Crystal structures and dynamical properties of dense CO2.

Structural polymorphism in dense carbon dioxide (CO2) has attracted significant attention in high-pressure physics and chemistry for the past two decades. Here, we have performed high-pressure experiments and first-principles theoretical calculations to investigate the stability, structure, and dynamical properties of dense CO2 We found evidence that CO2-V with the 4-coordinated extended structure can be quenched to ambient pressure below 200 K-the melting temperature of CO2-I. CO2-V is a fully coordinated structure formed from a molecular solid at high pressure and recovered at ambient pressure. Apart from confirming the metastability of CO2-V (I-42d) at ambient pressure at low temperature, results of ab initio molecular dynamics and metadynamics (MD) simulations provided insights into the transformation processes and structural relationship from the molecular to the extended phases. In addition, the simulation also predicted a phase V'(Pna21) in the stability region of CO2-V with a diffraction pattern similar to that previously assigned to the CO2-V (P212121) structure. Both CO2-V and -V' are predicted to be recoverable and hard with a Vicker hardness of ∼20 GPa. Significantly, MD simulations found that the CO2 in phase IV exhibits large-amplitude bending motions at finite temperatures and high pressures. This finding helps to explain the discrepancy between earlier predicted static structures and experiments. MD simulations clearly indicate temperature effects are critical to understanding the high-pressure behaviors of dense CO2 structures-highlighting the significance of chemical kinetics associated with the transformations.

Phase Diagram of Dense H_{2}-He Mixtures: Evidence for Strong Chemical Association, Miscibility, and Structural Change.

The phase diagram of hydrogen-helium mixtures is presented to 75 GPa, underscoring the formation of metastable H_{2}-rich crystallite in He-rich fluid mixtures and the structural phase transition in He lattice at ∼52 GPa. The Raman data also indicate a significant level of mixing between H_{2} and He even in solids, giving rise to new vibrational bands in He-rich solid at ∼2400 cm^{-1} for H-He stretching and 140 cm^{-1} for the lattice phonon of H_{2} incorporated hcp He. Therefore, the present result signifies unexpected, strong chemical association of the interstitial-filled guest molecules (H_{2} or He) with the host lattice (hcp He or H_{2}) in this quantum solid mixture.

Transformation of hydrazinium azide to molecular N8 at 40 GPa.

Hydrazinium azide (HA) has been investigated at high pressures to 68 GPa using confocal micro-Raman spectroscopy and synchrotron powder x-ray diffraction. The results show that HA undergoes structural phase transitions from solid HA-I to HA-II at 13 GPa, associated with the strengthening of hydrogen bonding, and then to N8 at 40 GPa. The transformation of HA to recently predicted N8 (N≡N+-N--N=N--N-+N≡N) is evident by the emergence of new peaks at 2384 cm-1, 1665 cm-1, and 1165 cm-1, arising from the terminal N≡N stretching, the central N=N stretching, and the N-N stretching, respectively. However, upon decompression, N8 decomposes to ε-N2 below 25 GPa, but the remnant can be seen as low as 3 GPa.

Phase diagram of ammonium perchlorate: Raman spectroscopic constrains at high pressures and temperatures.

We present the pressure-temperature (PT) induced physical and chemical transformations in ammonium perchlorates (APs) up to 50 GPa and 450 °C, using diamond anvil cells and confocal micro-Raman spectroscopy, which provide new constraints for the phase diagram of AP. The results show spectral evidences for three new polymorphs (III, IV, and VI) of AP, in addition to two previously known phases (I and II), at various PT conditions with varying degrees of hydrogen bonding and lack of strong spectral evidence for previously known high-temperature cubic phase (phase V). Upon further heating, AP chemically decomposes to N2, N2O, and H2O. The present phase diagram is, therefore, in sharp contrast to the previous one, underscoring a rich polymorphism, a large stability field for solids, and a replacement of the melt with a decomposition line.

Pressure-induced phase and chemical transformations of lithium peroxide (Li2O2).

