Structure of a new dense amorphous ice

The detailed structure of a new dense amorphous ice, VHDA, is determined by isotope substitution neutron diffraction. Its structure is characterized by a doubled occupancy of the stabilizing interstitial location that was found in high density amorphous ice, HDA. As would be expected for a thermally activated unlocking of the stabilizing "interstitial," the transition from VHDA to LDA (low-density amorphous ice) is very sharp. Although its higher density makes VHDA a better candidate than HDA for a physical manifestation of the second putative liquid phase of water, as for the HDA case, the VHDA to LDA transition also appears to be kinetically controlled.     

X-ray Raman spectroscopic study of water in the condensed phases

Oxygen K-edge x-ray absorption spectra of high-density amorphous (HDA) ice, low-density amorphous ice Ic, ice Ih, normal and deuterated liquid water were measured with the synchrotron x-ray Raman scattering method under almost identical experimental conditions by in situ heating of an HDA ice sample. The distinct preedge structure previously reported in water was observed in all the spectra. The results show that core-hole excitations are localized and not strongly affected by the local environment. Therefore, the existence of the preedge feature is not a concise indicator of the magnitude of local disorder within the hydrogen bonded network. The intensity of the near-edge absorption shifts into the postedge region when the hydrogen bond network becomes more ordered. This observation is interpreted as an enhancement of Wannier over Frenkel excitations in an ordered crystal.     

Nature of the polyamorphic transition in ice under pressure

We present a neutron diffraction study of the transition between low-density and high-density amorphous ice (LDA and HDA, respectively) under pressure at approximately 0.3 GPa, at 130 K. All the intermediate diffraction patterns can be accurately decomposed into a linear combination of the patterns of pure LDA and HDA. This progressive transformation of one distinct phase to another, with phase coexistence at constant pressure and temperature, gives direct evidence of a classical first-order transition. In situ Raman measurements and visual observation of the reverse transition strongly support these conclusions, which have implications for models of water and the proposed second critical point in the undercooled region of liquid water.     

Amorphous ice: stepwise formation of very-high-density amorphous ice from low-density amorphous ice at 125 K

On compressing low-density amorphous ice (LDA) at 125 K up to 1.6 GPa, two distinct density steps accompanied by heat evolution are observable in pressure-density curves. Samples recovered to 77 K and 1 bar after the first and second steps show the x-ray diffraction pattern of high-density amorphous ice (HDA) and very HDA (VHDA), respectively. The compression of the once formed HDA takes place linearly in density up to 0.95 GPa, where nonlinear densification and HDA --> VHDA conversion is initiated. This implies a stepwise formation process LDA--> HDA --> VHDA at 125 K, which is to the best of our knowledge the first observation of a stepwise amorphous-amorphous-amorphous transformation sequence. We infer that the relation of HDA and VHDA is very similar to the relation between LDA and HDA except for a higher activation barrier between the former. We discuss the two options of thermodynamic versus kinetic origin of the phenomenon.     

Polyamorphism of ice at low temperatures from constant-pressure simulations

We report results of molecular dynamics simulations of amorphous ice in the pressure range 0-22.5 kbar. The high-density amorphous (HDA) ice prepared by compression of Ih ice at T=80 K is annealed to T=170 K at intermediate pressures in order to generate relaxed states. We confirm the existence of recently observed phenomena, the very high-density amorphous ice, and a continuum of HDA forms. We suggest that both phenomena have their origin in the evolution of the network topology of the annealed HDA phase with decreasing volume, resulting at low temperatures in the metastability of a range of densities.     

Crystal-like high frequency phonons in the amorphous phases of solid water

The high frequency dynamics of low-density (LDA) and high-density (HDA) amorphous ice and of cubic ice ( I(c)) has been measured by inelastic x-ray scattering in the 1-15 nm(-1) momentum transfer ( Q) range. Sharp phononlike excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8 nm(-1) in LDA and I(c) and up to 5 nm(-1) in HDA. The narrow width of these excitations is in sharp contrast to the broad features observed in all amorphous systems studied so far. The "crystal-like" behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to soundlike excitations which is interpreted as a sign of low local disorder.     

Relation between the high density phase and the very-high density phase of amorphous solid water

It has been suggested that high-density amorphous (HDA) ice is a structurally arrested form of high-density liquid (HDL) water, while low-density amorphous ice is a structurally arrested form of low-density liquid (LDL) water. Recent experiments and simulations have been interpreted to support the possibility of a second distinct high-density structural state, named very high-density amorphous (VHDA) ice, questioning the LDL-HDL hypothesis. We test this interpretation using extensive computer simulations and find that VHDA is a more stable form of HDA and that, in fact, VHDA should be considered as the amorphous ice of the quenched HDL.     

