Neutron scattering studies of structural transformations and vibrational spectra of ice after high pressure treatment

Abstract

The transitions of the recovered high-pressure phase ice VIII first to high-density amorphous (hda) and low-density amorphous ices, and finally to cubic Ic, and hexagonal Ih ice were observed at heating using real-time neutron diffraction. Inelastic incoherent neutron scattering measurements on the hdu ice, ice Ih and high-pressure phase ice VI revealed similarity between the amorphous phase and crystalline ice VI and led to the new proposition that hda ice consists of two interpenetrating hydrogen-bounded networks with no hydrogen bonds between “sublattices”.     

Vibrational spectra of high- and low-density amorphous ice,ice Ic,and ice Ih by neutron scattering

Abstract

The vibrational spectra of high- and low-density amorphous ice made ice Ic and Ice Ih have been obtained at 15 K using inelastic incoherent neutron scattering     

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

Lattice dynamical calculations have been carried out for ice II based on the force field constructed for ice Ih. In order to fully understand ice II inelastic neutron scattering spectra, the decomposed phonon density of states was shown mode by mode. Calculated results have shown that the hydrogen bond force constant between the six-molecule rings is significantly weaker, 75eV/nm², compared with the force constant, 220eV/nm², within the rings. Inelastic neutron scattering spectra of clathrate hydrate H₂O+He are almost the same as ice II. This means that the absorption of He atoms cannot affect the bond strengths of the ice II host lattice. Based on the force field model for ice II, the van der Waals interactions between water molecules and helium atoms are considered. The results obtained are consistent with experimental data. Lattice dynamical calculations have been carried out for ice II using seven rigid pairwise potentials. It was found that MCY makes the stretching and bending interactions in ice II too weak and makes the O-O bond length too long (～5%), thus its lattice densities are obviously lower than other potential lattices or experimental values.     

IR spectroscopic study of surface properties of amorphous water ice

The state of the surface of amorphous ice with a specific surface area of about 160 m²/g obtained by the condensation of water vapor at 77 K is studied by IR spectroscopy. As the temperature increases to 130–160 K, absorption bands of surface hydroxyl groups vanish, whereas changes in bands characteristic of hydroxyl groups in the bulk of ice are indicative of a phase transition of ice from amorphous to the polycrystalline structure. The surface sites of amorphous ice are characterized with low-temperature adsorption of carbon monoxide. It is shown that there are two types of CO adsorption sites, free hydroxyl groups and oxygen atoms of surface coordinately unsaturated water molecules. Upon adsorption of nitrogen, methane, and carbon monoxide, in addition to the perturbation of surface OH groups, reversible changes in the spectrum are observed in the region of vibrations of bulk hydroxyls, which indicate that the strength of hydrogen bonds between water molecules in the surface layer of icy particles increases approaching the strength of these bonds in the crystal and that the ice surface becomes less amorphous. These results indicate that the properties of the ice surface layer substantially depend on the presence of adsorbed molecules.     

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.     

Vibrational states of glassy and crystalline orthoterphenyl

Low-frequency vibrations of glassy and crystalline orthoterphenyl are studied by means of neutron scattering. Phonon dispersions are measured along the main axes of a single crystal, and the corresponding longitudinal and transversal sound velocities are obtained. For glassy and polycrystalline samples, a density of vibrational states is determined and cross-checked against other dynamic observables. In the crystal, low-lying zone-boundary modes lead to an excess over the Debye density of states. In the glass, the boson peak is located at even lower frequencies. With increasing temperature, both glass and crystal show anharmonicity. Received 7 December 1999     

The O–H stretching band in ice Ih derived via eV neutron spectroscopy on VESUVIO using the new very low angle detector bank

Strong demand exists for an experimental facility enabling new experimental investigations on condensed matter systems based on epithermal neutron scattering at high energy and low momentum transfers. This need will be met by the very low angle detector (VLAD) bank, to be installed on the VESUVIO spectrometer at the ISIS spallation neutron source. The equipment will operate in the scattering angular range 1°<2θ<5°. Scattering measurements from a polycrystalline ice sample using a VLAD prototype demonstrates the effectiveness of the detection technique adopted for the construction of the full detector array. The resulting density of states in ice is 9±2 atoms/cell, in agreement with previous measurements. PACS 61.12.Ex; 63.20.Dj; 63.50.+x     

Range of recoil atoms in isotropic stopping materials

A calculation of the stopping power, range, and range straggling of low energetic atoms (E?1 MeV) is reported. The computation applied the biatomic repulsive potential of Firsov and thus had to be carried out numerically. The results, however, are presented in an analytical form. — Experimental checks were performed with recoil atoms. 96 keV Ra-224 atoms from the α-decay of Th-228 were used to measure the range distributions in several light gases. — The fast neutron induced reactions, such as (n, p)-or (n, α)-reactions, led to recoils of higher energies (up to more than 1 MeV). Their mean ranges in polycrystalline metals or amorphous oxide layers could be obtained by measuring the fraction of radioactive recoils which had left the target surface. — The experimental data are in good agreement with the theory, except at the highest energies where the observed ranges are slightly larger than predicted.     

