Exceptional thermal stability of carbonic acid-rich very high-density amorphous ice

: Similar to water ice, clathrate hydrates also undergo amorphization under increasing pressure at low temperature. This phenomenon was also recently reported for carbon dioxide (CO2) clathrate hydrate and identified as a crucial step for triggering the formation of carbonic acid (H2CO3) molecules and their subsequent arrangement into different crystalline polymorphs upon heating. Here, we present a combined x-ray diffraction and Raman study of the amorphization of CO2 clathrate hydrate into a high-density amorphous ice, followed by its evolution into a very-high-density amorphous (VHDA) structure. The main peculiarity of this VHDA mixed ice lies in the incorporation of a large amount of H2CO3 molecules, which confer exceptional stability to the amorphous phase, extending even above ambient temperature and thus largely exceeding the stability of all the amorphous ices, both pure water ice and mixed or salty ices, reported so far. Remarkably, crystallization occurs in two steps (at 4 GPa and 273 K): it first leads to the nucleation of the recently discovered metastable ice XXI, which suddenly converts to phase VII a few degrees above, allowing the nucleation of the hydrated ϵm-H2CO3 structure.