We present the pressure-induced phase/chemical changes of lithium peroxide (Li2O2) to 63 GPa using diamond anvil cells, confocal micro-Raman spectroscopy, and synchrotron x-ray diffraction. The Raman data show the emergence of the major vibrational peaks associated with O2 above 30 GPa, indicating the subsequent pressure-induced reversible chemical decomposition (disassociation) in dense Li2O2. The x-ray diffraction data of Li2O2, on the other hand, show no dramatic structural change but remain well within a P63/mmc structure to 63 GPa. Nevertheless, the Rietveld refinement indicates a subtle change in the structural order parameter z of the oxygen position O (13, 23, z) at around 35 GPa, which can be considered as a second-order, isostructural phase transition. The nearest oxygen-oxygen distance collapses from 1.56 Å at ambient condition to 1.48 Å at 63 GPa, resulting in a more ionic character of this layered crystal lattice, 3Li(+)+(LiO2)3 (3-). This structural change in turn advocates that Li2O2 decomposes to 2Li and O2, further augmented by the densification in specific molar volumes.

Superconductivity in highly disordered dense carbon disulfide.

High pressure plays an increasingly important role in both understanding superconductivity and the development of new superconducting materials. New superconductors were found in metallic and metal oxide systems at high pressure. However, because of the filled close-shell configuration, the superconductivity in molecular systems has been limited to charge-transferred salts and metal-doped carbon species with relatively low superconducting transition temperatures. Here, we report the low-temperature superconducting phase observed in diamagnetic carbon disulfide under high pressure. The superconductivity arises from a highly disordered extended state (CS4 phase or phase III[CS4]) at ~6.2 K over a broad pressure range from 50 to 172 GPa. Based on the X-ray scattering data, we suggest that the local structural change from a tetrahedral to an octahedral configuration is responsible for the observed superconductivity.

Pressure-induced symmetry-lowering transition in dense nitrogen to layered polymeric nitrogen (LP-N) with colossal Raman intensity.

We present the discovery of a novel nitrogen phase synthesized using laser-heated diamond anvil cells at pressures between 120-180 GPa well above the stability field of cubic gauche (cg)-N. This new phase is characterized by its singly bonded, layered polymeric (LP) structure similar to the predicted Pba2 and two colossal Raman bands (at ∼1000 and 1300 cm^{-1} at 150 GPa), arising from two groups of highly polarized nitrogen atoms in the bulk and surface of the layer, respectively. The present result also provides a new constraint for the nitrogen phase diagram, highlighting an unusual symmetry-lowering 3D cg-N to 2D LP-N transition and thereby the enhanced electrostatic contribution to the stabilization of this densely packed LP-N (ρ=4.85 g/cm^{3} at 120 GPa).

Hydrogen bonding induced proton exchange reactions in dense D2-NH3 and D2-CH4 mixtures.

We have investigated high-pressure behaviors of simple binary mixtures of NH3 and D2 to 50 GPa and CH4 and D2 to 30 GPa using confocal micro-Raman spectroscopy. The spectral data indicate strong proton exchange reactions occur in dense D2-NH3 mixture, producing different isotopes of ammonia such as NH3, NH2D, NHD2, and ND3. In contrast, the proton exchange process in dense D2-CH4 mixture is highly limited, and no vibration feature is apparent for deuterated methane. The vibrational modes of H2 isotopes in D2-NH3 are blue shifted from those of pure H2 isotopes, whereas the modes of D2-CH4 show overall agreement with those in pure D2 and CH4. In turn, this result advocates the presence of strong repulsion and thereby internal pressure in D2-NH3 mixture, which are absent in D2-CH4. In fact, the bond length of hydrogen molecules in D2-NH3, calculated from the present spectral data, is shorter than that observed in pure hydrogen - supporting the enhanced intermolecular interaction in the mixture. Comparing the present spectral results with those previously observed in D2-H2O mixtures further suggests that the strength of repulsive interaction or the magnitude of internal pressure in the mixtures is proportional to the strength of hydrogen bonding in H2O, NH3, and CH4 in decreasing order. Hence, we suggest that the proton exchange is assisted by hydrogen bonding in these molecules.

