Laboratory-based x-ray computed tomography for 3D imaging of samples in a diamond anvil cell in situ at high pressures.

Three-dimensional (3D) visualization of a material under pressure can provide a great deal of information about its physical and chemical properties. We developed a technique combining in-house x-ray computed tomography (XCT) and a diamond anvil cell to observe the 3D geometry of a sample in situ at high pressure with a spatial resolution of about 610 nm. We realized observations of the 3D morphology and its evolution in minerals up to a pressure of 55.6 GPa, which is comparable to the pressure conditions reported in a previous synchrotron XCT study. The new technique developed here can be applied to a variety of materials under high pressures and has the potential to provide new insights for high-pressure science and technology.

New Insights into the Cs Adsorption on Montmorillonite Clay from 133Cs Solid-State NMR and Density Functional Theory Calculations.

The adsorption sites of Cs on montmorillonite clays were investigated by theoretical 133Cs chemical shift calculations, 133Cs magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and X-ray diffraction under controlled relative humidity. The theoretical calculations were carried out for structures with three stacking variations in the clay layers, where hexagonal cavities formed with Si-O bonds in the tetrahedral layers were aligned as monoclinic, parallel, alternated; with various d-spacings. After structural optimization, all Cs atoms were positioned around the center of hexagonal cavities in the upper or lower tetrahedral sheets. The calculated 133Cs chemical shifts were highly sensitive to the tetrahedral Al (AlT)-Cs distance and d-spacing, rather than to the Cs coordination number. Accordingly, three peaks observed in our theoretical spectra were interpreted to be adsorbed Cs around the center of hexagonal cavity with or without AlT and on the surface in the open nanospace. In a series of 133Cs MAS NMR spectral changes for partial Cs substituted samples, the Cs atoms are preferentially adsorbed at sites near AlT for low Cs substituted montmorillonites. The presence of nonhydrated Cs was also confirmed in partially Cs substituted samples, even after being hydrated under high relative humidity.

Prediction of physical properties of water under extremely supercritical conditions: a molecular dynamics study.

The physical properties of water under a wide range of pressure and temperature conditions are important in fundamental physics, chemistry, and geoscience. Molecular simulations are useful for predicting and understanding the physical properties of water at phases extremely different from ambient conditions. In this study, we developed a new five-site flexible induced point charge model to predict the density, static dielectric constant, and transport properties of water in the extremely supercritical phase at high temperatures and pressures of up to 2000 K and 2000 MPa. The model satisfactorily reproduced the density, radial distribution function, static dielectric constant, reorientation time, and self-diffusion coefficients of water above the critical points. We also developed a database of the static dielectric constant, which is useful for discussing the electrical conductivity of aqueous fluids in the earth's crust and mantle.

Molecular dynamics simulations of nucleation from vapor to solid composed of Lennard-Jones molecules.

We performed molecular dynamics (MD) simulations of nucleation from vapor at temperatures below the triple point for systems consisting of 10(4)-10(5) Lennard-Jones (L-J) type molecules in order to test nucleation theories at relatively low temperatures. Simulations are performed for a wide range of initial supersaturation ratio (S(0) ≃ 10-10(8)) and temperature (kT = 0.2-0.6ε), where ε and k are the depth of the L-J potential and the Boltzmann constant, respectively. Clusters are nucleated as supercooled liquid droplets because of their small size. Crystallization of the supercooled liquid nuclei is observed after their growth slows. The classical nucleation theory (CNT) significantly underestimates the nucleation rates (or the number density of critical clusters) in the low-T region. The semi-phenomenological (SP) model, which corrects the CNT prediction of the formation energy of clusters using the second virial coefficient of a vapor, reproduces the nucleation rate and the cluster size distributions with good accuracy in the low-T region, as well as in the higher-T cases considered in our previous study. The sticking probability of vapor molecules onto the clusters is also obtained in the present MD simulations. Using the obtained values of sticking probability in the SP model, we can further refine the accuracy of the SP model.

Structure and dynamics of empty cages in xenon clathrate hydrate.

We performed molecular dynamics calculations of xenon clathrate hydrate to investigate the effects of empty cages on the structure and dynamics of the surrounding lattice. The distinct structure and dynamics of the empty cages, and cages including Xe, which coexist in the lattice, were analyzed. The results show that the ellipsoidal tetrakaidecahedral cage shrinks along the minor (100) axis and expands along the major (100) axis due to the absence of Xe from the cage, whereas the dodecahedral cage shrinks isotropically. These distortions of the empty cages cause a reduction in the lattice constant and an enhancement of the thermal vibrations of the surrounding lattice. The vibrational density of states shows that the hydrogen bonds consisting of the tetrakaidecahedral cage are strengthened by the absence of Xe, whereas those of the dodecahedral cage are weakened. These results show differing mechanisms of guest-host interaction for the two types of cages including Xe. Repulsion is the dominant guest-host interaction for the dodecahedral cage, as proposed by previous studies. For the tetrakaidecahedral cage, however, attractive interaction is dominant along the major (100) axis, whereas repulsive interaction is dominant along the minor (100) axis. The present results suggest that a small number of empty cages can affect not only the local structures but also the macroscopic properties of the crystal. It is concluded that the distortions of the empty cages are one of the important factors governing the density and phase equilibrium of clathrate hydrates.

Tests of the homogeneous nucleation theory with molecular-dynamics simulations. I. Lennard-Jones molecules.

Two kinds of the homogeneous nucleation theory exist at the present: the classical nucleation theory and the semiphenomenological model. To test them, we performed molecular-dynamics (MD) simulations of nucleation from vapor to liquid with 5000-20,000 Lennard-Jones-type molecules. Simulations were done for various values of supersaturation ratios (from 2 to 10) and temperatures (from 80 to 120 K). We compared the size distribution of clusters in MD simulations with those in the theoretical models because the number density of critical clusters governs the nucleation rate. We found that the semiphenomenological model achieves excellent agreements in size distributions of the clusters with all MD simulations we done. The classical theory underestimates the number density of the clusters in the temperature range of 80-100 K, but overestimates in 100-120 K. The semiphenomenological model also predicts well the nucleation rate in MD simulations, while the classical nucleation theory does not. Our results confirmed the validity of the semiphenomenological model for Lennard-Jones-type molecules.

Molecular-dynamics studies of surface of ice Ih.

We performed molecular dynamics calculations of surface of ice Ih in order to investigate formation mechanism of melting layer on the surface. The results showed that the vibrational amplitude of the atoms in the surface layer greatly depends on the crystal orientation, whereas that in the ice bulk is isotropic. The anisotropy of the vibration is due to a dangling motion of the free O-H bonds exist at the surface layer. The dangling motion enhances the rotational motion of the water molecules. The vibrational density of state showed a coupling between the rotational vibration and the lattice vibration of the water molecules in the surface layer. The coupling of the vibrations causes a distortion of ice lattice. Through the hydrogen-bonding network, the distortion transmits to the interior of the crystal. We conclude that the dangling motion of the free O-H bonds exist at the surface layer is one of the dominant factors governing the surface melting of ice crystal.

Post-perovskite phase transition in MgSiO3.

In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D" seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D" discontinuity.