Infrared fundamentals and phase transitions in CO2 up to 50 GPa

Egg-derived porous plasma modified clay composite for wastewater remediation

Clays are often envisaged as an alternative to activated carbon for wastewater pollutant adsorption. However, conclusive results have only been obtained for clays heavily chemically modified. In this study, a greener approach is proposed to improve the retention capacity of clays. It consists in mixing clay (C) with eggshell (ES) and calcine, and then exposing to gliding arc plasma (ESC-800/PL). The resulting materials were characterized by nitrogen physisorption, FTIR, XRD, TGA/DTG, and point of zero charge analyses. The preparation gives porous platelet agglomerates resulting from the kaolinite-metakaolinite transition, thereby increasing their internal specific surface area and capacity to retain pollutants. This granular distribution is kept stable by partial pozzolanic reactions avoiding deagglomeration. The specific surface area and total pore volume increased respectively from 14 m² g^-1 and 0.049 cm³ g^-1 to 89 m² g^-1 and 0.061 cm³ g^-1 leading to an enhanced removal efficiency of Fast Green and Orange G dyes from polluted water. The maximum adsorption capacity occurred at 298 K attaining values of 32.34 and 14.78 mg g^-1 for OG and FG, respectively. The pH plays a crucial role in the maximum sorption of dyes, and the experimental data were successfully adjusted to pseudo-first-order kinetic and Liu isotherm model.

Phase transitions and chemical reactions of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine under high pressure and high temperature

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most important energetic materials. Investigations on its phase transitions and chemical reactions under extreme conditions are very important to understand the explosion process and design new energetic materials. By using a diamond anvil cell combined with in situ Raman, IR and X-ray diffraction techniques up to a pressure of ∼40 GPa, we found that β-HMX undergoes four reversible phase transitions without any chemical reaction under external pressure at room temperature. Isostructural phase transitions emerge around 5 GPa (ζ-HMX) and 10–13 GPa (ε-HMX), and another two phases emerge at 16 GPa (η-HMX) and 27 GPa (ϕ-HMX). The unit cells of ζ-HMX and ε-HMX were determined as a = 6.215 Å, b = 10.417 Å, c = 8.272 Å, β = 124.88°, P2₁/c at 6.2 GPa and a = 6.130 Å, b = 9.846 Å, c = 8.258 Å, β = 125.06°, P2₁/c at 12.6 GPa, respectively. The crystal structures of β, ζ, and ε-HMX were obtained by Rietveld refinement, based on which the rotations of NO₂ groups were found to be related to the phase transition at 5 GPa. Additionally, HMX decomposes at 8.7 GPa and 300 °C. Carbon dioxide, hydroxyl, imino and hydroxyimino groups were detected in the IR spectrum, which indicates that the reaction contains a hydrogen transfer process. Our investigation uncovers the structural variation of β-HMX under external pressure and identifies the decomposition products under extreme conditions, which provides new insight to understand the detonation process of energetic materials.

Phase transitions, crystal structures and chemical reactions of β-HMX under extreme conditions were described systematically.

Electronic structure of carbon dioxide under pressure and insights into the molecular-to-nonmolecular transition

Knowledge of the high-pressure behavior of carbon dioxide (CO2), an important planetary material found in Venus, Earth, and Mars, is vital to the study of the evolution and dynamics of the planetary interiors as well as to the fundamental understanding of the C-O bonding and interaction between the molecules. Recent studies have revealed a number of crystalline polymorphs (CO2-I to -VII) and an amorphous phase under high pressure-temperature conditions. Nevertheless, the reported phase stability field and transition pressures at room temperature are poorly defined, especially for the amorphous phase. Here we shed light on the successive pressure-induced local structural changes and the molecular-to-nonmolecular transition of CO2 at room temperature by performing an in situ study of the local electronic structure using X-ray Raman scattering, aided by first-principle exciton calculations. We show that the transition from CO2-I to CO2-III was initiated at around 7.4 GPa, and completed at about 17 GPa. The present study also shows that at ~37 GPa, molecular CO2 starts to polymerize to an extended structure with fourfold coordinated carbon and minor CO3 and CO-like species. The observed pressure is more than 10 GPa below previously reported. The disappearance of the minority species at 63(± 3) GPa suggests that a previously unknown phase transition within the nonmolecular phase of CO2 has occurred.

