Samuele Fanetti

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.

We formed two-dimensional pseudo-hexagonally ordered diamond-like carbon nano-threads by laser heating dense molecular benzene, the archetypal aromatic compound, to temperatures exceeding 1000 K at about 40 GPa in diamond anvil cells. The heated samples were then rapidly (<1 s) temperature quenched. Instead, in previous studies, carbon nano-threads were obtained by subjecting benzene to slow compression–decompression cycles (<2−3 GPa/h) at room temperature, reaching maximum pressures of 23–24 GPa. Our pressure–temperature–time reaction path for this kinetically driven transformation differs substantially from previous studies, as it is significantly more extreme and faster. Synchrotron x-ray diffraction measurements of temperature-quenched products were performed during a compression–decompression cycle from 40 to 54 to 0 GPa, allowing the determination of a one-dimensional equation of state for the characteristic interlayer spacing. This spacing reflects the van der Waals distance between neighboring threads. Comparison with previously reported ab initio equations of state constrains the local internal structure of the threads, which is otherwise elusive at high pressures due to disorder along the chain axis.

We report the results of an EXAFS (extended x-ray absorption fine structure) study of Fe3+, Ni2+, and Cu2+ aqueous solutions under high pressures. EXAFS experiments were performed using synchrotron radiation at room temperature and up to pressures of about 1.2 GPa using a diamond anvil cell. Data analysis has been performed using advanced multiple-scattering simulations, and information about the evolution of the first hydration shell around the metal ions has been obtained. It is shown that Fe3+ and Ni2+ solutions retain a local octahedral structure up to the highest pressure, while Cu2+ solutions show a predominant distorted pyramidal fivefold structure with two oxygen distances. The first-neighbor metal–oxygen distances show a different behavior with pressure in the three solutions, being gradually shortened for Ni2+ solutions or elongated in Fe3+ solutions (by ∼−0.01 and ∼0.02 Å respectively), while in Cu2+ solutions, the difference between average equatorial and axial Cu–O distances is gradually reduced. The present results show that pressure does not act as a simple isotropic perturbation on ionic hydration, which is found to be dependent on the bonding mechanisms and ligand-field anisotropy of transition-metal ions.

The structural and electronic properties of bibenzyl (1,2-diphenylethane) are investigated as a function of pressure using single-crystal X-ray diffraction (SC-XRD) and two-photon-induced fluorescence. Bibenzyl is an appealing pseudostilbene compound for the high-pressure synthesis of mixed carbon nanothreads with tunable optical properties, due to its fully saturated linking group. Crystal structures are refined from ambient pressure up to 37 GPa by synchrotron XRD, and the appearance of pseudosymmetry traits at 13 GPa reveals a phase transition that consists of molecular distortions leading to the tripling of the a axis and asymmetric unit while maintaining the unit cell's point group symmetry. Two-photon-induced fluorescence up to 18 GPa provides insights into the structural changes where the strengthening of π–π interactions at the phase transition stabilizes the electronic ground state and drives the molecular distortions that occur at that pressure. Coupling structural and electronic properties rationalizes the high-pressure reactivity compared to other pseudostilbenes, showing that the molecular evolution from the added torsional degree of freedom and establishment of π–π interactions at the phase transition lead the molecular conformation to an unfavorable state for pressure-induced polymerization.

Hydrated carbonic acid polymorphs can be synthesised from CO ₂ clathrate hydrate in p , T conditions typical of subduction processes in icy planets and satellites. These results have implication for both astrochemistry and fundamental science.

This study addresses azobenzene's structural compression and reactivity under hydrostatic high-pressure conditions. Synchrotron X-ray diffraction data of single crystals compressed with neon as the pressure-transmitting medium allowed the refinement of the crystal structure up to 28 GPa, at which the onset of the reaction was observed. Analysis of the pressure-dependent lattice parameters reveals a first-order isostructural phase transition at 13 GPa. We have solved the crystal structure of the high-pressure phase of azobenzene offering a key insight into the strong contribution of stress on the structural compression mechanism and crystal's reaction chemistry at elevated pressures. While the collapse of the b cell parameter, previously observed under non-hydrostatic conditions, was identified as the crucial step toward the formation of azobenzene-derived double-core nanothreads, under quasi-hydrostatic conditions the compression of the cell parameters up to 33 GPa followed a different route. The evolution of the cell parameters and the refinement of the crystal structure close to the onset of the reaction identified a topochemical polymerization path, corroborated by reaction kinetics data by infrared spectroscopy and by computed polymer structures, suggesting a complex growth process, resulting in a distinctly different material compared to that formed upon non-hydrostatic compression. These findings underscore the pivotal role of compression conditions in determining the reaction pathways of azobenzene, providing novel insights for its application in nanomaterial synthesis.

Carbon dioxide (CO ₂ ) is widespread in astrochemically relevant environments, often coexisting with water (H ₂ O) ices and thus triggering a great interest regarding the possible formation of their adducts under various thermodynamic conditions. Amongst them, solid carbonic acid (H ₂ CO ₃ ) remains elusive, yet being widely studied. Synthetic routes followed for its production have always been characterised by drastic irradiation on solid ice mixtures or complex procedures on fluid samples (such as laser heating at moderate to high pressures). Here we report about a simpler yet effective synthetic route to obtain two diverse carbonic acid crystal structures from the fast, cold compression of pristine clathrate hydrate samples. The two distinct polymorphs we obtained, differing in the water content, have been deeply characterised via spectroscopic and structural techniques to assess their composition and their astonishing pressure stability, checked up to half a megabar, also highlighting the complex correlations between them so to compile a detailed phase diagram of this system. These results may have a profound impact on the prediction and modelisation of the complex chemistry which characterises many icy bodies of our Solar System.

Carbon dioxide (CO ₂ ) is widespread in astrochemically relevant environments, often coexisting with water (H ₂ O) ices and thus triggering a great interest regarding the possible formation of their adducts under various thermodynamic conditions. Amongst them, solid carbonic acid (H ₂ CO ₃ ) remains elusive, yet being widely studied. Synthetic routes followed for its production have always been characterised by drastic irradiation on solid ice mixtures or complex procedures on fluid samples (such as laser heating at moderate to high pressures). Here we report about a simpler yet effective synthetic route to obtain two diverse carbonic acid crystal structures from the fast, cold compression of pristine clathrate hydrate samples. The two distinct polymorphs we obtained, differing in the water content, have been deeply characterised via spectroscopic and structural techniques to assess their composition and their astonishing pressure stability, checked up to half a megabar, also highlighting the complex correlations between them so to compile a detailed phase diagram of this system. These results may have a profound impact on the prediction and modelisation of the complex chemistry which characterises many icy bodies of our Solar System.