Ih Structure Research Articles

High Resolution X-ray Diffraction Studies of the Natural Minerals of Gas Hydrates and Occurrence of Mixed Phases

The types and concentrations of gas hydrates collected from four different geological sites were analyzed using high-resolution angular dispersive X-ray powder diffraction, with synchrotron radiation as the source. Measurements were taken across a temperature range of 80 to 300 K and a pressure range of 0.1 to 80 MPa. All four gas hydrate samples showed the presence of three mixed crystal structures: structures I, II, and H. Additionally, the ice Ih structure was inherently present in all the natural gas hydrate minerals. The variation in the types and concentrations of gas hydrates can be attributed to the diversity of natural gases at different geographic locations.

Porous silicate cement membrane: Unraveling the effect of triethanolamine on ice crystal changing, pore structure, and performance

Triethanolamine (TEA) has been broadly used as the organic amine conditioning agent to modify the workability of cement-based materials. However, the effect of TEA on the structure and property of porous silicate cement membranes has not been reported. In this paper, TEA was applied to study its effect on parameters of porous silicate cement membranes based on the preparation method combining freeze casting and heat-dry curing, because it is easy to form complexes with cement particles. The effect and acting mechanism of TEA on porous silicate cement membranes were researched using a series of characterizations and molecular dynamics (MD) simulation. Results showed that TEA significantly improved the performance of porous silicate cement membranes. The addition of TEA changed the pore morphology of porous silicate cement membranes from lamellar structures to cellular structures, with mesoporous size distribution focusing on 2.5–4.5 nm at TEA doping ratio of 0.16 wt%. The oil-water separation performance could reach up to 99.6 %. Through comprehensive comparison, the porous silicate cement membrane with 1.0 wt% TEA showed better holistic preponderance of oil-water separation and permeation flux. Quantum chemistry calculation and MD simulation were used to briefly explain the influence of TEA on phase transformation. MD showed that adding TEA changed the morphology and structure of ice crystals. The percentage of the Ih structure significantly increased from 53 % to 86 % after adding TEA. This study might expand the application of TEA in other aspects and enlighten relevant workers in new experimental designs.

Coalescing Dynamics between Ag55 and Cu55 Clusters as Well as Thermodynamics during Cooling the Coalesced Clusters from Atomic Simulations.

Molecular dynamics simulations are performed to investigate the coalescing processes between a Cu55 cluster with a liquid, FCC, or Ih structure and a Ag55 cluster in liquid, as well as the structural changes of the coalesced clusters during the cooling process. The simulation results show that the initial structure of the Ag and Cu clusters significantly affects the coalescence stages and the structures after coalescence. There are apparent rotations of the Ag cluster with the liquid structure relative to the Cu cluster with the liquid structure when they are approaching. Before the formation of a neck, the Cu cluster with the Ih structure is more stable and less likely to lose its structure compared to the Cu cluster with the FCC structure. During the cooling process, the coalesced clusters will form different packing structures, including Ih and metastable core/shell structures. The Lode-Nadai values reveal the loading states on the atoms when the two clusters collide. The thermodynamic behaviors during the cooling process were investigated to better understand the order degree of the packing structures and the structural transition processes.

The short-range order in liquid water and amorphous ice

The short-range order in water and ice was determined from experimentally measured partial radial distribution functions by applying the Quasi Crystalline Model (QCM). Partial radial distribution functions were analyzed for water at several pressures and temperatures, crystalline ice, and for the three known phases of amorphous ice: Low-Density Amorphous (LDA), High-Density Amorphous (HDA), and Very-High-Density Amorphous (VHDA). It was found that at low temperatures and pressures, the short-range order of water is similar to that of the hexagonal ice (Ih) structure. At higher pressures and low temperatures, the short-range order of water becomes similar to that of tetragonal ice III structures with a c/a ratio of 0.8. At higher temperatures of 573 K, the short-range order obtained was similar to that of rhombohedral ice II (α = 113°). As for the amorphous ices, we conclude from the QCM analysis that these three forms are structurally distinct with short-range orders corresponding to ice Ih, ice III, and ice II for LDA, HDA, and VHDA ices, respectively.