Origin of low-frequency local vibrational modes in high density amorphous ice

Incoherent-inelastic neutron scattering data are obtained from 5-80 K for high-density amorphous (hda) ice in the region 0-135 cm(-1). An excess contribution to the vibrational density of states is identified near 20 cm(-1). The origin of these vibrations has been identified by lattice dynamics calculations on an "experimental" structure derived from reverse Monte Carlo analysis of hda ice neutron diffraction data. An interpretation that localized oscillations of short chains and isolated water molecules are responsible for the excess low-frequency modes is consistent with our data.     

Ab initio study of the structure and dynamical properties of crystalline ice

We investigated the structural and dynamical properties of a tetrahedrally coordinated crystalline ice from first principles based on density functional theory within the generalized gradient approximation with the projected augmented wave method. First, we report the structural behaviour of ice at finite temperatures based on the analysis of radial distribution functions obtained by molecular dynamics simulations. The results show how the ordering of the hydrogen bonding breaks down in the tetrahedral network of ice with entropy increase, in agreement with the neutron diffraction data. We also calculated the phonon spectra of ice in a 3× 1× 1 supercell using the direct method. So far, due to the direct method used in this calculation, the phonon spectra are obtained without taking into account the effect of polarization arising from dipole–dipole interactions of water molecules, which is expected to yield the splitting of longitudinal and transverse optic modes at the Γ point. The calculated longitudinal acoustic velocities from the initial slopes of the acoustic mode are in reasonable agreement with the neutron scattering data. Analysis of the vibrational density of states shows the existence of a boson peak at low energy of the translational region, a characteristic common to amorphous systems.     

Stress sensitivity coefficient in polycrystalline and amorphous ferromagnetic materials below room temperature

Summary The piezomagnetic coupling coefficient (the ?stress sensitivity?) is quantitatively investigated below room temperature and a comparison is made among polycrystalline samples and amorphous ones. The technique used is an original one. It experimentally consists in the measurement of the amplitude of magnetoelastic waves, their resonance frequency and differential susceptibility; theoretically, it consists in coupling Newton’s law of force with the piezomagnetic state equations. The results clearly show different responses of polycrystalline samples with respect to the amorphous ones; in particular the nonmonotonic behaviour of the ?stress sensitivity? for metallic glasses is interesting for establishing their actual magnetic states at low temperature.     

A structural comparison of supercooled water and intermediate density amorphous ices

New data are presented on neutron diffraction in ultrapure bulk supercooled heavy water measured down to 262?K. The data are analysed in terms of the trends observed in the first sharp diffraction peak (FSDP) parameters, the feature which dominates the measured neutron spectra. The neutron FSDP position, height and width are compared to literature data for supercooled water, water under pressure and to the same parameters obtained for recently discovered intermediate density amorphous ices. It is found that the FSDP parameters in supercooled water and the amorphous ices generally exhibit a similar behaviour, suggesting a new structural regime may occur in deeply supercooled water below Q ₀?～?1.83?Å^?1 (T?～?251?K) associated with increased intermediate range ordering. It is argued that this structural regime may be linked to a similar trend in the density which appears when the density is plotted as a function of FSDP position. A detailed comparison of the neutron and X-ray structure factors for supercooled water and intermediate density amorphous ices with the same FSDP positions is also made. The diffraction data show that although the overall general structures are qualitatively very similar, the amorphous ice correlations are considerably sharper and extend to much higher radial distances.     

The role of material microstructure in the magnetic behavior of amorphous and polycrystalline Co<Subscript>x</Subscript>Si<Subscript>1-x</Subscript> lines

The role of material microstructure in the magnetic anisotropy of real nanostructures has been studied by the comparison of the magnetic behavior of arrays of amorphous and polycrystalline Co_xSi_1-x lines. From the experimental measurements of angular dependences of remanences parallel and perpendicular to the applied field we determine the angular dispersion of effective local easy axis of anisotropy. We have proved that amorphous lines have a dispersion of effective anisotropy axis much smaller than the polycrystalline samples. As a consequence, amorphous lines have a better defined magnetic behaviour, pointing the interest of the fabrication of nanostructures made of amorphous materials.     

