Ice Polymorphs Research Articles

The anomalously high melting temperature of bilayer ice

Confinement of water usually depresses its melting temperature. Here we use molecular dynamics simulations to determine the liquid-crystal equilibrium temperature for water confined between parallel hydrophobic or mildly hydrophilic plates as a function of the distance between the surfaces. We find that bilayer ice, an ice polymorph in which the local environment of each water molecule strongly departs from the most stable tetrahedral structure, has the highest melting temperature (T(m)) of the series of l-layer ices. The melting temperature of bilayer ice is not only unusually high compared to the other confined ices, but also above the melting point of bulk hexagonal ice. Recent force microscopy experiments of water confined between graphite and a tungsten tip reveal the formation of ice at room temperature [K. B. Jinesh and J. W. M. Frenken, Phys. Rev. Lett. 101, 036101 (2008)]. Our results suggest that bilayer ice, for which we compute a T(m) as high as 310 K in hydrophobic confinement, is the crystal formed in those experiments.

Phase behaviour and thermoelastic properties of perdeuterated ammonia hydrate and ice polymorphs from 0 to 2 GPa

The results are described of a series of neutron powder diffraction experiments over the pressure and temperature ranges 0 <P< 2 GPa, 150 <T< 240 K, which were carried out with the objective of determining the phase behaviour and thermoelastic properties of perdeuterated ammonia dihydrate (ND3·2D2O). In addition to the low-pressure cubic crystalline phase, ADH I, two closely related monoclinic polymorphs of ammonia dihydrate have been identified, which commonly occur as a composite in the range 450–550 MPa at 175 K; these are labelled ADH IIaand IIb, and each has unit-cell volumeV≃ 310 Å3and number of formula units per unit cellZ= 4. It has been determined that this composite dissociates to a mixture of ammonia monohydrate (ND3·D2O) phase II (AMH II) and ice II when warmed to ∼190 K at 550 MPa, which in turn partially melts to ice II + liquid atT= 196 K; AMH II has a large orthorhombic unit cell (V≃ 890 Å3,Z= 16). Above 600 MPa, an orthorhombic polymorph of ammonia dihydrate (withV≃ 530 Å3,Z= 8), which has been referred to previously as ADH IV, persists to pressures greater than 2 GPa and appears to be the liquidus phase over this whole pressure range. This phase has been observed co-existing with ice II, ice VI and AMH II. The most plausible synthesis of the high-pressure phase behaviour is described here. This model explains the reported observations, and provides measurements of the densities, thermal expansion, bulk moduli and crystal growth kinetics of the high-pressure ammonia dihydrate, ammonia monohydrate and ice polymorphs.

No Confinement Needed: Observation of a Metastable Hydrophobic Wetting Two-Layer Ice on Graphene

The structure of water at interfaces is crucial for processes ranging from photocatalysis to protein folding. Here, we investigate the structure and lattice dynamics of two-layer crystalline ice films grown on a hydrophobic substrate, graphene on Pt(111), with low energy electron diffraction, reflection-absorption infrared spectroscopy, rare-gas adsorption/desorption, and ab initio molecular dynamics. Unlike hexagonal ice, which consists of stacks of puckered hexagonal "bilayers", this new ice polymorph consists of two flat hexagonal sheets of water molecules in which the hexagons in each sheet are stacked directly on top of each other. Such two-layer ices have been predicted for water confined between hydrophobic walls, but not previously observed experimentally. Our results show that the two-layer ice forms even at zero pressure at a single hydrophobic interface by maximizing the number of hydrogen bonds at the expense of adopting a nontetrahedral geometry with weakened hydrogen bonds.