Melting and phase transitions of nitrogen under high pressures and temperatures.

Dense nitrogen exhibits fascinating molecular and extended polymorphs as well as an anomalous melt maximum at high temperatures. However, the exact solid-liquid phase boundary is still the subject of debate, as both creating and probing hot dense nitrogen, solid and fluid alike, poses unique experimental challenges. Raman studies of nitrogen were performed to investigate the melting curve and solid-solid phase transitions in the pressure-temperature range of 25 to 103 GPa and 300 to 2000 K. The solid-liquid phase boundary has been probed with time-resolved Raman spectroscopy on ramp heated nitrogen in diamond anvil cell (DAC), showing a melting maximum at 73 GPa and 1690 K. The solid-solid phase boundaries have been measured with spatially resolved micro-confocal Raman spectroscopy on resistively heated DAC, probing the δ-ɛ phase line to 47 GPa and 914 K. At higher pressures the θ-phase was produced upon a repeated thermal heating of the ζ-phase, yet no evidence was found for the ι-phase. Hence, the present results signify the path dependence of dense nitrogen phases and provide new constraints for the phase diagram.

High density amorphous ice at room temperature.

The phase diagram of water is both unusual and complex, exhibiting a wide range of polymorphs including proton-ordered or disordered forms. In addition, a variety of stable and metastable forms are observed. The richness of H(2)O phases attests the versatility of hydrogen-bonded network structures that include kinetically stable amorphous ices. Information of the amorphous solids, however, is rarely available especially for the stability field and transformation dynamics--but all reported to exist below the crystallization temperature of approximately 150-170 K below 4-5 GPa. Here, we present the evidence of high density amorphous (HDA) ice formed well above the crystallization temperature at 1 GPa--well inside the so-called "no-man's land." It is formed from metastable ice VII in the stability field of ice VI under rapid compression using dynamic-diamond anvil cell (d-DAC) and results from structural similarities between HDA and ice VII. The formation follows an interfacial growth mechanism unlike the melting process. Nevertheless, the occurrence of HDA along the extrapolated melt line of ice VII resembles the ice Ih-to-HDA transition, indicating that structural instabilities of parent ice VII and Ih drive the pressure-induced amorphization.

Physical and chemical transformations of highly compressed carbon dioxide at bond energies.

Carbon dioxide exhibits a richness of high-pressure polymorphs with a great diversity in intermolecular interaction, chemical bonding, and crystal structures. It ranges from typical molecular solids to fully extended covalent solids with crystal structures similar to those of SiO2. These extended solids of carbon dioxide are fundamentally new materials exhibiting interesting optical nonlinearity, low compressibility and high energy density. Furthermore, the large disparity in chemical bonding between the extended network and molecular structures results in a broad metastability domain for these phases to room temperature and almost to ambient pressure and thereby offers enhanced opportunities for novel materials developments. Broadly speaking, these molecular-to-non-molecular transitions occur due to electron delocalization manifested as a rapid increase in electron kinetic energy at high density. The detailed mechanisms, however, are more complex with phase metastabilities, path-dependent phases and phase boundaries, and large lattice strains and structural distortions - all of which are controlled by well beyond thermodynamic constraints to chemical kinetics associated with the governing phases and transitions. As a result, the equilibrium phase boundary is difficult to locate precisely (experimentally or theoretically) and is often obscured by the presence of metastable phases (ordered or disordered). This paper will review the pressure-induced transformations observed in highly compressed carbon dioxide and present chemistry perspectives on those molecular-to-non-molecular transformations that can be applied to other low-Z molecular solids at Mbar pressures where the compression energy rivals the chemical bond energies.

Phase diagram of ammonium nitrate.

