We report a detailed experimental characterization of the infrared absorption properties of the high pressure, red phase of oxygen. Spectra are measured in the far infrared, down to $100{\mathrm{cm}}^{-1\mathrm{}}$, in the fundamental vibration and in the overtone regions up to 63 GPa. A new, strong peak is observed at about $300{\mathrm{cm}}^{-1\mathrm{}}$. Samples less than 0.5 μm thick were produced to measure the strong vibron mode avoiding saturation. We find that the diatomic oxygen molecule is not a good basis to describe the vibrational spectrum of the $\epsilon$ phase. The spectra can be interpreted in a straightforward way on the basis of an association among the oxygen molecules leading to the formation of ${\mathrm{O}}_4$ units.

The high-pressure structural properties of benzene have been investigated up to the occurrence of the chemical reaction by means of x-ray diffraction and infrared absorption techniques. The infrared spectra show that sample annealing above 500 K produces pure phase-II crystals. X-ray diffraction patterns collected at 540 K on these crystals allowed the equation of state of benzene to be obtained. These results indicate the stability of phase II up to the pressure where benzene reacts and no evidence of the III and ${\mathrm{III}}^{\prime }$ crystal structures is gained. On these bases the existing phase diagram of benzene is discussed and a simplified one is proposed.

A review of experimental and theoretical studies performed over the past three decades on high pressure chemistry of solid CO₂, at 0–80 GPa and 40–3000 K, is presented. Emphasis is placed on the recently discovered non-molecular covalent crystalline phase V, and its glassy counterpart a-CO₂, along with other molecular phases, whose interpretation is crucial for determining the reaction path to non-molecular CO₂. The matter is still under debate, and many open issues are outlined, such as the true reaction mechanism for forming phase V. Finally, we propose arguments to stimulate possible future research in a more extended P–T range. This work is a tutorial review and should be of general interest both for solid state chemistry and condensed matter physics communities.

The discovery of nonmolecular carbon dioxide under high-pressure conditions shows that there are remarkable analogies between this important substance and other group IV oxides. A natural and long-standing question is whether compounds between CO ₂ and SiO ₂ are possible. Under ambient conditions, CO ₂ and SiO ₂ are thermodynamically stable and do not react with each other. We show that reactions occur at high pressures indicating that silica can behave in a manner similar to ionic metal oxides that form carbonates at room pressure. A silicon carbonate phase was synthesized by reacting silicalite, a microporous SiO ₂ zeolite, and molecular CO ₂ that fills the pores, in diamond anvil cells at 18–26 GPa and 600–980 K; the compound was then temperature quenched. The material was characterized by Raman and IR spectroscopy, and synchrotron X-ray diffraction. The experiments reveal unique oxide chemistry at high pressures and the potential for synthesis of a class of previously uncharacterized materials. There are also potential implications for CO ₂ segregation in planetary interiors and for CO ₂ storage.