DFT and Local-MP2 Periodic Study of the Structure and Stability of Two Proton-Ordered Polymorphs of Ice

The equilibrium geometry and the formation energy of two periodic polymorphs of Ice have been theoretically studied: the former (Ice XI, crystal group Cmc2(1)) is experimentally observed as the most stable structure at low temperature and pressure; the latter (crystal group Pna2(1)) is the simplest proton-ordered model of ordinary ice. With the Crystal code, the problem is solved using Hartree-Fock (HF), pure Kohn-Sham (PW91), or hybrid (B3LYP) one-electron Hamiltonians. The B3LYP results are those in best agreement with the experiment. Using the B3LYP-optimized geometry and starting from the corresponding HF Crystal solution, the energetics of the two polymorphs have been investigated at an ab initio MP2 level using the Cryscor code, based on a local-correlation approach: these calculations have allowed us not only to confirm the excellent B3LYP results as concerns the formation energy and the relative stability of the two structures but also to analyze the role in this respect of the intra- and intermolecular contributions to the correlation energy. Since both Crystal and Cryscor adopt a basis set of localized Gaussian-type functions and since very small energy differences are involved, utter attention has been paid to correcting for the basis set superposition error in the calculation of formation energies.

An icy mineralogy package (IMP) for in-situ studies of Titan’s surface

We describe the scientific case for and preliminary design of an instrument whose primary goal is to determine the chemistry (element abundance) and mineralogy (compound identity and abundance) of Titan’s surface using a combination of energy dispersive X-ray fluorescence spectroscopy (EDXRF) and X-ray diffraction (XRD). XRD is capable of identifying any crystalline substance present on Titan’s surface at relative abundances greater than ∼1 wt%, allowing unambiguous identification of, for example, structure I and II clathrates (even in the presence of ice), and various organic solids, which may include C 2H 2, C 2H 4, C 4H 2, HCN, CH 3CN, HC 3N, and C 4N 2). The XRF component of the instrument will obtain elemental abundances for 16 < Z < 60 with minimum detection limits better than 10 ppm (including detection of atmospheric noble gas isotopes), and may achieve detection limits of 0.01–1% for lighter elements down to Z = 6 (carbon). The instrument is well suited to integration with other analytical tools as part of a light-weight surface chemistry and mineralogy package. Although considerably less sensitive to elemental abundance than GC–MS (10 −2 vs. 10 −8) it is likely to be significantly lighter (<0.5 kg vs. 10 kg). The instrument will be able to address the origin and evolution of Titan by measuring the mineralogy and chemistry of bulk crust (water ice/clathrates), excavated impact ejecta (possibly including XRD-identifiable high-pressure polymorphs of ice), and cryolava or ash (distinguishing between ammonia–methanol and sulfate–carbonate magmas). The instrument will also address atmospheric photochemistry and climate history by measuring the mineralogy and chemistry of organic sediments found in various environments, such as dunes, fluvial and peri-lacustrine areas (including possible evaporites), and possibly exhumed from deep reservoirs by mud volcanism. Lastly, the instrument will address Titan’s astrobiological evolution by enabling detection of more complex organic materials (crystalline carboxylic/amino acids or sugars) in areas where sediments have interacted with aqueous solutions.

Quantum chemical studies of the adsorption of single acetone molecules on hexagonal ice Ihand cubic ice Ic

The interaction energies of free acetone molecules with surfaces of two different ice polymorphs have been investigated by quantum chemical methods. Special emphasis has been given to sites for adsorption on the (0001) surface of hexagonal ice (I(h)) and the (1[combining macron]01) surface of cubic ice (I(c)), respectively. The structural optimisations made use of conventional electronic structure methods including HF and B3LYP using moderate basis sets up to 6-31+G(d) as well as local and ONIOM methods using 2 or 3 layers which were treated at different levels of theory. The adsorption energies at T = 0 K were calculated for the optimised adsorption geometries performing single points at the B3LYP, MP2 and LMP2 level in conjunction with valence triple-zeta basis sets up to 6-311+G(d,p). Including corrections for basis set superposition errors (BSSE) the most extensive calculations provide adsorption energies (T = 0 K) of -39.1 and -57.5 kJ mol(-1) for the energetically most favourable sites for adsorption of a single acetone molecule on ice I(h) and ice I(c), respectively. By vibrational analysis this can be transformed to adsorption enthalpies at around a temperature of 200 K yielding values of -31.5 for adsorption on ice I(h) and -49.9 kJ mol(-1) for adsorption on ice I(c). The current results support experimental observations of Behr et al. (J. Phys. Chem. A, 2006, 110, 8098) in which evidence was presented that acetone adsorbs on ice around 200 K at two different sites; each of which has a different adsorption enthalpy.

