earlier for lower pressures derived from neutron powder diffraction data.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2008 - Lehrbuch der Anorganischen Chemie

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