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

At a Glance

Metadata	Details
Publication Date	2025-06-25
Journal	Chemistry - A European Journal
Authors	Dominik Spahr, Elena Bykova, Lkhamsuren Bayarjargal, Maxim Bykov, Lukas Brüning
Institutions	Goethe University Frankfurt, Deutsches Elektronen-Synchrotron DESY
Citations	2
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Pressure Synthesis of Carbonic Acid (H${2}$CO${3}$)

This documentation analyzes the requirements and achievements of the research paper “Crystal Structure of Carbonic Acid (H${2}$CO${3}$) at Elevated Pressures from Single-Crystal Diffraction,” focusing on the critical role of high-quality diamond materials and connecting the experimental needs to 6CCVD’s core capabilities in MPCVD diamond synthesis.

Executive Summary

Novel Structure Determination: The first single-crystal structure solution of water-free carbonic acid (H${2}$CO${3}$) was achieved, identifying a new monoclinic polymorph (P2$_{1}$/n, Z=4).
High-Pressure Synthesis Method: H${2}$CO${3}$ was synthesized by reacting H${2}$O and CO${2}$ within a Laser-Heated Diamond Anvil Cell (LH-DAC) at moderate pressures (5-13 GPa) and temperatures (up to $\approx$800 K).
Critical Material Requirement: The success of this experiment relies entirely on the optical and mechanical integrity of the Single Crystal Diamond (SCD) anvils used in the DAC for simultaneous high-pressure generation, laser heating, and synchrotron X-ray transmission.
Structural Confirmation: The P2$_{1}$/n structure was solved via synchrotron single-crystal X-ray diffraction at $\approx$8 GPa and confirmed using Density Functional Theory (DFT) calculations and experimental Raman spectroscopy.
Scientific Impact: The established crystal structure is vital for lattice dynamical calculations, understanding the stability field of H${2}$CO${3}$ polymorphs, and enabling remote sensing identification of solid carbonic acid on icy moons and planets.

Technical Specifications

The following hard data points were extracted from the experimental results, highlighting the extreme conditions required for synthesis and analysis.

Parameter	Value	Unit	Context
Synthesis Pressure Range	5 to 13	GPa	Range for H${2}$O + CO${2}$ reaction
Diffraction Pressure	8(1)	GPa	Synchrotron X-ray analysis pressure
Synthesis Temperature	Up to 800	K	Laser heating maximum
Resulting Crystal Structure	Monoclinic P2$_{1}$/n	N/A	New H${2}$CO${3}$ polymorph (Z=4)
Unit Cell Volume (V)	176.7(1)	Å$^{3}$	Measured at 8 GPa
Lattice Parameter (a)	4.428(1)	Å	Measured at 8 GPa
Lattice Parameter (c)	9.034(4)	Å	Measured at 8 GPa
X-ray Beam Profile	$\approx$2 x 2	µm$^{2}$	Focused beam size for single-crystal diffraction
Bulk Modulus (K$_{0}$)	14.2(4)	GPa	Derived from DFT calculations
Characteristic Raman Mode	$\approx$620	cm$^{-1}$	Associated with H${2}$CO${3}$ atomic displacement

Key Methodologies

The synthesis and characterization of H${2}$CO${3}$-P2$_{1}$/n required precise control over pressure, temperature, and analytical techniques, all enabled by the use of high-performance diamond anvils.

High-Pressure Synthesis: A Laser-Heated Diamond Anvil Cell (LH-DAC) was employed to achieve the necessary pressure (5-13 GPa) and temperature ($\le$800 K) conditions for the H${2}$O + CO${2}$ reaction.
Sample Loading: CO${2}$ was cryogenically loaded as dry ice into the DAC sample chamber, ensuring the presence of H${2}$O-VII and CO$_{2}$-I starting phases.
In-situ Heating: Laser heating was performed from both sides for approximately 30 minutes, reaching temperatures up to 800 K, sufficient to induce the chemical reaction without causing phase transitions in the starting materials.
Phase Mapping (Raman Spectroscopy): Spatially resolved Raman spectroscopy was used before and after heating (at 9 GPa) to map the distribution of phases (H${2}$O-VII, CO${2}$-I, and the newly synthesized H${2}$CO${3}$-P2$_{1}$/n).
Single-Crystal X-ray Diffraction: Synchrotron radiation was used at 8(1) GPa with a highly focused $\approx$2 x 2 µm$^{2}$ beam to solve the crystal structure, successfully locating the hydrogen atom positions without relying on extensive constraints.
Computational Validation: DFT and DFPT calculations were performed to confirm the structural model, accurately reproduce the experimental Raman spectra, and verify the dynamic stability of the H${2}$CO${3}$-P2$_{1}$/n phase at 8 GPa.

6CCVD Solutions & Capabilities

The successful execution of high-pressure, high-temperature experiments like the synthesis of H${2}$CO${3}$ is fundamentally dependent on the quality and precision of the diamond anvils. 6CCVD specializes in providing the MPCVD diamond materials necessary to meet these extreme engineering demands.

Applicable Materials

To replicate or extend this high-pressure research, the primary requirement is for diamond material offering exceptional optical transparency, thermal stability, and mechanical strength.

Research Requirement	6CCVD Material Recommendation	Key Benefit
High-Pressure Anvils (5-13 GPa)	Optical Grade Single Crystal Diamond (SCD)	Highest purity (Type IIa equivalent) ensures maximum mechanical integrity and minimal stress-induced birefringence under extreme load.
Laser Heating & Raman Analysis	Optical Grade SCD	High transparency across the visible, IR, and X-ray spectra, crucial for simultaneous laser heating and spectroscopic/diffraction analysis.
Electrical/Resistive Heating	Boron-Doped Diamond (BDD) Wafers	For future experiments requiring electrical resistive heating elements or electrodes integrated into the DAC setup.

Customization Potential

High-pressure research often requires non-standard geometries and integrated functionalities (e.g., heating elements, electrodes). 6CCVD’s custom fabrication capabilities directly address these needs.

Custom Dimensions and Geometry: 6CCVD provides SCD plates and wafers up to 125mm, which can be laser-cut and polished to exacting specifications for DAC anvil fabrication (e.g., specific culet sizes, bevels, and table dimensions).
Ultra-Low Roughness Polishing: We guarantee SCD polishing to an average roughness (Ra) of < 1 nm, ensuring the critical anvil-to-anvil interface is perfectly flat for maximum pressure stability and minimal gasket extrusion.
Integrated Metalization Services: For advanced LH-DAC setups requiring integrated heating elements or electrical contacts (as used in similar high-pressure studies), 6CCVD offers in-house thin-film deposition of metals including Au, Pt, Pd, Ti, W, and Cu directly onto the diamond surface.
Thickness Control: We offer precise thickness control for SCD wafers (0.1 µm to 500 µm), allowing researchers to optimize the diamond path length for specific X-ray or neutron diffraction experiments, minimizing absorption and background noise.

Engineering Support

6CCVD’s in-house PhD team of material scientists and engineers is available to assist researchers in selecting and optimizing diamond materials for similar High-Pressure Synthesis and Structural Determination projects. We ensure that the diamond material properties (e.g., nitrogen concentration, defect density, and orientation) are perfectly matched to the specific demands of LH-DAC and synchrotron experiments.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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.