Physicochemical Properties of Structured Water at Material Surfaces

Physicochemical properties of thin film water on and between material surfaces have been reviewed. Infrared (IR) spectroscopy on thin film water sandwiched between various materials showed shifts of OH stretching vibration frequency maxima from 3400 cm−1 to about 3250 cm−1. These shifts vary with different materials, crystallographic orientations, film thickness (less than about 200 nm), dissolved ions and temperatures. The thin film water is supposed to have constrained structure close to ice and might have smaller diffusion coefficients and larger viscosity than the bulk liquid water. Standard molar volume, specific heat and entropy values for hydration water on some inorganic materials are different from these for the bulk liquid water and close to those for ice polymorphs. Sonic wave velocity measurements on a water-saturating rock suggest a larger P wave velocity for the thin film water than the value for bulk water. These structured thin water films with different properties from those of the bulk water can be present in the earth's interior and also in living materials such as cactus and human skin controlling dynamics of earth and life.

Polymorphic (NH): As Polyimidogen and Ammonium Azide Structure-types

AbstractThis paper reports a productive discussion of bonding principles in the non-metallic 2nd row hydrides. It suggests the inversion of a bonding character, potentially from hydrides of B &amp; C, which may display unsaturation and electronic deficiency accompanied by electronic delocalization in 1D or 2D or 3D. Contrasted with the opposite possibility, within the finite number of hydrides of N, O and F, that display apparently extensive H-bonding and subsequently proton delocalization in 1D and 2D (in HF &amp; ice polymorphs, respectively), and now potentially in 3D in a corresponding hydride of N called Rice's blue material, or perhaps polyimidogen. Where polyimidogen is a crystalline NH lattice that is a polymorph of the ammonium azide structure-type thus.

Structural and thermodynamic properties of different phases of supercooled liquid water

Computer simulation results are reported for a realistic polarizable potential model of water in the supercooled region. Three states, corresponding to the low density amorphous ice, high density amorphous ice, and very high density amorphous ice phases are chosen for the analyses. These states are located close to the liquid-liquid coexistence lines already shown to exist for the considered model. Thermodynamic and structural quantities are calculated, in order to characterize the properties of the three phases. The results point out the increasing relevance of the interstitial neighbors, which clearly appear in going from the low to the very high density amorphous phases. The interstitial neighbors are found to be, at the same time, also distant neighbors along the hydrogen bonded network of the molecules. The role of these interstitial neighbors has been discussed in connection with the interpretation of recent neutron scattering measurements. The structural properties of the systems are characterized by looking at the angular distribution of neighboring molecules, volume and face area distribution of the Voronoi polyhedra, and order parameters. The cumulative analysis of all the corresponding results confirms the assumption that a close similarity between the structural arrangement of molecules in the three explored amorphous phases and that of the ice polymorphs I(h), III, and VI exists.

Thermodynamic model for water and high-pressure ices up to 2.2GPa and down to the metastable domain

We propose a thermodynamic model of the properties of liquid water and ices I, III, V, and VI that can be used in the ranges of 0-2200 MPa and 180-360 K. This model is the first to be applicable to all H(2)O phases in these wide ranges, which exceed the stability domain of all phases. Developing empirical or semiempirical expressions for the specific volumes of liquid water or ices has been necessary. The model has been tested on available experimental data sets. The specific volume of liquid water is reproduced with an accuracy better than 1%. The error on the specific volume of ices remains within 2%. The model has also been used to describe the melting curves of high-pressure ice polymorphs and compared with new Simon equations fitting available data. Our calculations suggest a slight revision of the triple point positions in the H(2)O phase diagram. We have ensured the reliability of our model up to 1.5 GPa, and we have shown that it can be used with good confidence up to 2.2 GPa. In order to show the validity of this model in the low-temperature domains, the melting curve of ice Ih in the water-ammonia system has been modeled. This curve is reproduced with good accuracy down to 180 K, at a 1 bar pressure. It shows that this model can be used in further studies for modeling equilibriums involving liquid or solid phases of H(2)O under pressure and for investigating the effect of inhibitors in complex water-rich systems.