In situ Raman and optical microscopy of the relaxation behavior of amorphous ices under pressure

The transformation of low‐density amorphous (LDA) ice produced from high‐density amorphous (HDA) ice was studied up to 400 MPa as a function of temperature by in situ Raman spectroscopy and optical microscopy. Changes in these amorphous states of H₂O were directly tracked without using emulsions to just above the crystallization temperature T_x. The spectra show significant changes occurring above ∼125 K. The results are compared with data reported for the relaxation behavior of HDA, to form what we call relaxed HDA, or rHDA. We find a close connection with expanded HDA (eHDA), which is reported to exist as another metastable form in this P–T region. The observation of this temperature‐induced LDA transition under pressure complements the previously observed pressure‐induced reversible transition between LDA and HDA at 120–140 K. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface energy and surface proton order of ice Ih

Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disorder of the hydrogen atoms in the hydrogen bonded network of the lattice. Here we arrive at a first principles determination of the surface energy of ice Ih and suggest that the surface of ice is significantly more proton ordered than the bulk. We predict that the proton order-disorder transition, which occurs in the bulk at approximately 72 K, will not occur at the surface at any temperature below surface melting. An order parameter which defines the surface energy of ice Ih surfaces is also identified.     

Structural differences between unannealed and expanded high-density amorphous ice based on isotope substitution neutron diffraction

We here report isotope substitution neutron diffraction experiments on two variants of high-density amorphous ice (HDA): its unannealed form prepared via pressure-induced amorphization of hexagonal ice at 77?K, and its expanded form prepared via decompression of very-high density amorphous ice at 140?K. The latter is about 17?K more stable thermally, so that it can be heated beyond its glass-to-liquid transition to the ultraviscous liquid form at ambient pressure. The structural origin for this large thermal difference and the possibility to reach the deeply supercooled liquid state has not yet been understood. Here we reveal that the origin for this difference is found in the intermediate range structure, beyond about 3.6 Å. The hydration shell markedly differs at about 6 Å. The local order, by contrast, including the first as well as the interstitial space between first and second shell is very similar for both. ‘eHDA’ that is decompressed to 0.20?GPa instead of 0.07?GPa is here revealed to be rather far away from well-relaxed eHDA. Instead it turns out to be roughly halfway between VHDA and eHDA – stressing the importance for decompressing VHDA to at least 0.10?GPa to make an eHDA sample of good quality.     

The Effects of Charge Transfer Interactions on the Properties of Ice Ih

Potential models which include charge transfer are used to study ice/water coexistence properties and properties of the ice Ih phase. Two charge transfer models are used, one which is non-polarizable and one which is polarizable. These models transfer a discreet amount of charge for each hydrogen bond made and the net charge of a molecule is determined by the difference in the number of hydrogen bonds a molecule makes as a donor and as an acceptor. In ice Ih, this difference is very near zero and the net amount of charge transfer is correspondingly essentially zero. This differs from the amount of charge transfer in the liquid phase. The results for the polarizable charge transfer model confirm other studies that suggest the importance of polarizability in reproducing the high dielectric constant of ice Ih.     

Ground-state properties of crystalline ice from periodic hartree-fock calculations and a coupled-cluster-based many-body decomposition of the correlation energy

Ground state properties of crystalline ice Ih are investigated by combining periodic Hartree-Fock calculations with a many-body expansion for the electron correlation energy using second-order many-body perturbation theory and coupled-cluster techniques. Very good agreement with experimental data can already be achieved by considering two-body correlation contributions up to the third coordination shell in crystalline ice. This hints at the possibility to accurately simulate ab initio water by using periodic Hartree-Fock calculations together with a parametrized two-body correlation potential.     

Polyamorphism in water

Water, the most common and important liquid, has peculiar properties like the density maximum at 4 °C. Such properties are thought to stem from complex changes in the bonding-network structure of water molecules. And yet we cannot understand water. The discovery of the high-density amorphous ice (HDA) in 1984 and the discovery of the apparently discontinuous change in volume of amorphous ice in 1985 indicated experimentally clearly the existence of two kinds of disordered structure (polyamorphism) in a one-component condensed-matter system. This fact has changed our viewpoint concerning water and provided a basis for a new explanation; when cooled under pressure, water would separate into two liquids. The peculiar properties of water would be explained by the existence of the separation point: the liquid-liquid critical point (LLCP). Presently, accumulating evidences support this hypothesis. Here, I describe the process of my experimental studies from the discovery of HDA to the search for LLCP together with my thoughts which induced these experiments.     

