Pressure-induced dislocation amorphization of crystals

In terms of the isotropic elastic crystal model, it is shown that the formation of planar layers consisting of edge dislocation pileups is advantageous in energy if the shear modulus of the crystal is far lower than the bulk modulus. As pressure rises, the dislocation radius decreases, which can destroy the crystal structure.     

Energy landscape of clathrate hydrates

Clathrate hydrates are nanoporous crystalline materials made of a network of hydrogen-bonded water molecules (forming host cages) that is stabilized by the presence of foreign (generally hydrophobic) guest molecules. The natural existence of large quantities of hydrocarbon hydrates in deep oceans and permafrost is certainly at the origin of numerous applications in the broad areas of energy and environmental sciences and technologies (e.g. gas storage). At a fundamental level, their nanostructuration confers on these materials specific properties (e.g. their ??glass-like?? thermal conductivity) for which the host-guest interactions play a key role. These interactions occur on broad timescale and thus require the use of multi-technique approach in which neutron scattering brings unvaluable information. This work reviews the dynamical properties of clathrate hydrates, ranging from intramolecular vibrations to Brownian relaxations; it illustrates the contribution of neutron scattering in the understanding of the underlying factors governing chemical-physics properties specific to these nanoporous systems.     

Guest-host coupling and anharmonicity in clathrate hydrates

We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experimental results obtained with inelastic neutron scattering are compared with molecular-dynamics calculations. The vibrations of the guest molecules and their coupling to the cages is found to depend critically on the size, shape and electrostatic properties of the encaged guest. Atoms like xenon, that are large enough to fill the cages, show close-to-harmonic behaviour and couple strongly to the cage vibrations. Small atoms and molecules fully explore the anharmonicities of the potential within the cage, in particular at low frequencies and low temperatures. Their dynamic response is broad in energy and they couple weakly to the cage vibrations. The relevance of the microscopic dynamics for cage stability and the glass-like thermal conductivity is discussed. We equally place the observed dynamic peculiarities into the broader context of vibrations in disordered systems. Raman spectroscopic results on internal guest vibrations at high frequencies reflect also the influence of guest-host interactions and are discussed in the framework of the loose-cage tight-cage model.Received: 1 January 2003, Published online: 30 October 2003PACS: 82.75.-z Molecular sieves, zeolites, clathrates, and other complex solids - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 63.20.-e Phonons in crystal lattices     

Pressure-induced amorphization in silicon caused by the impact of electrosprayed nanodroplets

This Letter describes the shock-induced amorphization of single-crystal Si bombarded by nanodroplets. At impact velocities of several kilometers per second, the projectiles trigger strong compression pulses lasting tens of picoseconds. The phase transition, confirmed via transmission electron microscopy and electron backscatter diffraction, takes place when the projectile's stagnation pressure is approximately 15?GPa. We speculate that the amorphization results either from the decompression of the β-Sn phase or during the compression of the diamond phase.     

Novel results on structural investigations of natural minerals of clathrate hydrates

High-resolution powder X-ray diffraction measurements were carried out on gas hydrates in the angle-dispersive mode using a synchrotron radiation source. In contrast to the structural studies of laboratory-grown gas hydrates, this study has been performed on naturally grown clathrate hydrates obtained from the sea floor at different geographic locations. While the hydrate samples of the Cascadia Margin exhibit a preponderance of structure I, those from the Gulf of Mexico consist of mixed structures, namely structure II, structure I and structure H. Ice in structure Ih is inherently present in all the clathrate hydrate samples. PACS 61.10.Nz; 61.50.Ah; 61.66.Fn; 91.50.-r     

Influence of gravitational potential on the thermodynamic stability of pure and mixed clathrate hydrates

Clathrate hydrates are an ice-like material consisting of gas molecules confined within cavities in a crystalline water lattice. Phase equilibria of clathrate hydrates systems was described using the statistical mechanical theory of van der Waals and Platteeuw. This theory makes use of the fractional occupancy of cavities within the clathrate hydrate lattice in the determination of chemical equilibria. Classical density functional theory with intermolecular interactions restricted to the first hydration shell was employed to determine the fractional occupancy. In addition to the external field describing the gas-water interactions, the effect of a gravitational field was introduced. The results of the calculations show that although the gravitational potential term may be orders of magnitude smaller than the thermal kinetic energy of the gas species or the hydrogen-bond energy holding the clathrate lattice together, it can nevertheless influence the phase equilibrium of the clathrate hydrate system to some degree. The effect of the magnitudes of both the gravitational potential and the local gravitational field are considered too.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.     

On the application of binary correction factors in lattice distortion calculations for methane clathrate hydrates

The lattice distortion theory of Zele and co-workers is an attractive method for amending calculated phase equilibria of clathrate hydrates, since only two molecular computations are required. The perturbation energy between the empty and loaded clathrate hydrate lattice is the quantity of interest. The effect of binary correction factors applied to the Lorentz and Berthelot combining rules for the intermolecular interaction between gas and water particles is investigated. There are clear trends for the perturbation energy and lattice constant in terms of the binary correction factors, although there is significant sensitivity to the force field parameterization of the gas species.     