Disorder in ice polymorphs: A Monte Carlo simulation study

Two water molecules connected by hydrogen bond in hexagonal ice can have four possible configurations. These configurations are distinguished by the relative orientation of the two molecules and termed for obvious reasons as c-cis, h-cis, c-trans, and h-trans. The occurrence of symmetry permitted dimer orientations is a characteristic feature of each ice polymorph. In the proton-ordered structures the occurrence of orientations is strictly determined, while in the proton-disordered structures it can vary within certain limits. We performed Monte Carlo simulations using the so-called TIP5P-EW, TIP4P-EW and TIP4P-2005 interaction models to study this isomerism for the polymorphs of ice. We found that the variation of energy with the frequency of different dimer orientations in the proton-disordered phases is large enough to influence the results of phase stability studies. Knowing the distributions of dimer orientations of the ice IX–ice III ordered–disordered polymorph pairs, we could estimate the internal energy of ice IX using dimer energies assigned to certain orientations in the disordered phase of ice III. In agreement with experimental evidences at low temperatures the TIP4P-EW and TIP4P-2005 potentials predicted lower energy for ice VIII than for ice VII.

Structure of coexisting liquid phases of supercooled water: Analogy with ice polymorphs

The structural changes occurring in supercooled liquid water upon moving from one coexisting liquid phase to the other have been investigated by computer simulation using a polarizable interaction potential model. The obtained results favorably compare with recent neutron scattering data of high and low density water. In order to assess the physical origin of the observed structural changes, computer simulation of several ice polymorphs has also been carried out. Our results show that there is a strict analogy between the structure of various disordered (supercooled) and ordered (ice) phases of water, suggesting that the occurrence of several different phases of supercooled water is rooted in the same physical origin that is responsible for ice polymorphism.

Energetics of ice nanotubes and their encapsulation in carbon nanotubes from density-functional theory

We have performed total-energy electronic-structure calculations based on the density-functional theory that clarify atomic and electronic structures of ice nanotubes encapsulated in carbon nanotubes. We have found that three tubular structures of ice are stable in carbon nanotubes. The structural characteristics of these ice nanotubes is a straight stacking or helical arrangement of polygonal units consisting of water molecules. In these stable ice nanotubes, each water molecule is fourfold coordinated with neighboring water molecules through hydrogen bonds. The energy gain upon encapsulation of ice nanotubes in carbon nanotubes is found to be an order of $10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ per water molecule, whereas the energy gain to form ice nanotubes from isolated water molecules is about a half eV per molecule. Carbon nanotubes act as a template to form the new polymorph of ice. Detailed structural analysis has revealed a role of hydrogen-bond angles in determining energetics of the ice nanotubes. We have also clarified that energy gaps of ice nanotubes are comparable with the gap of the most stable ice polymorph Ih and that the electronic structure of ice nanotubes encapsulated in carbon nanotubes is a superposition of the energy bands of the constituents.

Classical interaction model for the water molecule

The authors propose a new classical model for the water molecule. The geometry of the molecule is built on the rigid TIP5P model and has the experimental gas phase dipole moment of water created by four equal point charges. The model preserves its rigidity but the size of the charges increases or decreases following the electric field created by the rest of the molecules. The polarization is expressed by an electric field dependent nonlinear polarization function. The increasing dipole of the molecule slightly increases the size of the water molecule expressed by the oxygen-centered sigma parameter of the Lennard-Jones interaction. After refining the adjustable parameters, the authors performed Monte Carlo simulations to check the ability of the new model in the ice, liquid, and gas phases. They determined the density and internal energy of several ice polymorphs, liquid water, and gaseous water and calculated the heat capacity, the isothermal compressibility, the isobar heat expansion coefficients, and the dielectric constant of ambient water. They also determined the pair-correlation functions of ambient water and calculated the energy of the water dimer. The accuracy of theirs results was satisfactory.

Modelling industrial scale high-pressure-low-temperature processes

The existence of metastable phases of ice I and liquid in the domain of ice III permits the use of optimized paths for pressure shift freezing (PSF) and pressure induced thawing (PIT) processes, crossing the ice III domain, without actual crystallization of this ice polymorph. The use of higher pressures and lower temperatures before pressure release, in the case of PSF, and the use of higher pressures for PIT, increases the real temperature gradients of both processes, therefore reducing the total processing time. In this paper, a computational model has been developed to simulate heat transfer phenomena during the precooling (in PSF) and the heating (in PIT) steps of these optimized processes. The high-pressure-low-temperature (HPLT) processes are assumed to be conducted in a high-pressure tubular reactor designed to work in a semi-continuous mode on an industrial scale. Results showed that important vessel lengths (40–370 m) are needed when the sample velocity ranges from 1 to 6 m/min. These vessel lengths are feasible thanks to the modular structure of the suggested high-pressure system.

