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.
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