Phonon dispersion of ice under pressure

We report measurements of the phonon dispersion of ice Ih under hydrostatic pressure up to 0.5 GPa, at 140 K, using inelastic neutron scattering. They reveal a pronounced softening of various low-energy modes, in particular, those of the transverse acoustic phonon branch in the [100] direction and polarization in the hexagonal plane. We demonstrate with the aid of a lattice dynamical model that these anomalous features in the phonon dispersion are at the origin of the negative thermal expansion (NTE) coefficient in ice below 60 K. Moreover, extrapolation to higher pressures shows that the mode frequencies responsible for the NTE approach zero at approximately 2.5 GPa, which explains the known pressure-induced amorphization (PIA) in ice. These results give the first clear experimental evidence that PIA in ice is due to a lattice instability, i.e., mechanical melting.     

Experimental evidence for a crossover between two distinct mechanisms of amorphization in ice Ih under pressure

We report neutron scattering data which reveal the central role of phonon softening leading to a negative melting line, solid-state amorphization, and negative thermal expansion of ice. We find that pressure-induced amorphization is due to mechanical melting at low temperatures, while at higher temperatures amorphization is governed by thermal melting (violations of Born's and Lindemann's criteria, respectively). This confirms earlier conjectures of a crossover between two distinct amorphization mechanisms and provides a natural explanation for the strong annealing observed in high-density amorphous ice.     

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.     

Correlated Tunneling in Hydrogen Bonds

We study the quantum nature of the protons participating in hydrogen bonds in several ice structures by analyzing the one particle density matrix. We find that in all cases, including ice Ih, the most common form of ice, and the high pressure phases, ice VIII, VII, and X, the system is ground-state dominated. However, while the dynamics is uncorrelated in the structures with standard asymmetric hydrogen bonds, such as ice Ih and VIII, local correlations among the protons characterize ice VII and, to a lesser extent, ice X in the so-called low barrier hydrogen bond regime. The correlations appear along the path to hydrogen bond symmetrization, when quantum fluctuations delocalize the proton on the two bond sides. The correlations derive from a strong requirement for local charge neutrality that favors concerted motion along the bonds. The resulting behavior deviates substantially from mean field theory, which would predict in ice VII coherent tunneling of the proton between the two bond sides, thereby causing an ionization catastrophe. Due to the correlations, the quantum state of the proton is entangled.     

利用拉曼光谱分析冰Ih相的表面薄层的氢键结构

水是生活中一种最基本且最重要的物质,由于它的一些奇特性质和反常物性,得到了广泛的研究,而拉曼光谱是研究水分子结构的一种非常合适的方法,它通过获得分子的振动和转动信息来理解分子结构和分子间的相互作用。在常压下测量了-20～-190 ℃温度范围内冰Ih相的表面薄层的拉曼光谱,实验结果发现随温度降低,冰Ih相的O∶H范德瓦尔斯键向高波数方向移动,而O-H极性共价键向低波数方向移动;且拉曼频移与温度呈线性关系,通过对不同振动模式的斜率进行比较,判断其键长的伸缩变化关系,从而证明了冰Ih相密度随温度的减小而增大,采用氢键理论(结构)给予了解释。同时,发现在-150 ℃时,O-H键反对称伸缩振动模式和O∶H键振动模式的拉曼峰强发生了突变,这表明冰Ih相发生了相变--冰Ⅺ相(冰Ih的质子有序相)。     

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.     

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.     

Theoretical model of ice nucleation induced by inertial acoustic cavitation. Part 2: Number of ice nuclei generated by a single bubble

In the preceding paper (part 1), the pressure and temperature fields close to a bubble undergoing inertial acoustic cavitation were presented. It was shown that extremely high liquid water pressures but quite moderate temperatures were attained near the bubble wall just after the collapse providing the necessary conditions for ice nucleation. In this paper (part 2), the nucleation rate and the nuclei number generated by a single collapsing bubble were determined. The calculations were performed for different driving acoustic pressures, liquid ambient temperatures and bubble initial radius. An optimal acoustic pressure range and a nucleation temperature threshold as function of bubble radius were determined. The capability of moderate power ultrasound to trigger ice nucleation at low undercooling level and for a wide distribution of bubble sizes has thus been assessed on the theoretical ground.     

Pressure amorphization through displacive disorder

After classifying amorphous materials according to their topology, we review a recently proposed theory of pressure amorphization (PA) that arises from some degree of displacive disorder while retaining a crystalline topology. That theory is based on the notion that one or more branches of the phonon spectrum become soft and flat with increasing pressure and is illustrated by a simple model that possesses the range of features displayed by many of the materials which undergo PA with displacive disorder. We report the results of Langevin simulations of the simple model which show how the probability of amorphization increases with the number of unit cells in the system and support our theory. We comment on how to generalize the model for the study of real systems. Received 29 march 2002     

Nature and stability of ice X

A study of pressure effects on the vibrational frequencies of ice X is performed to shed light on the existence and stability of this phase. The analysis reveals: (i) its stability range, (ii) the soft phonon nature of the transition at high pressures to the Pbcm structure proposed by Benoit et al. (M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, New high-pressure phase of ice, Phys. Rev. Lett. 76 (1996), pp. 2934–2936.) and (iii) the phonon collapse associated with the dynamical disordered structure at low pressures. Additionally, a topological analysis of the electron localization function and the electron density through the atoms-in-molecules formalism clarifies the chemical nature of ice X.     