Relation between the Grüneisen constant and Poisson’s ratio of vitreous systems

The lattice Grüneisen constant is calculated for amorphous polymers and glasses by using the data on Poisson’s ratio. While the thermodynamic Grüneisen constant of vitreous polymers characterizes the anharmonicity averaged over intrachain and other vibration modes, the lattice Grüneisen constant describes the anharmonicity of interchain vibrations associated with intermolecular interaction. For alkali-silicate glasses, the lattice Grüneisen constant characterizes the anharmonicity of vibrations of the ionic sublattice, which is formed by alkali metal ions and nonbridge oxygen ions.     

Fingerprints of amorphous icelike behavior in the vibrational density of states of protein hydration water

The low-frequency modes of protein hydration water are investigated by inelastic neutron scattering. Experiments on both protonated and fully deuterated maltose binding protein samples allow us to unambiguously single out the contribution from water. The low-energy vibrational density of states of hydration water at 100 K is similar to the density of states of high- and low-density amorphous ice, and quite different from that of simple forms of crystalline ice. This result can be related to the picture of hydration water mass density depending on the protein surface curvature, which supports its glassy behavior.     

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.     

Morphological transformations of vanadium oxide films during low-temperature reduction in hydrogen electron cyclotron resonance plasma

Morphological transformations of amorphous vanadium oxide films obtained by the sol-gel method and polycrystalline V₂O₅ films are studied during their low-temperature (295–623 K) reduction in a hydrogen electron cyclotron resonance plasma. The morphology of films is analyzed using atomic force microscopy and high-resolution electron microscopy. It is found that a homogeneous amorphous film during the reduction process transforms to an island film and then bulk amorphous islands of a regular shape appear. These islands resemble microcrystals, and their concentration depends on the temperature and the reduction time. The low-temperature reduction of polycrystalline V₂O₅ films leads to their amorphization; however, the microcrystals in the polycrystalline film do not change their shape in this process. A mechanism of the reduction process is proposed. This mechanism explains the regularities of morphological transformations in amorphous sol-gel films of vanadium oxides based on the suggestion of a competition between the ion-stimulated nucleation and growth of nuclei of the crystalline phase and the amorphization of the growing nuclei.     

Impurity lattice dynamics of Fe in Cr from precision Mössbauer fraction measurements

Precision 〈x²〉-values and their anharmonicity been measured for ⁵⁷Fe in Cr between 80 K and 600 K. At 294 K. $\text{? = 0.798 ± 0.005}$. A theoretical analysis based upon Cr neutron scattering data yields 0.8 for the impurity/host force constant ratio.     

Temperature dependence of the kinetic energy in the Zr<Subscript>40</Subscript>Be<Subscript>60</Subscript> amorphous alloy

The average kinetic energy 〈E(T)〉 of the atomic nucleus for each element of the amorphous alloy Zr₄₀Be₆₀ in the temperature range 10–300 K has been measured for the first time using VESUVIO spectrometer (ISIS). The experimental values of 〈E(T)〉 have been compared to the partial ZrBe spectra refined by a recursion method based on the data obtained with thermal neutron scattering. The satisfactory agreement has been reached with the calculations using partial spectra based on thermal neutron spectra obtained with recursion method. In addition, the experimental data have been compared to the Debye model. The measurements at different temperatures (10, 200, and 300 K) will provide an opportunity to evaluate the significance of anharmonicity in the dynamics of metallic glasses.     

Anharmonic effects on lattice resonant modes of impure metallic crystals

The effect of lattice anharmonicity on the resonant modes of dilute impurities for Au-Cu, Au-Ag and W-Cr metallic systems are studied from the poles of the double time temperature dependent impure Green’s function of these crystals. The third order force constants used in the present work are derived assuming the systems to obey the Lennard-Jones potential. The inclusion of lattice anharmonicity has been found to increase the resonant frequency which depends upon the mass defect, force constant change parameter and the impurity concentration terms. Some interesting features about the phase shift and the change in width of the vibrational spectrum are reported at room temperature for the isotopic defects and the defects interacting with host atoms. The results are found to be in qualitative agreement with the experiments on the systems considered.