Testing the adequacy of simple water models at the opposite ends of the phase diagram

The transferability of a few simple rigid non-polarizable water models were tested by Gibbs Ensemble Monte Carlo simulations to predict their vapor–liquid phase equilibria, and by isothermal–isobaric (Parrinello–Rahman) Monte Carlo simulations of the 13 known crystalline phases of ice. The temperature dependence of the corresponding second virial coefficients was also determined and then used to test the internal consistency of the simulated vapor-phase densities. The model predictions appear satisfactory for liquid water for ambient conditions, but they fail to mimic accurately the properties of the ice polymorphs and the orthobaric vapor phase. The major shortcomings of the models were in the overestimation by a factor of two (∼ 4–6 kJ/mol) of the internal energy difference between the high-pressure ice phases and the hexagonal phase. This unacceptable discrepancy is caused by the parameterization to reproduce the density of liquid water at ambient conditions, that accounts for the significant polarization effects in the condensed phases in terms of augmented dipole moments, with the consequent detrimental effect on the estimations of the vapor-phase properties.

Exploration of NVE classical trajectories as a tool for molecular crystal structure prediction, with tests on ice polymorphs

Following an initial Communication [Buch et al., J. Chem. Phys. 123, 051108 (2005)], a new molecular-dynamics-based approach is explored to search for candidate crystal structures of molecular solids corresponding to minima of the enthalpy. The approach is based on the observation of phase transitions in an artificial periodic system with a small unit cell and relies on the existence of an optimal energy range for observing freezing to low-lying minima in the course of classical trajectories. Tests are carried out for O structures of nine H2O-ice polymorphs. NVE trajectories for a range of preimposed box shapes display freezing to the different crystal polymorphs whenever the box dimensions approximate roughly the appropriate unit cell; the exception is ice II for which freezing requires unit cell dimensions close to the correct ones. In an alternate version of the algorithm, an initial box shape is picked at random and subsequently readjusted at short trajectory intervals by enthalpy minimization. Tests reveal the existence of ice forms which are "difficult" and "easy" to locate in this way. The former include ice IV, which is also difficult to crystallize experimentally from the liquid, and ice II, which does not interface with the liquid in the phase diagram. On the other hand, the latter crystal search procedure located successfully the remaining seven ice polymorphs, including ice V, which corresponds to the most complicated structure of all ice phases, with a monoclinic cell of 28 molecules.

A general purpose model for the condensed phases of water: TIP4P/2005

A potential model intended to be a general purpose model for the condensed phases of water is presented. TIP4P/2005 is a rigid four site model which consists of three fixed point charges and one Lennard-Jones center. The parametrization has been based on a fit of the temperature of maximum density (indirectly estimated from the melting point of hexagonal ice), the stability of several ice polymorphs and other commonly used target quantities. The calculated properties include a variety of thermodynamic properties of the liquid and solid phases, the phase diagram involving condensed phases, properties at melting and vaporization, dielectric constant, pair distribution function, and self-diffusion coefficient. These properties cover a temperature range from 123 to 573 K and pressures up to 40,000 bar. The model gives an impressive performance for this variety of properties and thermodynamic conditions. For example, it gives excellent predictions for the densities at 1 bar with a maximum density at 278 K and an averaged difference with experiment of 7 x 10(-4) g/cm3.

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.

Computational Free Energy Studies of a New Ice Polymorph Which Exhibits Greater Stability than Ice Ih.

The absolute free energies of several ice polymorphs were calculated using thermodynamic integration. These polymorphs are predicted by computer simulations using a variety of common water models to be stable at low pressures. A recently discovered ice polymorph that has as yet only been observed in computer simulations (Ice-i) was determined to be the stable crystalline state for all the water models investigated. Phase diagrams were generated, and phase coexistence lines were determined for all of the known low-pressure ice structures. Additionally, potential truncation was shown to play a role in the resulting shape of the free energy landscape.