Structural properties of the amorphous ices: an analysis in terms of distance-ranked neighbors and angular correlations

In this work we study through computer simulations the three known forms of amorphous ice, namely, the low density (LDA), the high density (HDA), and the very high density (VHDA) amorphous ices, at various pressures and temperatures. Adopting the TIP4P model, we are able to reproduce these three forms by following experiment-like procedures. Those systems are characterized from thermodynamic and structural points of view, in particular through an insightful analysis of the behavior of the second-shell neighbors in the various stages of the simulations. The distance-ranked neighbors are found to be the relevant markers of the differentiation mechanisms of the three forms, since their response to pressure induces specific distortions of the orientational correlations. We show that LDA, HDA, and VHDA are disordered forms whose local structures tend to a single tetrahedral network, to an ice-VII-like arrangement, and to a random-close-packed simple liquid-like structure, respectively. Despite the major structural deformations induced by pressure, the hydrogen bonds are still present in each of those forms, even though deformations of the tetrahedral angles are necessary to adapt to the denser structures.     

Electronic structures and hydrogen bond network of high-density and very high-density amorphous ices

Electronic structures of hexagonal ice (ice Ih), high-density amorphous ice (HDA), and very high-density amorphous ice (VHDA) are investigated using ab initio density functional theory (DFT) at 77 K under a pressure of 0.1 MPa, focusing on band structure, density of states (DOS), partial density of states (PDOS), and electron density. It is found that the integration intensity of the O-2p bonding band in HDA is 1.53 eV wider than that in the VHDA. Because more 2p electrons in HDA participate the 2p-1s hybridization of O-H. The classical molecular dynamics (MD) method has further been carried out to analyze the hydrogen bond network of HDA and VHDA with larger numbers of water molecules under the same temperature, pressure, and boundary conditions used as those during the DFT calculation. MD results show that there exists some water molecules with five hydrogen bonds in both HDA (4.1 +/- 0.1%) and VHDA (2.8 +/- 0.1%), as compared with the LDA, being consistent with the integration intensity results of PDOS. This result can be used to interpret the physical nature of the similar transition temperature of HDA and VHDA to LDA with different heating rates.     

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.     

Fifth-order two-dimensional Raman spectroscopy of liquid water, crystalline ice Ih and amorphous ices: sensitivity to anharmonic dynamics and local hydrogen bond network structure

The theoretical study of off-resonant fifth-order two-dimensional (2D)-Raman spectroscopy is made to analyze the intermolecular dynamics of liquid and solid water. The 2D-Raman spectroscopy is susceptible to the nonlinear anharmonic dynamics and local hydrogen bond structure in water. It is found that the distinct 2D-Raman response appears as the negative signal near the t(2) axis. The origin of this negative signal for t(2)<15 fs is from the nonlinear polarizability in the librational motions, whereas that for 30 fs相似文献   

Evolution of the structure of amorphous ice: from low-density amorphous through high-density amorphous to very high-density amorphous ice

We report results of molecular dynamics simulations of amorphous ice for pressures up to 22.5 kbar. The high-density amorphous ice (HDA) as prepared by pressure-induced amorphization of I(h) ice at T=80 K is annealed to T=170 K at various pressures to allow for relaxation. Upon increase of pressure, relaxed amorphous ice undergoes a pronounced change of structure, ranging from the low-density amorphous ice at p=0, through a continuum of HDA states to the limiting very high-density amorphous ice (VHDA) regime above 10 kbar. The main part of the overall structural change takes place within the HDA megabasin, which includes a variety of structures with quite different local and medium-range order as well as network topology and spans a broad range of densities. The VHDA represents the limit to densification by adapting the hydrogen-bonded network topology, without creating interpenetrating networks. The connection between structure and metastability of various forms upon decompression and heating is studied and discussed. We also discuss the analogy with amorphous and crystalline silica. Finally, some conclusions concerning the relation between amorphous ice and supercooled water are drawn.     

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.     

Regions of stability for LDA,HDA, and VHDA amorphous ices

Model structures have been created for ice Ih and for low density (LDA), high density (HDA), and very high density (VHDA) amorphous ices using the procedure for determining the equilibrium configurations of molecules in amorphous phases. The chemical potentials of these ices were calculated for wide ranges of temperature and pressure. The curves of equilibrium phase transitions have been plotted. It is shown that at high pressures, VHDA ice is more stable than Ih, while HDA ice is metastable with respect to VHDA over the whole range of pressures and temperatures. These calculations provide an explanation to the experimentally observed transition of HDA into a higher density phase (VHDA) during isobaric heating.     

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.     

