Optical studies of diatomic molecules at extreme conditions

The formidable progress achieved in the research at extreme conditions led to important discoveries of many unusual and interesting physical and chemical phenomena. Materials with high compressibility were and still are of particular interest due to a significant reduction of volume which could result in unexpected changes of bonding and/or electronic properties. Among highly compressible materials simple diatomic molecules such as H2, N2, and O2 are particularly interesting because they form new types of solids at high pressure. Hydrogen, being the most abundant element in the universe, possesses simple electronic structure, therefore, the study of hydrogen systems is of special interest. In the last three decades, there were subsequently explored and described several high-pressure phases of hydrogen up to 400 GPa. However, there is still a vast area of unexplained effects, which requires further analysis. The contributed work discusses Raman experiments in a wide pressure and temperature range where rotational and lattice phonon excitations have been measured in the Raman spectrum of solid H2 and D2 at 10, 77, 150 and 300 K from 2 to 180 GPa and up to 380 GPa at 300 K. Analysis of the Raman spectra allows to model how the rotational modes change with pressure and temperature and how the mass scaling laws evolve as the density increases in both hydrogen and deuterium. Comparison of vibrational frequencies of the isotopes appears to be extremely useful for estimation of equivalent pressures for both isotopes. Nitrogen and oxygen are archetypal elements possessing unique features such as extremely strong triple bond in case of N2 and magnetic moment in O2 . Both N2 and O2 exhibit rich polymorphism, with additional phases of O2 derived from its electronic and magnetic properties. N2 /O2 mixtures (for example, 20.9% O2 and 78% N2 mixture is air that we breathe) have been studied up to 12 GPa at 300 K experimentally and explored up to 500 GPa at 0 K theoretically. In the current project, N2 /O2 molecular systems are examined at 300 K up to 150 GPa. Rich polymorphism is observed, with seven phases exhibiting drastically different Raman spectra for concentrations below 45% of O2 and a more stable area with three phases in the concentration range from 45% to 80% of oxygen at pressures above 12 GPa. Moreover, characteristic Raman spectra obtained for the mix with 25% O2 after laser heating to approximately 2000 K at 25 and 96 GPa reveals pronounced peaks indicating the potential formation of new compounds

Synthesis and stability of hydrogen iodide at high pressures

Through high-pressure Raman spectroscopy and x-ray diffraction experiments, we have investigated the formation, stability field, and structure of hydrogen iodide (HI). Hydrogen iodide is synthesized by the reaction of molecular hydrogen and iodine at room temperature and at a pressure of 0.2 GPa. Upon compression, HI solidifies into cubic phase I, and we present evidence for the emergence of a phase I′ above 3.8 GPa. Across the wide temperature regime presented here, HI is unstable under compression (11 GPa at 300 K, 18 GPa at 77 K), decomposing into its constituent elements, after which no further reaction between hydrogen and iodine was observed up to pressures of 60 GPa. This study provides both the constraints on the phase diagram of HI and its kinetic stabilit

Raman signal from a hindered hydrogen rotor

We present a method for calculation of Raman modes of the quantum solid phase I hydrogen and deuterium. We use the mean-field assumption that the quantized excitations are localized on one molecule. This is done by explicit solution of the time-dependent Schroedinger equation in an angle-dependent potential, and direct calculation of the polarization. We show that in the free rotor limit, the H₂ and D₂ frequencies differ by a factor of 2, which evolves toward √2 as the modes acquire librational character due to stronger interactions. The ratio overshoots √2 if anharmonic terms weaken the harmonic potential. We also use density functional theory and molecular dynamics to calculate the E_(2g) optical phonon frequency and the Raman linewidths. The molecular dynamics shows that the molecules are not free rotors except at very low pressure and high temperature, and become like oscillators as phase II is approached. We fit the interaction strengths to experimental frequencies, but good agreement for intensities requires us to also include strong preferred orientation and stimulated Raman effects between S₀ (1) and S₀ (0) contributions. The experimental Raman spectrum for phase II cannot be reproduced, suggesting that the mean-field assumption is invalid in that case

Polymerized 4-Fold Coordinated Carbonate Melts in the Deep Mantle

International audienceOur understanding of the deep carbon cycle has witnessed amazing advances in the last decade, including the discovery of tetrahedrally coordinated high pressure (P) carbonate phases. However, little is known about the physical properties of their molten counterpart at moderate depths, while their properties at lower mantle conditions remain unexplored. Here, we report the structure and density of FeCO3 melts and glasses from 44 to 110 GPa by means of in situ x-ray synchrotron diffraction, and ex situ Raman and x-ray Raman spectroscopies. Carbon is fully transformed to 4-fold coordination, a bond change recoverable at ambient P. While low P melts react with silica, resulting in the formation of silico-carbonate glasses, high P melts are not contaminated but still quench as glasses. Carbonate melts are therefore polymerized, highly viscous and poorly reacting with silicates in the lower mantle, in stark opposition with their low P properties