Ammonium Nitrate (AN) is a fertilizer, yet becomes an explosive upon a small addition of chemical impurities. The origin of enhanced chemical sensitivity in impure AN (or AN mixtures) is not well understood, posing significant safety issues in using AN even today. To remedy the situation, we have carried out an extensive study to investigate the phase stability of AN and its mixtures with hexane (ANFO-AN mixed with fuel oil) and Aluminum (Ammonal) at high pressures and temperatures, using diamond anvil cells (DAC) and micro-Raman spectroscopy. The results indicate that pure AN decomposes to N2, N2O, and H2O at the onset of the melt, whereas the mixtures, ANFO and Ammonal, decompose at substantially lower temperatures. The present results also confirm the recently proposed phase IV-IV' transition above 17 GPa and provide new constraints for the melting and phase diagram of AN to 40 GPa and 400°C.

Pressure-induced phase transition and polymerization of tetracyanoethylene (TCNE).

We have studied the pressure-induced physical and chemical transformations of tetracyanoethylene (TCNE or C6N4) in diamond anvil cells using micro-Raman spectroscopy, laser-heating, emission spectroscopy, and synchrotron x-ray diffraction. The results indicate that TCNE in a quasi-hydrostatic condition undergoes a shear-induced phase transition at 10 GPa and then a chemical change to two-dimensional (2D) C=N polymers above 14 GPa. These phase and chemical transformations depend strongly on the state of stress in the sample and occur sluggishly in non-hydrostatic conditions over a large pressure range between 7 and 14 GPa. The x-ray diffraction data indicate that the phase transition occurs isostructurally within the monoclinic structure (P21∕c) without any apparent volume discontinuity and the C=N polymer is highly disordered but remains stable to 60 GPa-the maximum pressure studied. On the other hand, laser-heating of the C=N polymer above 25 GPa further converts to a theoretically predicted 3D C-N network structure, evident from an emergence of new Raman νs(C-N) at 1404 cm(-1) at 25 GPa and the visual appearance of translucent solid. The C-N product is, however, unstable upon pressure unloading below 10 GPa, resulting in a grayish powder that can be considered as nano-diamonds with high-nitrogen content at ambient pressure. The C-N product shows a strong emission line centered at 640 nm at 30 GPa, which linearly shifts toward shorter wavelength at the rate of -1.38 nm∕GPa. We conjecture that the observed red shift upon unloading pressure is due to increase of defects in the C-N product and thereby weakening of C-N bonds.

Formation and phase transitions of methane hydrates under dynamic loadings: compression rate dependent kinetics.

We describe high-pressure kinetic studies of the formation and phase transitions of methane hydrates (MH) under dynamic loading conditions, using a dynamic-diamond anvil cell (d-DAC) coupled with time-resolved confocal micro-Raman spectroscopy and high-speed microphotography. The time-resolved spectra and dynamic pressure responses exhibit profound compression-rate dependences associated with both the formation and the solid-solid phase transitions of MH-I to II and MH-II to III. Under dynamic loading conditions, MH forms only from super-compressed water and liquid methane in a narrow pressure range between 0.9 and 1.6 GPa at the one-dimensional (1D) growth rate of 42 µm/s. MH-I to II phase transition occurs at the onset of water solidification 0.9 GPa, following a diffusion controlled mechanism. We estimated the activation volume to be -109±29 Å(3), primarily associated with relatively slow methane diffusion which follows the rapid interfacial reconstruction, or martensitic displacements of atomic positions and hydrogen bonds, of 5(12)6(2) water cages in MH-I to 4(3)5(12)6(3) cages in MH-II. MH-II to III transition, on the other hand, occurs over a broad pressure range between 1.5 and 2.2 GPa, following a reconstructive mechanism from super-compressed MH-II clathrates to a broken ice-filled viscoelastic solid of MH-III. It is found that the profound dynamic effects observed in the MH formation and phase transitions are primarily governed by the stability of water and ice phases at the relevant pressures.

Evidence for superionic H2O and diffusive He-H2O at high temperature and high pressure.

