Computer simulation of the 13 crystalline phases of ice

As a reference for follow-up studies toward more accurate model parametrizations, we performed molecular-dynamics and Monte Carlo simulations for all known crystalline phases of ice, as described by the simple point-charge/extended and TIP4P water models. We started from the measured structures, densities, and temperatures, and carried out classical canonical simulations for all these arrangements. All simulated samples were cooled down close to 0 K to facilitate the comparison with theoretical estimates. We determined configurational internal energies as well as pressures, and monitored how accurately the measured configurations were preserved during the simulations. While these two models predicted very similar thermophysical and structural properties for water at ambient conditions, the predicted features for the corresponding ice polymorphs may differ significantly.     

Formation of high density amorphous ice by decompression of ice VII and ice VIII at 135 K

Monte Carlo computer simulations of ice VII and ice VIII phases have been undertaken using the four-point transferable intermolecular potential model of water. By following thermodynamic paths similar to those used experimentally, ice is decompressed resulting in an amorphous phase. These phases are compared to the high density amorphous phase formed upon compression of ice Ih and are found to have very similar structures. By cooling liquid water along the water/Ih melting line a high density amorphous phase was also generated.     

Investigation of vapor-deposited amorphous ice and irradiated ice by molecular dynamics simulation

With the purpose of clarifying a number of points raised in the experimental literature, we investigate by molecular dynamics simulation the thermodynamics, the structure and the vibrational properties of vapor-deposited amorphous ice (ASW) as well as the phase transformations experienced by crystalline and vitreous ice under ion bombardment. Concerning ASW, we have shown that by changing the conditions of the deposition process, it is possible to form either a nonmicroporous amorphous deposit whose density (approximately 1.0 g/cm3) is essentially invariant with the temperature of deposition, or a microporous sample whose density varies drastically upon temperature annealing. We find that ASW is energetically different from glassy water except at the glass transition temperature and above. Moreover, the molecular dynamics simulation shows no evidence for the formation of a high-density phase when depositing water molecules at very low temperature. In order to model the processing of interstellar ices by cosmic ray protons and heavy ions coming from the magnetospheric radiation environment around the giant planets, we bombarded samples of vitreous ice and cubic ice with 35 eV water molecules. After irradiation the recovered samples were found to be densified, the lower the temperature, the higher the density of the recovered sample. The analysis of the structure and vibrational properties of this new high-density phase of amorphous ice shows a close relationship with those of high-density amorphous ice obtained by pressure-induced amorphization.     

Computer simulation of two new solid phases of water: Ice XIII and ice XIV

NpT Monte Carlo simulations have been performed for two recently discovered solid phases of water which have been denoted as ice XIII and ice XIV C. G. Salzmann et al. [Science311, 1758 (2006)]. Several potential models of water were considered, namely, the traditional SPC/E, TIP4P, and TIP5P and the more recent TIP5P-E, TIP4P-Ew, TIP4P/Ice, and TIP4P/2005 models. Significant differences in density and oxygen-oxygen radial distribution functions are found between the predictions of the SPC/E, TIP5P, and the models of the TIP4P family. The models TIP4P/Ice and TIP4P/2005 provide the best estimates of the density.     

Experimental and modeling study of ice melting

Differential scanning calorimetry (DSC) is applied to analyse the process of ice melting. Experimental results were compared to those obtained by a numerical simulation in which a conventional enthalpy formulation was applied. The effects of various parameters on the kinetics of transformations and therefore the shape of curves has been analysed and the importance of temperature gradients inside the sample evaluated.     

Computer simulation of quantum melting in hydrogen clusters.

We introduce a new criterion, based on multipole dynamical correlations calculated within reptation quantum Monte Carlo, to discriminate between a melting versus freezing behavior in quantum clusters. This criterion is applied to small clusters of para-hydrogen molecules (both pristine and doped with a CO chromophore), for cluster sizes of around twelve molecules. This is a magic size at which para-hydrogen clusters display an icosahedral structure and a large stability. Despite the similar geometric structure of CO@(pH2)12 and (pH2)13, the first system has a rigid, crystalline, behavior; the second behaves more like a superfluid (or, possibly, a supersolid).     

The simulation of the growth of colonies in the spinodal decomposition of metastable phases

The generalized continual hole gas model and Monte Carlo simulations were used to study the growth of colonies and lamellar segregations in the decomposition of metastable phases. The morphology of segregations qualitatively changed as the temperature decreased and depended substantially on the initial state (the presence of grain boundaries and critical equilibrium phase nuclei). The simulation results were compared with the kinetics of growth of eutectic colonies and morphologies of the decomposition of metastable austenite.     