CO2-helium and CO2-neon mixtures at high pressures

The properties of mixtures of carbon dioxide with helium or neon have been investigated as a function of CO(2) concentration and pressure up to 30 GPa at room temperature. The binary phase diagrams of these mixtures are determined over the full range of CO(2) concentrations using visual observations and Raman scattering measurements. Both diagrams are of eutectic type, with a fluid-fluid miscibility gap for CO(2) concentrations in the range [5, 75] mol. % for He and [8, 55] mol. % for Ne, and a complete separation between the two components in the solid phase. The absence of alloys or stoichiometric compounds for these two binary systems is consistent with the Hume-Rothery rules of hard sphere mixtures. The Raman spectra and x-ray diffraction patterns of solid CO(2) embedded in He or Ne for various initial concentrations have been measured up to 30 GPa and 12 GPa, respectively. The frequencies of the Raman modes and the volume of solid phase I are identical, within error bars, to those reported for 100% CO(2) samples, thus confirming the total immiscibility of CO(2) with He and Ne in the solid phase. These results demonstrate the possibility to perform high-pressure experiments on solid CO(2) under (quasi-)hydrostatic conditions using He or Ne as pressure transmitting medium.

High pressure-high temperature decomposition of γ-cyclotrimethylene trinitramine

Decomposition of γ-cyclotrimethylene trinitramine (γ-RDX) under high pressure-high temperature conditions was examined to elucidate the reactive behavior of RDX crystals. Vibrational spectroscopy measurements were obtained for single crystals in a diamond anvil cell (DAC) at pressures from 6 to 12 GPa and temperatures up to 600 K. Global decomposition rates, activation energies, and activation volumes at several pressures and temperatures below the P-T locus for the γ-RDX decomposition were obtained. Similar to ε-RDX, but in contrast to α-RDX, we found that pressure decelerates the decomposition of γ-RDX. The decomposition deceleration with pressure in the γ-phase can be attributed to pressure-inhibiting bond homolysis step(s). The main decomposition species were identified as N(2)O, CO(2), and H(2)O, in accord with the species reported for the α-phase decomposition at high pressures. This work complements previous studies on RDX at HP-HT conditions and provides comprehensive results on the reactive behavior of γ-RDX; the γ-phase plays a key role in RDX decomposition at P-T conditions relevant to shock wave initiation.

In situ high P-T Raman spectroscopy and laser heating of carbon dioxide

In situ high P-T Raman spectra of solid CO(2) up to 67 GPa and 1,660 K have been measured, using a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system in diamond anvil cells. A metallic foil was employed to efficiently absorb the incoming Nd:YLF laser and heat the sample. The average sample temperature was accurately determined by detailed balance from the anti-Stokes/Stokes ratio, and was compared to the temperature of the absorber determined by fitting the thermal radiation spectrum to the Planck radiation law. The transformation temperature threshold and the transformation dynamics from the molecular phases III and II to the polymeric phase V, previously investigated only by means of temperature quench experiments, was determined at different pressures. The P-T range of the transformation, between 640 and 1,100 K in the 33-65 GPa pressure interval, was assessed to be a kinetic barrier rather than a phase boundary. These findings lead to a new interpretation of the high P-T phase diagram of carbon dioxide. Furthermore, our approach opens a new way to perform quantitative in situ Raman measurements under extremely high pressures and temperatures, providing unique information about phase relations and structural and thermodynamic properties of materials under these conditions.

