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The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. [ABSTRACT FROM AUTHOR]

The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. [ABSTRACT FROM AUTHOR]

We have studied the dynamical properties of doubly and singly occupied structure II N[SUB2] clathrate hydrates by performing a series of classical molecular dynamics calculations. The intermolecular vibrational density of states of the guest molecules show the most explicit changes when going from single to double occupancies. Instead of the narrow peak for single occupancy (at 20 cm[SUP-1]), a broad range of frequencies occurs between 0 and 200 cm[SUP-1] with four modes, which have been assigned to the possible vibrations in the cage. This spectrum provides a unique fingerprint of double occupancy. For the host lattice, the libron band and the optic phonon branch show lower frequencies as compared to single fillings. In contrast, the acoustic phonon branch shifts to lower frequencies for a single filling of the large cases. We find that there is a large vibrational guest-host coupling for both single and double occupancies, although the frequency range is broader in the latter case. There is a significant translational diffusion of the N[SUB2] molecules in the doubly occupied cages. Occasionally both molecules simultaneously migrate from one local energy minimum to another. An orientational order of the N[SUB2] molecules occurs only within the small cages. However, the relative orientation of the two molecules in the doubly occupied cages is preserved in time. [ABSTRACT FROM AUTHOR]

We have performed molecular dynamics calculations to obtain a number of properties of a doubly occupied structure II N[sub 2] clathrate hydrate, in particular to study its behavior under higher pressures. The calculated neutron diffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (in either single or double occupancy) on the calculated pattern is demonstrated and discussed. The calculated Raman spectra show that the average vibrational frequency of the N[sub 2] molecules decreases in going from the singly occupied small cages to the doubly occupied large cages and then to the singly occupied large cages, respectively. The frequency distributions are explained in terms of radial distribution functions. When applying large pressures at low temperatures, a clathrate-amorphous transition occurs for a partially doubly occupied clathrate. The transition occurs at about the same pressure as for single occupations, but the densification is larger for the latter. In both cases, the transition is reversible. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ice I transforms to a high-density amorphous phase when pressed to 10 kbar at 77 K. Similar transformations in structure I and structure II clathrate hydrates have been studied by pressing samples in a piston-cylinder apparatus and by molecular dynamics simulations. The simulations were also carried out on structure I and II empty lattices. The hydrates and the empty lattices were found to transform to high-density phases under pressure at 77 K. The high-density phases of the empty lattices could be recovered at zero pressure, as is possible in the case of high-density amorphous phase of ice. However, it was not possible to recover the high-density phases of the hydrates at zero pressure. Instead, they reverted back to their original crystalline structures when the pressure was released. The molecular dynamics results suggest that under pressure the water molecules in the hydrates collapse around the guest molecules, and the repulsive forces between the guest and the water molecules are mainly responsible for the reversible transition to the original structure when the pressure is released. [ABSTRACT FROM AUTHOR]

The dynamics of Kr atoms as guests in dense clathrate hydrate structures are investigated using site specific 83Kr nuclear resonant inelastic x-ray scattering (NRIXS) spectroscopy in combination with molecular dynamics simulations. The dense structure H hydrate and filled-ice structures are studied at high pressures in a diamond anvil high-pressure cell. The dynamics of Kr in the structure H clathrate hydrate quench recovered at 77 K is also investigated. The Kr phonon density of states obtained from the experimental NRIXS data are compared with molecular dynamics simulations. The temperature and pressure dependence of the phonon spectra provide details of the Kr dynamics in the clathrate hydrate cages. Comparison with the dynamics of Kr atoms in the low-pressure structure II obtained previously was made. The Lamb-Mossbauer factor obtained from NRIXS experiments and molecular dynamics calculations are in excellent agreement and are shown to yield unique information on the strength and temperature dependence of guest-host interactions. [ABSTRACT FROM AUTHOR]

The anomalous glass-like thermal conductivity of crystalline clathrates has been suggested to be the result of the scattering of thermal phonons of the framework by ‘rattling’ motions of the guests in the clathrate cages. Using the site-specific 83Kr nuclear resonant inelastic scattering spectroscopy in combination with conventional incoherent inelastic neutron scattering and molecular-dynamics simulations, we provide unambiguous evidence and characterization of the effects on these guest–host interactions in a structure-II Kr clathrate hydrate. The resonant scattering of phonons led to unprecedented large anharmonic motions of the guest atoms. The anharmonic interaction underlies the anomalous thermal transport in this system. Clathrates are prototypical models for a class of crystalline framework materials with glass-like thermal conductivity. The explanation of the unusual molecular dynamics has a wide implication for the understanding of the thermal properties of disordered solids and structural glasses. [ABSTRACT FROM AUTHOR]

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N[sub 2] guest molecules in order to investigate the possibility of double occupancy, i.e., two N[sub 2] molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O-N[sub 2] radial distribution function, and therefore the structure in the vicinity of the N[sub 2] molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates. All these results indicate that, upon filling the large cages with pairs of molecules, the stresses on the host lattice are comparable to those in single occupation. Small changes in the intermolecular interactions do not affect the results. The clathrate hydrate destabilizes upon increasing the size of the guest molecules by more than about 10%. We also find that the clathrate hydrate remains stable when experimental conditions are closely followed, i.e., for partially doubly occupied clathrates with a pressure dependent filling fraction. The calculated compressibility is the same as in experiments. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]