The vaporization rate of ice at temperatures near its melting point

The first study of free vaporization kinetics of ice at temperatures near its melting point is reported. The experimental approach employed is based on a unique combination of thermal desorption spectroscopy, microcalorimetry, and time-of-flight mass spectrometry, making it possible to overcome challenges associated with the introduction of volatile solids into a high vacuum environment. Measurements of the vaporization rate of polycrystalline ice demonstrate that the vaporization kinetics deviate dramatically from those predicted by a simple mobile precursor mechanism. The vaporization rate follows Arrhenius behavior from -40 to 0 degrees C with an effective activation energy of 50+/-4 kJ/mol, which is significantly higher than the value predicted by the simple mobile precursor mechanism. Extrapolation of earlier measurements conducted below -40 degrees C yields a value of approximately 0.02 at 0 degrees C for the vaporization coefficient alphav. In contrast, experimentally determined vaporization coefficient is found to be 0.7+/-0.3 and shows a weak dependence on temperature up to the bulk melting point. The role of possible surface phase transitions in the mechanisms of release and uptake of H2O and other chemical species by ice surfaces is discussed.     

DSC study and computer modelling of the melting process in ice slurry

In order to understand the non-isothermal melting kinetics in the ice slurry, a differential scanning calorimetry (DSC) was used. Experimental results were compared to those obtained by a numerical simulation in which a general enthalpy method was applied. In this work the ice slurry studied consists of ice particles uniformly dispersed within a water-antifreeze liquid mixture. The effects of the heating rate and the initial antifreeze mass fraction are discussed. It has been found that the temperature gradients inside the sample of the solution become important if either heating rate increases or initial antifreeze mass fraction decreases.     

Quantum path integral simulation of isotope effects in the melting temperature of ice Ih

The isotope effect in the melting temperature of ice Ih has been studied by free energy calculations within the path integral formulation of statistical mechanics. Free energy differences between isotopes are related to the dependence of their kinetic energy on the isotope mass. The water simulations were performed by using the q-TIP4P/F model, a point charge empirical potential that includes molecular flexibility and anharmonicity in the OH stretch of the water molecule. The reported melting temperature at ambient pressure of this model (T=251?K) increases by 6.5±0.5 and 8.2±0.5?K upon isotopic substitution of hydrogen by deuterium and tritium, respectively. These temperature shifts are larger than the experimental ones (3.8 and 4.5 K, respectively). In the classical limit, the melting temperature is nearly the same as that for tritiated ice. This unexpected behavior is rationalized by the coupling between intermolecular interactions and molecular flexibility. This coupling makes the kinetic energy of the OH stretching modes larger in the liquid than in the solid phase. However, the opposite behavior is found for intramolecular modes, which display larger kinetic energy in ice than in liquid water.     

Computer simulation of chiral liquid crystal phases VIII. Blue phases of the chiral Gay-Berne fluid

The phase diagram of chiral calamitic liquid crystals was studied in the temperature-chirality parameter plane by means of computer simulation. Bulk systems composed of N = 2048 uniaxial chiral calamitic Gay-Berne molecules, i.e. with interactions described by the Gay-Berne potential and an additive term for the energy of the chiral interaction, were investigated using Monte Carlo (MC) simulations in the canonical ensemble (NVT). A rich polymorphism of chiral liquid crystal phases was observed along an isotherm with increasing chirality parameter describing the strength of the chiral interaction. In addition to the cholesteric phase (N*), for the first time a blue phase I (BP I) could be proven by computer simulation of a many-particle system based on model intermolecular interactions. Additionally, at high values of the chirality parameter, a phase with randomly oriented squirming double twist tubes was found as characteristic for the so-called spaghetti model for blue phase III (BP III). The structures of all phases were characterized by order parameters, a set of scalar and pseudoscalar orientational correlation functions, and especially by visual representations of selected configurations.     

Computer simulation of chiral liquid crystal phases VIII. Blue phases of the chiral Gay-Berne fluid

The phase diagram of chiral calamitic liquid crystals was studied in the temperature-chirality parameter plane by means of computer simulation. Bulk systems composed of N = 2048 uniaxial chiral calamitic Gay-Berne molecules, i.e. with interactions described by the Gay-Berne potential and an additive term for the energy of the chiral interaction, were investigated using Monte Carlo (MC) simulations in the canonical ensemble (NVT). A rich polymorphism of chiral liquid crystal phases was observed along an isotherm with increasing chirality parameter describing the strength of the chiral interaction. In addition to the cholesteric phase (N*), for the first time a blue phase I (BP I) could be proven by computer simulation of a many-particle system based on model intermolecular interactions. Additionally, at high values of the chirality parameter, a phase with randomly oriented squirming double twist tubes was found as characteristic for the so-called spaghetti model for blue phase III (BP III). The structures of all phases were characterized by order parameters, a set of scalar and pseudoscalar orientational correlation functions, and especially by visual representations of selected configurations.     