The structure and crystallization of thin water films on Pt(111)

When water is adsorbed on Pt(111) above 135 K several different ice structures crystallize, depending on the thickness of the ice layer. At low coverage water forms extended islands of ice with a (square root(37) x square root(37))R25(o) unit cell, which compresses as the monolayer saturates to form a (square root(39) x square root(39))R16(o) structure. The square root(39) low-energy electron diffraction (LEED) pattern becomes more intense as the second layer grows, remaining bright for films up of 10-15 layers and then fading and disappearing for films more than ca. 40 layers thick. The ice multilayer consists of an ordered square root(39) wetting layer, on which ice grows as a crystalline film which progressively loses its registry to the wetting layer. Ice films more than ca. 50 layers thick develop a hexagonal LEED pattern, the entire film and wetting layer reorienting to form an incommensurate bulk ice. These changes are reflected in the vibrational spectra which show changes in line shape and intensity associated with the different ice structures. Thin amorphous solid water films crystallize to form the same phases observed during growth, implying that these structures are thermodynamically stable and not kinetic phases formed during growth. The change from a square root(39) registry to incommensurate bulk ice at ca. 50 layers is associated with a change in crystallization kinetics from nucleation at the Pt(111) interface in thin films to nucleation of incommensurate bulk ice in amorphous solid water films more than 50 layers thick.     

Thermal transport coefficients for liquid and glassy water computed from a harmonic aqueous glass

We compute thermal transport coefficients for liquid and glassy water in terms of the vibrations of the quenched liquid. The thermal conductivity and thermal diffusivity are computed for H(2)O and D(2)O at densities from 0.93 to 1.2 g cm(-3). The computed thermal diffusivity of liquid water is in reasonable agreement with measured values and is found to increase with increasing temperature due largely to the thermal accessibility of delocalized librational modes. The influence of structure and density on the thermal conductivity of amorphous ices is investigated. The calculations reveal that density alone is unable to explain the measured thermal conductivity of amorphous ices, particularly low-density amorphous ices, for which the thermal conductivity decreases with increasing temperature near 100 K. To investigate the influence of structure on thermal transport in amorphous ices we have computed the thermal transport coefficients for low-density amorphous ices prepared in two different ways, one formed by quenching the liquid at 0.93 g cm(-3) and the other by distortion of cubic ice at the same density. The computed thermal conductivity of the latter is higher, but the structures of both forms are too disordered for the thermal conductivity to exhibit the unusual variation observed experimentally.     

Radiation effects in water ice: a near-edge x-ray absorption fine structure study

The changes in the structure and composition of vapor-deposited ice films irradiated at 20 K with soft x-ray photons (3-900 eV) and their subsequent evolution with temperatures between 20 and 150 K have been investigated by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K edge. We observe the hydroxyl OH, the atomic oxygen O, and the hydroperoxyl HO(2) radicals, as well as the oxygen O(2) and hydrogen peroxide H(2)O(2) molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (I(cryst)) ice films. The evolution of their concentrations with the temperature indicates that HO(2), O(2), and H(2)O(2) result from a simple step reaction fuelled by OH, where O(2) is a product of HO(2) and HO(2) a product of H(2)O(2). The local order of ice is also modified, whatever the initial structure is. The crystalline ice I(cryst) becomes amorphous. The high-density amorphous phase (I(a)h) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amorphous ice (I(a)l). The phase I(a)h is thus peculiar to irradiated ice and does not exist in the as-deposited ice films. A new "very high density" amorphous phase-we call I(a)vh-is obtained after warming at 50 K the irradiated p-ASW ice. This phase is stable up to 90 K and partially transforms into crystalline ice at 150 K.     

High-pressure gas hydrates

It has long been known that crystalline hydrates are formed by many simple gases that do not interact strongly with water, and in most cases the gas molecules or atoms occupy 'cages' formed by a framework of water molecules. The majority of these gas hydrates adopt one of two cubic cage structures and are called clathrate hydrates. Notable exceptions are hydrogen and helium which form 'exotic' hydrates with structures based on ice structures, rather than clathrate hydrates, even at low pressures. Clathrate hydrates have been extensively studied because they occur widely in nature, have important industrial applications, and provide insight into water-guest hydrophobic interactions. Until recently, the expectation-based on calculations-had been that all clathrate hydrates were dissociated into ice and gas by the application of pressures of 1 GPa or so. However, over the past five years, studies have shown that this view is incorrect. Instead, all the systems so far studied undergo structural rearrangement to other, new types of hydrate structure that remain stable to much higher pressures than had been thought possible. In this paper we review work on gas hydrates at pressures above 0.5 GPa, identify common trends in transformations and structures, and note areas of uncertainty where further work is needed.     