Anomalous behavior of proton zero point motion in water confined in carbon nanotubes

The momentum distribution of the protons in ice Ih, ice VI, high density amorphous ice, and water in carbon nanotubes has been measured using deep inelastic neutron scattering. We find that at 5 K the kinetic energy of the protons is 35 meV less than that in ice Ih at the same temperature, and the high momentum tail of the distribution, characteristic of the molecular covalent bond, is not present. We observe a phase transition between 230 and 268 K to a phase that does resemble ice Ih. Although there is yet no model for water that explains the low temperature momentum distribution, our data reveal that the protons in the hydrogen bonds are coherently delocalized and that the low temperature phase is a qualitatively new phase of ice.     

Slow amorphization of zeolites

The dynamics of amorphization in two zeolites with different densities is investigated using high-pressure Raman spectroscopy. Slow amorphization of the denser zeolite under pressure leads to the formation of a low-density amorphous (LDA) phase that transforms into a more disordered high-density amorphous (HDA) phase with a further increase in the pressure. It is revealed that the LDA-HDA transformation is a first order phase transition occurring with an increase in the silicon coordination.     

Phase diagram of water under an applied electric field

Simulations are used to investigate for the first time the anisotropy of the dielectric response and the effects of an applied electric field E(ex) on the phase diagram of water. In the presence of electric fields ice II disappears from the phase diagram. When E(ex) is applied in the direction perpendicular to the ac crystallographic plane the melting temperatures of ices III and V increase whereas that of ice Ih is hardly affected. Ice III also disappears as a stable phase when E(ex) is applied in the direction perpendicular to the ab plane. E(ex) increases by a small amount the critical temperature and reduces slightly the temperature of the maximum density of liquid water. The presence E(ex) modifies all phase transitions of water but its effect on solid-solid and solid-fluid transitions seems to be more important and different depending on the direction of E(ex).     

NEUTRON SCATTERING AND LATTICE DYNAMICAL STUDIES OF THE HIGH-PRESSURE PHASE ICE (I)

Lattice dynamical calculations of ice VIII have been carried out by using a slightly modified set of force constants obtained recently for ice Ih (Li J C and Ross D K 1993 Nature 365 327). A weak interaction was introduced between the two interpenetrated sublattices in the ice VIII structure. The calculated results for H₂O and D₂O ice VIII are in reasonable agreement with the measured inelastic neutron scattering spectra. The eigenvectors of phonon modes in the range of translational and librational bands have been studied in order to understand the properties of the vibrational modes. It is found that the third peak at 26.7meV in the translation results from weak hydrogen bond interactions, and the first peak (14.7meV) is much higher than it is in ice Ih (～7.1meV), which is partially due to the interactions between the two sublattices.     

Theoretical investigation of structures,compositions, and phase transitions of neon hydrates based on ices Ih and II

Thermodynamic conditions of existence in the p-T plane and the composition of neon hydrates based on ices Ih and II are determined. The occupancy of neon in cages (channels) of ices Ih and II at temperatures below 0°C is calculated. It is shown that the occupancy of neon in hydrate based on ices cages decreases with growing temperature. Lines of monovariant equilibria between gas phase (neon)-neon hydrate based on ice Ih-liquid water (or ice II) and neon-gas phase (neon)-hydrate based on ice II-liquidwater (or ice II) are found. The line of divariant equilibria between neon hydrate based on ice Ih-neon hydrate based on ice II has been also calculated. The possibility of ice stabilization due to inclusion of neon into ice cages (channels) is shown.     

Polyamorphism in silicon nanocrystals under pressure

Many works have been devoted to describing mechanisms of pressure-induced polyamorphism. This phenomenon is apparent in the phase transition between low- and high-density amorphous states (LDA and HDA) upon the application of pressure, resulting in substantial changes in the structure and physical properties of the amorphous state. The HDA–LDA transition in Si nanocrystals is observed when recording Raman spectra in situ during decompression at 6.68 GPa.     

Dielectric relaxation of low-density amorphous ice under pressure

In situ studies at high pressures show that the dielectric relaxation time tau of low-density amorphous (LDA) ice is more than an order of magnitude longer than that of high density amorphous ice. The increase in tau at the transformation to LDA ice with a simultaneous large density decrease shows that, despite an increase in the average intermolecular distance, the structural change leads to restriction for the orientational diffusion of H2O. The origin is most likely the same as in ice I, i.e., due to the ice rules. This result further stresses the crystallinelike nature of LDA ice.