Dynamical instabilities of ice X

Water ice X is stable in the 120-400 GPa pressure range, as obtained from lattice dynamical calculations performed using the density-functional theory. Below 120 GPa, it is characterized by one unstable flat phonon band, which generates the disordered ice X structure. Above 400 GPa, ice X has an unstable phonon mode in M, which leads to the Pbcm orthorhombic structure obtained in previous molecular-dynamics calculations [M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, Phys. Rev. Lett. 76, 2934 (1996)10.1103/PhysRevLett.76.2934]. Therefore, based on lattice dynamics, we propose that the high-pressure low-temperature phase-transition sequence in H2O ice is ice VIII-disordered ice X-ordered ice X-ice Pbcm.     

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.     

Structure and energetics of molecular point defects in ice Ih

We present a first-principles study of the molecular vacancy and three distinct molecular interstitial structures in ice Ih. The results indicate that, due to its bonding to the surrounding hydrogen-bond network, the bond-center (Bc) configuration is the favored molecular interstitial in ice Ih. A comparison between the vacancy and the Bc interstitial suggests that the former is the predominant molecular point defect for T approximately < 200K although a crossover scenario in which the latter becomes favored below the melting point is conceivable.     

高压下有序晶态合金Fe3Pt的低能声子不稳定性及磁性反常

有序晶态Fe₃Pt因瓦合金处于一种特殊的磁临界状态, 这种磁临界状态下体系的晶格动力学稳定性对压力极为敏感. 基于密度泛函理论的第一性原理的投影缀加平面波方法研究了不同晶态合金的Fe₃Pt的焓和磁性随压力的变化规律, 结果表明, 在压力小于18.54 GPa下, P4/mbm结构是热力学稳定的相. Pm3m结构、I4/mmm结构、DO₂₂结构的Fe₃Pt在铁磁性坍塌临界压力附近体系的总磁矩急剧下降并具有振荡现象, 且I4/mmm结构和DO₂₂结构的Fe₃Pt 在临界压力附近出现了Fe1原子磁矩反转现象. 在43 GPa下, DO₂₂结构的Fe₃Pt出现了亚铁磁微观磁特性突然增强且伴随着体积突然增大的现象. 在高压下, 对Pm3m结构Fe₃Pt的晶格动力学计算表明, 压力小于26.95 GPa的铁磁态下体系的自发磁化诱导了体系横向声学支声子软化, 表明体系中存在很强的自发体积磁致伸缩. 特别是在铁磁性坍塌临界压力41.9 GPa至磁性完全消失的57.25 GPa压力区间, 晶格动力学稳定性对压力更加敏感. 压力大于57.25 GPa时, 压力诱导了体系声子谱的稳定.     

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.     

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.     

New observations on hydrogen bonding in ice by density functional theory simulations

In this paper, we report on a series of computational simulations on hydrogen bonding in two ice phases （Ih and Ic） using CASTEP with PW91 and RPBE exchange-correlation based on ab initio density functional theory. The strength of the H-bond is correlated with intramolecular O-H stretching, and the energy splitting exists for both the H-bond and covalent O-H stretching. By analyzing the dispersion relationship of to（q）, we observe the separation of the longitudinal optic （LO） mode from transverse optic （TO） mode at the gamma point, seemingly interpreting the controversial two H-bond peaks in the vibrational spectrum of ice recorded by inelastic incoherent neutron scattering experiments. The test of ambient environment on phonon density of sates （PDOS） shows that the relaxed tetrahedral structure is the most stable structural configuration for water clusters.     

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.     

Collapse pressure measurement of single hollow glass microsphere using single-beam acoustic tweezer

Microbubbles are widely used in medical ultrasound imaging and drug delivery. Many studies have attempted to quantify the collapse pressure of microbubbles using methods that vary depending on the type and population of bubbles and the frequency band of the ultrasound. However, accurate measurement of collapse pressure is difficult as a result of non-acoustic pressure factors generated by physical and chemical reactions such as dissolution, cavitation, and interaction between bubbles. In this study, we developed a method for accurately measuring collapse pressure using only ultrasound pulse acoustic pressure. Under the proposed method, the collapse pressure of a single hollow glass microsphere (HGM) is measured using a high-frequency (20–40 MHz) single-beam acoustic tweezer (SBAT), thereby eliminating the influence of additional factors. Based on these measurements, the collapse pressure is derived as a function of the HGM size using the microspheres’ true density. We also developed a method for estimating high-frequency acoustic pressure, whose measurement using current hydrophone equipment is complicated by limitations in the size of the active aperture. By recording the transmit voltage at the moment of collapse and referencing it against the corresponding pressure, it is possible to estimate the acoustic pressure at the given transmit condition. These results of this study suggest a method for quantifying high-frequency acoustic pressure, provide a potential reference for the characterization of bubble collapse pressure, and demonstrate the potential use of acoustic tweezers as a tool for measuring the elastic properties of particles/cells.     

Phonons and their dispersion in model ferroelastics Hg2Hal2

Dispersion relations of the acoustic and optical phonon frequencies have been calculated and plotted, and the density of states of the phonon spectrum of Hg₂Cl₂ and Hg₂Br₂ crystals has been derived. The effect of hydrostatic pressure on the frequencies of acoustic and optical phonons and their dispersion has been theoretically analyzed. It has been found that an increase in the pressure leads to a strong softening of the slowest acoustic TA branch (the soft mode) at the X point of the Brillouin zone boundary, which is consistent with the phenomenological Landau theory and correlates with experiment.