High-pressure synthesis of carbonic acid polymorphs from carbon dioxide clathrate hydrate

Carbon dioxide (CO₂) is largely present in diverse astrochemically relevant environments, quite often co-existing with water (H₂O) ices. Their simultaneous presence has triggered a great interest regarding the stabilization of CO₂ clathrate hydrates and the possible formation of adducts under various thermodynamic conditions. Amongst these adducts, solid carbonic acid (H₂CO₃) remains elusive. All the synthetic routes followed up to now for its production required quite drastic conditions (from high energy protonation of solid CO₂ to laser heating at high pressure on fluid mixtures of CO₂ and H₂O).

In our study, we discovered a highly reproducible, simpler and effective way to synthesize two diverse carbonic acid crystal structures upon the fast, cold compression of pristine CO₂ clathrate hydrates. We found that the products of this reaction strictly depend on the starting pressures, resulting in three different reaction pathways. In the first pathway, for pressures lower than 2.7 GPa, pristine CO₂ clathrate hydrate simply decomposes into its constituents, as expected from previous studies. For intermediate pressures (between 2.7 and 4.8 GPa), a first crystalline phase is observed, characterized by a well-defined lattice phonon region (see Figure 1a, green spectrum) and a specific diffraction pattern. For pressures exceeding 4.8 GPa, the formation of an amorphous product is observed, characterized by a broad, unstructured band in the lattice phonon region (see Figure 1a, black spectrum). Both the two products feature an intense, quite broad Raman band at about 1050 cm^-1, a reported signature band for carbonate-based systems and, also, carbonic acid (see Figure 1b). We found that the high pressure, amorphous product (called a-ε) transformed upon decompression down to 4.8 GPa or heating at higher pressures into a distinct, much more structured crystalline phase characterized by 10 lattice phonons (see Figure 1a, red spectrum) and sharper internal Raman bands (Figure 1b, red spectrum). This structure was found to be that already reported by Abramson and co-authors in a recent paper, where it was obtained in much more drastic conditions (from fluid CO₂ and H₂O upon resistive heating): we called this phase ε-H₂CO₃.

Figure 1: Raman spectra of ε-H₂CO₃ (red), a-ε b(lack) and ε_m-H₂CO₃ (green).

We studied the ε-phase upon compression and decompression using both spectroscopic (Raman, performed at LENS) and structural (X-ray diffraction, XRD) techniques. Powder X-Ray diffraction measurements were carried out at the Xpress beamline of Elettra, exploiting the high brilliance of the micrometric beam to fully characterize our samples. The patterns were Rietveld refined to follow the cell parameters behavior with pressure (see Figure 2). Our results attest to the remarkable stability of the ε-carbonic acid up to half a megabar. We also observed how the intermediate pressure polymorph, called ε_m-H₂CO₃ and reported here for the very first time, is stable up to very high pressures at room temperature but converts into the already discussed ε-H₂CO₃ upon increasing temperature. This led us to identify this species as metastable, thus calling it ε_m (where m stands for metastable). The comparison of spectroscopic and structural data, along with symmetry and Group Theory considerations, led us to find the main difference between ε_m and ε-H₂CO₃ in their water content, the ε phase reportedly being monohydrated carbonic acid while ε_m is characterized by no (or disordered) water molecules in its unit cell. This interpretation is further supported by the red-shift shown by some internal modes of ε polymorph upon compression, which is not observed in the analogous modes of ε_m (in agreement with theoretical calculations) and by the presence of extra –OH stretching bands in the spectrum of ε-H₂CO₃ that are not visible in ε_m-H₂CO₃ (highlighted by red stars in Figure 1c).

Figure 2: Examples of Rieveld refinements for XRD patterns acquired on ε-H₂CO₃ at the Xpress beamline of Elettra.

These results attest to the great complexity hidden in the behavior of a relatively simple ice mixture such as that composed of H₂O and CO₂. The conditions observed in our work for the synthesis of the diverse H₂CO₃ structures are quite similar to the (p,T) conditions that may be found in many extraterrestrial environments, thus making this process probable to occur, for instance, through the subduction of surface material into the interior of icy planets. This study therefore provides novel information to support predictions and modeling of the chemistry occurring on several icy bodies of our solar system.

This research was conducted by the following research team:

Selene Berni¹, Demetrio Scelta^1,2, Sebastiano Romi^1,3, Samuele Fanetti^1,2,  F. Alabarse⁴, Marco Pagliai³ and Roberto Bini^1,2,3
1 LENS- European Laboratory for Non-Linear Spectroscopy, Sesto F.no (FI), Italy.
² ICCOM-CNR, Istituto di Chimica dei Composti OrganoMetallici, Sesto F.no (FI), Italy.
³ Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Sesto F.no (FI), Italy.
⁴ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy.

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