Formation and structural anomaly of the metastable phases in an immiscible Ag-Mo system studied by ion beam mixing and molecular dynamics simulation

For the equilibrium immiscible Ag-Mo system characterized by a large positive heat of formation, the nanosized Ag-Mo multilayered samples are designed and prepared to include sufficient interfacial free energy to elevate their initial energetic states to be higher than that of either the amorphous phase or solid solution and then subject to 200 keV xenon ion irradiation. The results show that a uniform amorphous alloy can be obtained within a composition range, at least, from 25 to 88 atom % of Mo. Interestingly, in the intermediate stage of ion irradiation, a bcc phase, an amorphous phase, and an order (bcc)-disorder coexisting state appear simultaneously in the Ag12Mo88 multilayered sample and extend over the entire bright field image with unanimously homogeneous composition. In thermodynamic modeling, a Gibbs free energy diagram of the Ag-Mo system is constructed, based on Miedema's model, and suggests that within a narrow composition regime of 85-90 atom % of Mo, the energy difference between the bcc and the amorphous phases is extremely small, which is probably the very reason for the order-disorder coexisting state to appear. In atomistic modeling, an ab initio derived Ag-Mo potential is applied to perform molecular dynamics simulations. The simulations not only determine an intrinsic glass-forming ability/range (GFA/GFR) of the Ag-Mo system to be from 10 to 88 atom % of Mo but also reveal the possibility of the formation/appearance of a crystalline and amorphous mixture in a narrow composition regime of 88-92 atom % of Mo. Apparently, the theoretical results are in excellent agreement and/or compatible with the experimental observations in ion beam mixing.     

Trapping and release of CO2 guest molecules by amorphous ice

Interactions of 13CO2 guest molecules with vapor-deposited porous H2O ices have been examined using temperature-programmed desorption (TPD) and Fourier transform infrared (FTIR) techniques. Specifically, the trapping and release of 13CO2 by amorphous solid water (ASW) has been studied. The use of 13CO2 eliminates problems with background CO2. Samples were prepared by (i) depositing 13CO2 on top of ASW, (ii) depositing 13CO2 underneath ASW, and (iii) codepositing 13CO2 and H2O during ASW formation. Some of the deposited 13CO2 becomes trapped when the ice film is annealed. The amount of 13CO2 trapped in the film depends on the deposition method. The release of trapped molecules occurs in two stages. The majority of the trapped 13CO2 escapes during the ASW-to-cubic ice phase transition at 165 K, and the rest desorbs together with the cubic ice film at 185 K. We speculate that the presence of 13CO2 at temperatures up to 185 K is due to 13CO2 that is trapped in cavities within the ASW film. These cavities are similar to ones that trap the 13CO2 that is released during crystallization. The difference is that 13CO2 that remains at temperatures up to 185 K does not have access to escape pathways to the surface during crystallization.     

In situ high-pressure x-ray diffraction study of H2O ice VII

Ice VII was examined over the entire range of its pressure stability by a suite of x-ray diffraction techniques in order to understand a number of unexplained characteristics of its high-pressure behavior. Axial and radial polycrystalline (diamond anvil cell) x-ray diffraction measurements reveal a splitting of diffraction lines accompanied by changes in sample texture and elastic anisotropy. In situ laser heating of polycrystalline samples resulted in the sharpening of diffraction peaks due to release of nonhydrostatic stresses but did not remove the splitting. Radial diffraction measurements indicate changes in strength of the material at this pressure. Taken together, these observations provide evidence for a transition in ice VII near 14 GPa involving changes in the character of the proton order/disorder. The results are consistent with previous reports of changes in phase boundaries and equation of state at this pressure. The transition can be interpreted as ferroelastic with the appearance of spontaneous strain that vanishes at the hydrogen bond symmetrization transition near 60 GPa.     

Topological defects and bulk melting of hexagonal ice

We use classical molecular dynamics combined with the recently developed metadynamics method [Laio, A.; Parrinello, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 20] to study the process of bulk melting in hexagonal ice. Our simulations show that bulk melting is mediated by the formation of topological defects which preserve the coordination of the tetrahedral network. Such defects cluster to form a defective region involving about 50 molecules with a surprisingly long lifetime. The subsequent formation of coordination defects triggers the transition to the liquid state.     

An experimental study of crystallization and crystal growth of methane hydrates from melting ice

An experiment with well defined gas-water interfacial surface area was developed to study the crystallization and crystal growth of methane hydrates. Measurable formation rates were observed only when melting ice was involved. No hydrates nucleated from liquid water or from non-melting ice. It is concluded that melting ice, which like hydrate water is hydrogen-bonded, provides a template for hydrate nucleation as well as providing a heat sink for absorbing the heat of formation during hydrate growth. The experiment was conducted in the absence of mixing so that hydrate crystals grew under quiescent conditions.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Solid phases of cyclopentane: combined experimental and simulation study

The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.