导致冰面极低摩擦系数的原因:研究进展及模型分析

低摩擦系数是冰的重要特性。人们将冰面的低摩擦系数归因于冰表面存在液态水。本文首先综述了液态水产生的三大假说,通过计算证明压力融解和摩擦融解并非冰面具有低摩擦系数的主要原因。通过分析前人结果,进一步明确冰表面存在液态水并非冰面低摩擦系数的唯一原因。提出在冰的表面存在由冰晶和液态水构成的过渡层结构的新模型,指出过渡层中小冰晶在受力切线方向上的水平滑动或者滚动,以及液态水的润滑都是冰面低摩擦系数的原因。     

High density amorphous ices: disordered water towards close packing

The structure of amorphous ice under pressure has been studied by molecular dynamics at 160 K. The starting low-density phase undergoes significant changes as the density increases, and at rho=1.51 g/cm(3) our calculated g(OO)(r) is in excellent agreement with in situ neutron diffraction data obtained at 1.8 GPa and 100 K on very high density amorphous ice made at 150 K. As the system is further compressed, in the theoretical simulations, up to rho=1.90 g/cm(3), the structural modifications are continuous up to the highest density. The analysis of orientational distributions reveals that dense amorphous ice is characterized by major distortions of the tetrahedral geometry, and that the pressure structural changes, already observed experimentally at lower densities, can be interpreted as a trend towards a disordered closed-packed structure.     

How many amorphous ices are there?

Many acronyms are used in the literature for describing different kinds of amorphous ice, mainly because many different preparation routes and many different sample histories need to be distinguished. We here introduce these amorphous ices and discuss the question of how many of these forms are of relevance in the context of polyamorphism. We employ the criterion of reversible transitions between amorphous "states" in finite intervals of pressure and temperature to discriminate between independent metastable amorphous "states" and between "substates" of the same amorphous "state". We argue that the experimental evidence suggests we should consider there to be three polyamorphic "states" of ice, namely low-(LDA), high-(HDA) and very high-density amorphous ice (VHDA). In addition to the realization of reversible transitions between them, they differ in terms of their properties, e.g., compressibility, or number of "interstitial" water molecules. Thus they cannot be regarded as structurally relaxed variants of each other and so we suggest considering them as three distinct megabasins in an energy landscape visualization.     

Pressure amorphized ices--an atomistic perspective

We offer our viewpoint on the nature of amorphous ices produced by pressurization of crystalline ice Ih and the inter-relationship between them from an atomistic perspective. We argue that the transformation of high density amorphous (HDA) ice from crystalline ice is due to a mechanical process arising from the instability of the ice Ih structure. The densification of HDA upon thermal annealing under pressure is a relaxation process. The conversion of the densified amorphous ice to a lower density form (LDA) upon the release of pressure can be attributed to a similar process. It is speculated that amorphous ices are metastable frustrated structures due to the large activation barriers associated with proton reorientation in the formation of the underlying stable crystalline ice polymorphs.     

Negative thermal expansion of water in hydrophobic nanospaces

The density and intermolecular structure of water in carbon micropores (w = 1.36 nm) are investigated by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements between 20 K and 298 K. The SAXS results suggest that the density of the water in the micropores increased with increasing temperature over a wide temperature range (20-277 K). The density changed by 10%, which is comparable to the density change of 7% between bulk ice (I(c)) at 20 K and water at 277 K. The results of XRD at low temperatures (less than 200 K) show that the water forms the cubic ice (I(c)) structure, although its peak shape and radial distribution functions changed continuously to those of a liquid-like structure with increasing temperature. The SAXS and XRD results both showed that the water in the hydrophobic nanospaces had no phase transition point. The continuous structural change from ice I(c) to liquid with increasing temperature suggests that water shows negative thermal expansion over a wide temperature range in hydrophobic nanospaces. The combination of XRD and SAXS measurements makes it possible to describe confined systems in nanospaces with intermolecular structure and density of adsorbed molecular assemblies.     

限制在疏水板中的新的六边形-四边形三层冰结构

对限制在两个光滑的疏水板间的水进行了分子动力学模拟,观察到了两种晶体结构,都满足冰规则．在1GPa的压强和1．0nm的板间距下获得的新的冰相是平坦的六边形-四边形三层冰．在此结构中,靠近板的两层（外层）中的水分子形成六边形环,中间层的水分子形成四边形环．对于外层的水分子,其四个氢键中的三个在同一层中,另一个氢键与中间层连接．对于中间层的水分子,四个氢键中的两个在同。层中,而另外两个氢键与两个不同的外层相连．虽然三层的形状不同,但其面密度却接近相等．另一种结构是在0．8nm的板问距和100MPa的侧向压下获得的平坦的六边形双层冰．模拟中的相变既有一阶相变,也有连续相变．     

Reflection absorption infrared spectroscopy and temperature-programmed desorption studies of the adsorption and desorption of amorphous and crystalline water on a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two- and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.     

The glass transition behaviors of low-density amorphous ice films with different thicknesses

The glass transition behaviors of amorphous ice with different thicknesses are studied by determining the heat capacity of low-density amorphous ice without crystallization using first principle molecular dynamics (FP-MD) and classical MD methods. The behaviors are also studied by analyzing hydrogen-bond network, the radial distribution functions, and relationship between hydrogen bond and electronic structures. It is found that the glass transition temperature (T(g)) in the range of 90 K < T < 100 K for 4 nm amorphous ice film by FP-MD method, and 120 K < T(g) < 130 K for 8 nm amorphous ice film by MD method. Meanwhile, T(g) decreases with the decreasing thickness of amorphous ice film, which is also validated by the theoretical model.