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Infrared spectroscopy has been applied to the study of solid oxygen at high pressure and low temperature. An absorption line in the vibron fundamental mode region has been observed, and its behavior with pressure measured up to the transition to the $\ensuremath{\epsilon}$ phase. This absorption, not expected according to the symmetry of the $\ensuremath{\delta}$ phase, indicates the existence of a new crystal structure between 2 and 8 GPa at low temperature. The comparison of the infrared and Raman frequency evolution with pressure is done quantitatively on the basis of a simple vibrational coupling model and the intermolecular coupling force constant is obtained. In contrast to solid hydrogen, here the coupling constant is positive and shows a strong dependence on the intermolecular distance which can indicate that significant charge transfer processes are already present in this low-pressure phase.

Fourier-transform infrared and Raman studies are performed on $\mathrm{Ar}({\mathrm{H}}_{2}{)}_{2}$ compound single crystals over a large pressure (5--30 GPa) and temperature (30--300 K) range in a diamond-anvil cell. The effect of polarization on the infrared absorption is investigated. On the basis of the spectroscopic features and comparing to the cases of pure solid ${\mathrm{H}}_{2}$ and of matrix isolated ${\mathrm{H}}_{2}$ in argon crystal, we assign the quite complex infrared spectra to the pure vibron and vibron plus phonons and vibron plus rotons combination bands. The difference of the infrared and Raman vibron frequency, as a function of pressure, is interpreted quantitatively with reference to vibrational coupling between hydrogen molecules, as in the hydrogen crystal case. Information on the structural properties can thus be derived. Finally, this study highlights the similarity in the molecular problems for the $\mathrm{Ar}({\mathrm{H}}_{2}{)}_{2}$ compound and for solid hydrogen.

Rutile-type ${\mathrm{GeO}}_{2}$ was found to undergo a proper ferroelastic transition at 26.7(2) GPa from Raman spectroscopic measurements. The ${B}_{1g}$ optic mode softens up to this pressure and then becomes a hard ${A}_{g}$ mode. The square of these mode frequencies varies linearly with pressure in accordance with the soft-mode theory of second-order phase transitions. The present results have enabled a systematic relationship between the square of the soft-mode frequency at ambient pressure and the critical pressure in the homologous series ${\mathrm{SiO}}_{2},$ ${\mathrm{GeO}}_{2},$ and ${\mathrm{SnO}}_{2}$ to be identified.

High-pressure infrared spectra of solid methane are reported up to 30 GPa between 50 and 300 K. The symmetric stretching mode (${\ensuremath{\nu}}_{1}$) was successfully used as a probe of the phase transitions. Seven different phases have been identified. Pressure and temperature-dependent studies allowed us to outline all the phase boundaries in this portion of the diagram. A high-pressure phase (HP), stable in all the temperature range analyzed, has been identified. The transition to this phase occurs at about 8 GPa at 50 K, and 25 GPa at 300 K. The wide range of stability of this phase suggests a single site-ordered structure. Group-theoretical and qualitative arguments point to hcp (${\mathrm{D}}_{6\mathrm{h}}$ factor group) as the favored crystal structure of the HP phase. The knowledge of the phase diagram allows us to outline the evolution of the crystal structure and of the site symmetries as the pressure increases. The low-pressure fcc crystalline modifications transform to the fully ordered hcp structure through intermediate tetragonal phases. Competition between molecular and crystalline fields determines a complex site-symmetry evolution. Similarities with analogous fcc-hcp evolution observed in rare gases and atomic systems support our conclusions.

The antiferromagnetic $\ensuremath{\alpha}$ and $\ensuremath{\delta}$ phases of solid oxygen have been investigated by means of infrared absorption spectroscopy along several isotherms, decreasing pressure at several temperatures, in the pressure range from 0.6 to 9.1 GPa between 30 and 298 K. The path followed in the $P\text{\ensuremath{-}}T$ plane also crosses the recently proposed hypothetical phase boundary between the antiferromagnetic $\ensuremath{\delta}\text{-I}$ phase and the nonmagnetic $\ensuremath{\delta}\text{-II}$ phases. No evidence of this phase transition has been observed in the spectra and, more importantly, strong evidence of antiferromagnetic ordering of the molecular spins has been observed in the region assigned to the nonmagnetic $\ensuremath{\delta}\text{-II}$ phase.

In this work we report a study in the near infrared spectral region, at low temperature and high pressure, of solid $\ensuremath{\delta}\ensuremath{-}{O}_{2}$ and $\ensuremath{\beta}\ensuremath{-}{O}_{2},$ concerning electronic excitations between the ground state ${}^{3}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}$ and the lowest excited states ${}^{1}{\ensuremath{\Delta}}_{g}$ and ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}.$ These transitions are essentially due to the simultaneous creation of an exciton, a magnon, and a vibron, and confirm the antiferromagnetic order of the $\ensuremath{\delta}$ phase and the short-range antiferromagnetic order of $\ensuremath{\beta}\ensuremath{-}{O}_{2}.$ Strong phonon sidebands are also observed. A simple model let us obtain, from the frequency position of the observed bands, the exchange integral between nearest-neighbor molecules as a function of pressure, i.e., of the intermolecular distance. This result is compared with the available theoretical calculations at high pressure and other experimental data at ambient pressure. The comparison makes it possible to estimate the spin-correlation function in the $\ensuremath{\beta}$ p hase. Finally, we measure a dramatic change of the spectrum at the $\ensuremath{\delta}\ensuremath{-}\ensuremath{\epsilon}$ phase transition, which is consistently interpreted on the basis of the formation of the ${O}_{4}$ molecule, confirming previous vibrational data. The antiferromagnetic coupling in solid oxygen appears to be the driving force leading to the formation of the diamagnetic ${O}_{4}$ molecule.

The infrared spectrum of the \ensuremath{\epsilon} phase of solid oxygen has been studied between room temperature and 20 K as a function of pressure up to 63 GPa. Besides the strong absorption in the fundamental ${\mathrm{O}}_{2}$ vibron mode and the broad doublet in the overtone region, another peak is detected in the far infrared region. The analysis of the overtone bands allows the determination of the density of states of the ${\mathrm{O}}_{2}$ vibron region which consists of two separated energy regions, including one the infrared and the other the Raman bands observed in the $1500\char21{}1650 {\mathrm{cm}}^{\ensuremath{-}1}$ range. This result, consistent with the analysis of the other Raman and infrared bands at lower frequency, is interpreted on the basis of a crystal composed by a molecular unit formed by four oxygen atoms. This hypothesis explains the strong infrared absorption which is in contrast with the model of a crystal composed by diatomic oxygen molecules. Very thin crystalline slabs $(l~0.4 \ensuremath{\mu}\mathrm{m})$ allowed to measure the intensity of the strong infrared absorption at $1500\char21{}1550 {\mathrm{cm}}^{\ensuremath{-}1}.$ The measurement of the Raman spectrum as a function of the incident power and of the laser excitation frequency shows how the intensity and the frequency of the Raman lines are affected by the experimental conditions. Finally, a simple chain model provides indirect proof of our assignment of the low-frequency infrared mode and allows to rule out an association in polymeric units formed by more than four atoms even at pressures close to the insulator-metal transition.