We present the evidence of superionic phase formed in H2O and, for the first time, diffusive H2O-He phase, based on time-resolved x-ray diffraction experiments performed on ramp-laser-heated samples in diamond anvil cells. The diffraction results signify a similar bcc-like structure of superionic H2O and diffusive He-H2O, while following different transition dynamics. Based on time and temperature evolution of the lattice parameter, the superionic H2O phase forms gradually in pure H2O over the temperature range of 1350-1400 K at 23 GPa, but the diffusive He-H2O phase forms abruptly at 1300 K at 26 GPa. We suggest that the faster dynamics and lower transition temperature in He-H2O are due to a larger diffusion coefficient of interstitial-filled He than that of more strongly bound H atoms. This conjecture is then consistent with He disordered diffusive phase predicted at lower temperatures, rather than H-disordered superionic phase in He-H2O.

Pressure induced isostructural metastable phase transition of ammonium nitrate.

The energetic material ammonium nitrate (AN, NH(4)NO(3)) has been studied under both hydrostatic and nonhydrostatic conditions using diamond anvil cells combined with micro-Raman spectroscopy and synchrotron X-ray powder diffraction. The refined powder X-ray data indicates that under hydrostatic conditions AN-IV (orthorhombic, Pmmn) is stable to above 40 GPa. In one nonhydrostatic compression experiment a volume collapse was observed, suggesting an isostructural phase transition to a "metastable" phase IV' between 17 and 28 GPa. The structures of phase IV and IV' are similar with the subtle difference in the hydrogen-bonding network; that is, a noticeably shorter N1···O1 distance seen in phase IV'. This hydrogen bond has a significant component along the b-axis, which proves to be the most compressible until cell axis over the entire pressure range. It is likely that the shear stress of the nonhydrostatic experiment drives the phase IV-to-IV' transition to occur. We compare the present isotherms of phase IV and IV' in both static and nonhydrostatic conditions with the previously obtained Hugoniot and find that the nonhydrostatic isotherm approximately matches the Hugoniot. On the basis of this comparison, we conjecture that a chemical reaction or phase transition may occur in AN under dynamic pressure conditions at 22 GPa.

Highly repulsive interaction in novel inclusion D2-N2 compound at high pressure: Raman and x-ray evidence.

We present spectral and structural evidences for the formation of a homogeneous cubic δ-N(2)-like, noncrystalline solid and an incommensuratelike hexagonal (P6(3)22) inclusion compound (N(2))(12)D(2), formed by compressing a nitrogen-rich mixture to 5.5 and 10 GPa, respectively. A strong repulsive coupling in (N(2))(12)D(2) is evident from a blue shift, discontinuous changes, and the absence of turnover of the D(2) vibron to 70 GPa--all in sharp contrast to both pure D(2) and other inclusion compounds. This repulsive interaction is responsible to the observed incommensuratelike structure and large internal pressure.

H2O and D2 mixtures under pressure: spectroscopy and proton exchange kinetics.

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D(2)-H(2)O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D(2)-H(2)O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D(2) molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 Å at 6 GPa--slightly shorter than that in pure--attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D(2) and H(2)O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k ~ 0.085 h(-1) at 0.2 GPa in fluid D(2) and water mixtures, k ~ 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D(2)-ice mixtures, and k ~ 10(-3) h(-1) at 8 GPa in solid D(2) and ice mixtures.

"Stubborn" triaminotrinitrobenzene: unusually high chemical stability of a molecular solid to 150 GPa.

We report an unexpectedly high chemical stability of molecular solid 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under static high pressures. In contrast to the high-pressure behavior of the majority of molecular solids, TATB remains both chemically stable and an insulator to 150 GPa--well above the predicted metallization pressure of 120 GPa. Single crystal studies have shown that TATB exhibits pressure-induced Raman changes associated with two subtle structural phase transitions at 28 and 56 GPa. These phase transitions are accompanied by remarkable color changes, from yellow to orange and to dark red with increasing pressure. We suggest that the high-stability of TATB arises as a result of its hydrogen-bonded aromatic two-dimensional (2D) layered structure and highly repulsive interlayer interaction, hindering the formation of 3D networks or metallic states.