Application of low field NMR T2 measurements to clathrate hydrates

Low field (2 MHz) Nuclear Magnetic Resonance (NMR) proton spin–spin relaxation time (T₂) distribution measurements were employed to investigate tetrahydrofuran (THF)—deuterium oxide (D₂O) clathrate hydrate formation and dissociation processes. In particular, T₂ distributions were obtained at the point of hydrate phase transition as a function of the co-existing solid/liquid ratios. Because T₂ of the target molecules reflect the structural arrangements of the molecules surrounding them, T₂ changes of THF in D₂O during hydrate formation and dissociation should yield insights into the hydrate mechanisms on a molecular level. This work demonstrated that such T₂ measurements could easily distinguish THF in the solid hydrate phase from THF in the coexisting liquid phase. To our knowledge, this is the first time that T₂ of guest molecules in hydrate cages has been measured using this low frequency NMR T₂ distribution technique. At this low frequency, results also proved that the technique can accurately capture the percentages of THF molecules residing in the solid and liquid phases and quantify the hydrate conversion progress. Therefore, an extension of this technique can be applied to measure hydrate kinetics. It was found that T₂ of THF in the liquid phase changed as hydrate formation/dissociation progressed, implying that the presence of solid hydrate influenced the coexisting fluid structure. The rotational activation measured from the proton response of THF in the hydrate phase was 31 KJ/mole, which agreed with values reported in the literature.     

Theoretical investigation of structures and compositions of double neon-methane clathrate hydrates,depending on gas phase composition and pressure

The region of existence of neon clathrate hydrates is an actual problem of hydrate chemistry. The current work presents theoretical study of the equilibrium formation conditions of pure neon clathrate hydrates and double clathrate hydrates of neon-methane mixture. The structures and properties of double clathrate hydrates were described within the scope of the previously developed molecular clathrate hydrate model that takes into account the influence of guest molecules on the host lattice, interaction of guest molecules between themselves, and the possibility of multiple filling of host lattice cages by guest molecules. The model makes it possible to find an equilibrium state and thermodynamic properties of clathrate hydrates at given values of p and T. In the present work, we considered the properties of double clathrate hydrates in the range of pressures from 0 to 4 kbar at 250 K. The results of modeling have shown that the mass fraction of neon in double clathrate hydrate of Ne and CH₄ mixture of cubic structure I (sI) can reach 26%, and 22.5% in double hydrate of cubic structure II (sII) even at a low methane concentration (1%) in gas phase, at high pressure. It is shown that in double clathrate hydrates of the Ne and CH₄ mixture at high pressures, phase transition sII-sI can occur.     

Slow amorphization of zeolites

The dynamics of amorphization in two zeolites with different densities is investigated using high-pressure Raman spectroscopy. Slow amorphization of the denser zeolite under pressure leads to the formation of a low-density amorphous (LDA) phase that transforms into a more disordered high-density amorphous (HDA) phase with a further increase in the pressure. It is revealed that the LDA-HDA transformation is a first order phase transition occurring with an increase in the silicon coordination.     

Pressure-induced new chemistry

It has long been recognized that the valence electrons of an atom dominate the chemical properties, while the innershell electrons or outer empty orbital do not participate in chemical reactions. Pressure, as a fundamental thermodynamic variable, plays an important role in the preparation of new materials. More recently, pressure stabilized a series of unconventional stoichiometric compounds with new oxidation states, in which the inner-shell electrons or outer empty orbital become chemically active. Here, we mainly focus on the recent advances in high-pressure new chemistry including novel chemical bonding and new oxidation state, identified by first-principles swarm intelligence structural search calculations.The aim of this review is to provide an up-to-date research progress on the chemical bonding with inner-shell electrons or outer empty orbital, abnormal interatomic charge transfer, hypervalent compounds, and chemical reactivity of noble gases.Personal outlook on the challenge and opportunity in this field are proposed in the conclusion.     

Pressure-induced ferromagnetism of fullerenes

Ferromagnetic behavior of polymerized fullerenes appears quite close to the boundary where the fullerene molecules are destroyed. However, when the presence of a disordered or graphitic phase is detectable by the standard characterisation methods, the ferromagnetic features quickly disappear.     

Pressure-induced crystallization of triacylglycerides

We report on the impact of high pressures on the crystallization of pure triolein and triolein–trilaurin mixtures. We detect growth rates and induction times of single crystals under a high pressure application with a polarization microscope. Statistical studies on the induction time were carried out studying light scattering and transmittance. Similar to supercooling in temperature-induced crystallization, we find a superpressuring in pressure-induced crystallization. In binary mixtures, the inverse induction time indicates a threshold concentration close to the melting point of one component, beyond which it depends linearly on concentration.