Synthesis and characterization of crystalline polymeric carbonic acid (H2CO3) with sp3-hybridized carbon at elevated pressures
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-08-08 |
| Journal | Communications Chemistry |
| Authors | Dominik Spahr, Lkhamsuren Bayarjargal, Lukas Brüning, Valentin Kovalev, Lena Wedek |
| Institutions | Dassault Systèmes (United Kingdom), Goethe University Frankfurt |
| Analysis | Full AI Review Included |
Synthesis and Characterization of Crystalline Polymeric Carbonic Acid (H2CO3) with sp3-Hybridized Carbon at Elevated Pressures
Section titled “Synthesis and Characterization of Crystalline Polymeric Carbonic Acid (H2CO3) with sp3-Hybridized Carbon at Elevated Pressures”Executive Summary
Section titled “Executive Summary”- Novel Synthesis: The first experimental synthesis and single-crystal structure determination of a high-pressure polymorph of carbonic acid (H2CO3-Cmc21) was achieved using a Laser-Heated Diamond Anvil Cell (LH-DAC).
- Extreme Conditions: The reaction between H2O and CO2 was successfully induced at extreme conditions: approximately 40 GPa pressure and 1000 K temperature.
- Structural Transformation: The resulting material exhibits a significant structural change, featuring sp3-hybridized carbon, moving away from the conventional sp2-hybridized carbonate structure.
- Polymerization: The crystal structure is characterized by the polymerization of [CO4]4- tetrahedra, which connect via corner sharing to form infinite chains along the c-axis.
- High Density/Stiffness: The polymeric phase (H2CO3-Cmc21) possesses a high bulk modulus (K0 = 50 GPa), indicating significantly greater stiffness compared to the low-pressure sp2-polymorph (K0 = 14.2 GPa).
- Astrophysical Relevance: The synthesis conditions suggest that this polymeric H2CO3 phase may exist within the H2O-rich ice shells of ice giants (e.g., Uranus and Neptune) and other icy planetary bodies containing H2O and CO2.
- Validation: The experimental single-crystal X-ray diffraction data (R1-value 4.6%) and Raman spectra were confirmed by Density Functional Theory (DFT) calculations, validating the structural model.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Synthesis Pressure | 40(2) | GPa | Target pressure for H2O + CO2 reaction |
| Synthesis Temperature | ≈1000(300) | K | Maximum temperature reached via laser heating |
| Crystal Structure | Orthorhombic, Cmc21 | N/A | High-pressure polymeric H2CO3 phase |
| Carbon Hybridization | sp3 | N/A | Characterized by [CO4]4- tetrahedral building blocks |
| Lattice Parameter (a) | 7.286(3) | A | Determined at 40 GPa by SC-XRD |
| Lattice Parameter (b) | 4.275(1) | A | Determined at 40 GPa by SC-XRD |
| Lattice Parameter (c) | 3.809(4) | A | Determined at 40 GPa by SC-XRD |
| Unit Cell Volume (V) | 118.6(1) | A3 | Determined at 40 GPa |
| SC-XRD R1-value | 4.6 | % | Reliability indicator for single crystal structure refinement |
| Bulk Modulus (K0) | 50(2) | GPa | Derived from DFT Equation of State (EoS) fitting |
| Pressure Derivative (Kp) | 5.2(1) | N/A | Derived from DFT EoS fitting |
| Characteristic Raman Mode 1 | ≈820 | cm-1 | Due to H atom displacement (perpendicular to O-H bond) |
| Characteristic Raman Mode 2 | ≈910 | cm-1 | Due to H atom displacement (with stretching contribution) |
| C-O Bond Length (Avg.) | 1.342 to 1.375 | A | Within the protonated [CO4]4- tetrahedron at 40 GPa |
| Hydrogen Bond Distance (O-H…O) | 2.444 | A | Indicative of very strong hydrogen bonds at 40 GPa |
| Anisotropic Compression (a-axis) | ≈10 | % | Compression between 30 and 90 GPa |
| Anisotropic Compression (b/c-axes) | ≈6 | % | Compression between 30 and 90 GPa |
Key Methodologies
Section titled “Key Methodologies”The synthesis and characterization relied on a combination of high-pressure experimental techniques and computational modeling:
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Diamond Anvil Cell (DAC) Preparation:
- A Boehler-Almax type DAC was used.
- Bidistilled H2O was added to the sample chamber, followed by partial evaporation.
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Cryogenic Loading:
- The DAC was cooled to ≈100 K.
- CO2 gas was condensed directly into the chamber as dry ice using a custom cryogenic loading system, ensuring a complete H2O + CO2 mixture.
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High-Pressure Compression:
- The mixture was compressed to the target pressure of 40(2) GPa. Pressure was monitored using the high-frequency edge of the diamond Raman band.
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Laser Heating (Synthesis):
- Double-sided heating was performed using a pulsed CO2 laser (λ = 10,600 nm) to induce the reaction, reaching temperatures up to ≈1000 K.
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Raman Spectroscopy and Mapping:
- An Oxford Instruments WITec alpha 300R microscope was used for in-situ Raman imaging (532 nm laser).
- 2D-Raman maps were generated to spatially resolve the distribution of the newly formed H2CO3-Cmc21 phase relative to the co-existing CO2 polymorphs (CO2-III and CO2-V).
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Synchrotron Single Crystal X-ray Diffraction (SC-XRD):
- Data was collected at the DESY PETRA III synchrotron (Extreme Conditions Beamline P02.2).
- A highly focused X-ray beam (≈2 x 2 µm2 FWHM) was used to collect diffraction data suitable for single-crystal analysis, leading to the structure solution (Cmc21).
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Density Functional Theory (DFT) Calculations:
- Calculations were performed using the CASTEP simulation package (PBE functional) with Tkatchenko and Scheffler v.d.W. corrections.
- Used to optimize the geometry, calculate the Equation of State (EoS) parameters (K0, Kp), and derive theoretical vibrational spectra (DFPT) for comparison with experimental Raman data.
Commercial Applications
Section titled “Commercial Applications”While this research is primarily fundamental high-pressure chemistry, the findings provide critical data and methodologies relevant to several engineering and commercial sectors dealing with extreme environments and carbon management:
- Extreme Environment Engineering: The synthesis methodology (LH-DAC) and the resulting material data (high bulk modulus, anisotropic compression) are essential for designing and modeling components intended for use in ultra-high pressure environments, such as deep-sea or deep-earth drilling equipment, or specialized sensors for planetary probes.
- High-Density Carbon Materials Synthesis: The successful pressure-induced sp2-to-sp3 hybridization in H2CO3 at relatively moderate pressures (40 GPa) offers a pathway for synthesizing novel, dense, and potentially ultra-hard sp3-hybridized carbon compounds that may have industrial applications.
- Geological Carbon Sequestration (CCS): The thermodynamic data (EoS) and structural information on high-pressure H2CO3 polymorphs improve predictive models for the long-term stability and phase behavior of CO2 and H2O mixtures under deep geological storage conditions.
- Astrochemical Modeling and Remote Sensing: The established Raman spectral signatures (≈820 cm-1 and ≈910 cm-1) for H2CO3-Cmc21 are crucial for creating spectral libraries, enabling the remote identification of high-pressure carbonic acid on icy planetary bodies using advanced spectroscopic instruments (e.g., those on JWST or future planetary missions).