Melting curve and fluid equation of state of carbon dioxide at high pressure and high temperature

The melting curve and fluid equation of state of carbon dioxide have been determined under high pressure in a resistively heated diamond anvil cell. The melting line was determined from room temperature up to 11.1+/-0.1 GPa and 800+/-5 K by visual observation of the solid-fluid equilibrium and in situ measurements of pressure and temperature. Raman spectroscopy was used to identify the solid phase in equilibrium with the melt, showing that solid I is the stable phase along the melting curve in the probed range. Interferometric and Brillouin scattering experiments were conducted to determine the refractive index and sound velocity of the fluid phase. A dispersion of the sound velocity between ultrasonic and Brillouin frequencies is evidenced and could be reproduced by postulating the presence of a thermal relaxation process. The Brillouin sound velocities were then transformed to thermodynamic values in order to calculate the equation of state of fluid CO2. An analytic formulation of the density with respect to pressure and temperature is proposed, suitable in the P-T range of 0.1-8 GPa and 300-700 K and accurate within 2%. Our results show that the fluid above 500 K is less compressible than predicted from various phenomenological models.

High pressure solid state chemistry of carbon dioxide

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.

Near-critical CO2 in mesoporous silica studied by in situ FTIR spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy was used to correlate the band shift of the nu2 vibrational band of carbon dioxide with the density of the fluid. Upon adsorption of CO2 on mesoporous silica and a nonporous SiO2 film, additional bands were detected due to interactions of CO2 with SiO2. Near the saturation pressure for the porous samples, the absorbance of the nu2 band increased strongly, which was concluded to be caused by liquidlike CO2 inside the pores. Integration of single-beam-sample-reference spectra between bulk CO2 and CO2 adsorbing on the mesoporous silica coated on one part of the internal reflection element revealed excess adsorption type isotherms with sharp maxima at 21 degrees C. A flatter curve shape could be observed at 25 degrees C, which allowed estimating the pore critical temperature. Moreover, the density of the fluid inside and outside the pores could be compared. Over the investigated ranges of pressure, temperature, and pore size, the results evidenced that the CO2 density was always higher in the silica pores than in the bulk, even under supercritical conditions. This has important consequences on the pressure dependence of dissolution power and diffusivity of fluids in mesoporous solids. An overview is given on the influences of fluid phase behavior in the bulk and in the pores at various conditions on solubility and diffusivity.

High-pressure molecular phases of solid carbon dioxide

We present a theoretical study of solid CO2 up to 50 GPa and 1500 K using first-principles calculations. In this pressure-temperature range, interpretations of recent experiments have suggested the existence of CO2 phases which are intermediate between molecular and covalent-bonded solids. We reexamine the concept of intermediate phases in the CO2 phase diagram and propose instead molecular structures, which provide an excellent agreement with measurements.

Nonlinear carbon dioxide at high pressures and temperatures

A nonlinear molecular carbon dioxide phase IV was discovered by laser heating CO2-III (Cmca) between 12 and 30 GPa, followed by quenching to 300 K. The Raman spectrum of quenched CO2-IV exhibits a triplet bending mode nu2(O = C = O) near 650 cm (-1), suggesting a broken inversion symmetry because of bending. The 650 cm (-1) bending modes soften with increasing pressure, indicating an enhanced intermolecular interaction among neighboring bent CO2 molecules. At 80 GPa, the low-frequency vibron collapses into high-frequency phonons, and CO2-IV becomes an extended amorphous solid.

Pressure-induced solid carbonates from molecular CO2 by computer simulation

A combination of ab initio molecular dynamic simulations and fully relaxed total energy calculations is used to predict that molecular CO2 should transform to nonmolecular carbonate phases based on CO4 tetrahedra at pressures in the range of 35 to 60 gigapascals. The simulation suggests a variety of competing phases, with a more facile transformation of the molecular phase at high temperatures. Thermodynamically, the most stable carbonate phase at high pressure is predicted to be isostructural to SiO2 alpha-quartz (low quartz). A class of carbonates, involving special arrangements of CO4 tetrahedra, is found to be more stable than all the other silica-like polymorphs.