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 properties of some forms of water ice reserve still intriguing surprises. Besides the several stable or metastable phases of pure ice, solid mixtures of water with gases are precursors of other ices, as in some cases they may be emptied, leaving a metastable hydrogen-bound water structure. We present here the first characterization of a new form of ice, obtained from the crystalline solid compound of water and molecular hydrogen called C₀-structure filled ice. By means of Raman spectroscopy, we measure the hydrogen release at different temperatures and succeed in rapidly removing all the hydrogen molecules, obtaining a new form of ice (ice XVII). Its structure is determined by means of neutron diffraction measurements. Of paramount interest is that the emptied crystal can adsorb again hydrogen and release it repeatedly, showing a temperature-dependent hysteresis.

Raman and infrared spectra of solid nitrogen have been collected between 25 K and room temperature up to 41 GPa. A careful analysis of the spectral band transformations occurring across the high pressure transitions among the δ, ${\delta }_{\mathrm{loc}},$ ε, and ζ phases allowed to define the phase diagram in the whole $\mathrm{P-T}$ region investigated. In particular, the transition between the ε and ζ phases has been observed in the range 30–230 K and the corresponding phase-boundary drawn. A significant metastability region (spanning about 10 GPa in pressure) hinders the transformation between the ε and ζ phases when pressure is varied at low temperature. Group theory arguments suggest a centrosymmetric structure for the ζ phase and the number of Raman and infrared ${\nu }_1$ and ${\nu }_2$ components can be reproduced both with cubic and tetragonal structures. An appreciable coupling among neighboring molecules is observed, at room temperature, only in the ε phase where the relative orientations of the molecules are fixed.

We have performed high-resolution inelastic neutron scattering studies on three samples of hydrogenated tetrahydrofuran-water clathrates, containing either ${\mathrm{H}}_2$ at different para and ortho concentrations, or HD. By a refined analysis of the data, we are able to assign the spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-${\mathrm{H}}_2$. The ${\mathrm{H}}_2$ molecule rotates almost freely, while performing a translational motion (rattling) in the nanometer-size cage, resulting a paradigmatic example of quantum dynamics in a nonharmonic potential well. Both the ${\mathrm{H}}_2$ rotational transition and the fundamental of the rattling transition split into triplets, having different separation. The splitting is a consequence of a substantial anisotropy of the environment with respect to the orientation of the molecule in the cage, in the first case, or with respect to the center-of-mass position inside the cage, in the second case. The values of the transition frequencies and band intensities have been quantitatively related to the details of the interaction potential between ${\mathrm{H}}_2$ and the water molecules, with very good agreement.

An extended temperature and pressure-dependent investigation is carried out on a ${\mathrm{L}\mathrm{a}}_{0.75}{\mathrm{C}\mathrm{a}}_{0.25}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ sample exploiting the infrared absorption technique coupled to a diamond anvil cell. The pressure dependence of the insulator to metal transition temperature $T_{\mathrm{I}\mathrm{M}}$ is determined for the first time up to 11.2 GPa. The $T_{\mathrm{I}\mathrm{M}}(P)$ curve we propose to model the present data has an exponential-like behavior with an associated characteristic pressure $P^{*}$ playing the role of a decay constant. It is found that the equivalence between an external and an internal (chemical) pressure holds over a limited range of pressure, namely, $P\le 2P^{*}$. Moreover, a certain universality character is associated with the proposed model curve in its ability to account for a large class of low-disorder manganites characterized by intermediate electron-phonon coupling.