Freezing mechanism of NaCl solution ultra-confined on surface of calcium-silicate-hydrate: A molecular dynamics study

Cement-based materials in cold regions usually suffer deicer-frost deterioration. To better understand the frozen behavior of the calcium silicate hydrate (C-S-H), molecular dynamics is utilized to investigate the freezing processes of gel surface NaCl solution. The presence of C-S-H substrate significantly reduces the freezing temperature of water molecules ultra-confined on the C-S-H surface, which is 17 K lower than that of bulk water. While majority of random distributed water molecules crystalize to hexagonal ice (Ih) structure, molecules within 0.6 nm of C-S-H substrate cannot form ordered ice crystal at 225 K. The non-icing water layer and lower freezing point are mainly due to oxygen atoms on the silicate chains that provide strong hydrogen bonds with neighboring water and restrict the water orientation. Ionic clusters formed in unfrozen solution are important influence factor for water freezing. Hopefully, this work can provide molecular insights of cement-based materials design in cold regions.

Estimation of the influence of icosahedric "magic" numbers of thermal stability of small silver nanoclusters

A comparative analysis of thermally induced structural transitions in silver nanoclusters, the number of atoms of which corresponded to the "magic" numbers of the icosahedral (Ih) structure with variation of their initial morphology, was carried out by the molecular dynamics method using the modified tight-binding potential TB-SMA. It is shown that, in the case of the initial fcc phase, the formation of the Ih modification, depending on the particle size, occurred either at the stage of preliminary thermal relaxation or during further heating. At the initial amorphous morphology, the nature of the structural transitions underwent significant changes. Thus, even in the case of Ag55 clusters, the icosahedral structure was formed only in 50-60% of the experiments performed. Based on the data obtained, it was concluded that to create a stable Ih structure, it is necessary to use the thermal cycling procedure. Keywords: nanoclusters, silver, computer simulation, "magic" numbers, structure of nanoclusters.

Оценка влияния икосаэдрических "магических" чисел на термическую стабильность малых нанокластеров серебра

A comparative analysis of thermally induced structural transitions in silver nanoclusters, the number of atoms of which corresponded to the “magic” numbers of the icosahedral (Ih) structure with variation of their initial morphology, was carried out by the molecular dynamics method using the modified tight-binding potential TB-SMA. It is shown that, in the case of the initial fcc phase, the formation of the Ih modification, depending on the particle size, occurred either at the stage of preliminary thermal relaxation or during further heating. At the initial amorphous morphology, the nature of the structural transitions underwent significant changes. Thus, even in the case of Ag55 clusters, the icosahedral structure was formed only in 50-60% of the experiments performed. Based on the data obtained, it was concluded that to create a stable Ih structure, it is necessary to use the thermal cycling procedure.

High-pressure phase transition of methane hydrate in water–methane–ammonia system

The phase transition of methane hydrate in water–methane–ammonia system was investigated under pressures up to 20 GPa using synchrotron X-ray powder diffraction (XRD) combined with diamond anvil cells. The XRD experiments revealed that the sI cage structure (MH-I) of methane hydrate transforms into an sH cage structure (MH-II) at approximately 1 GPa, further transforms into a filled-ice Ih structure (MH-III) at approximately 2 GPa, and remains in this structure under pressures up to at least 20 GPa. Ammonia was observed as ammonia hemihydrate phase-II above 3.8 GPa. It is therefore considered that methane hydrate can coexist with aqueous ammonia below 3.8 GPa and coexist with ammonia hemihydrate phase-II above 3.8 GPa. The transition pressures of methane hydrate in the investigated system were consistent with those in water–methane system. These results indicate that, although ammonia is thought to inhibit methane hydrate formation, methane hydrate can be stable in water–methane–ammonia system up to at least 20 GPa and at room temperature. The pressure range in this study covered the pressure conditions inside icy moons, indicating that methane hydrate has a potential to be the main constituent of them.

Crystal Structure Prediction via Basin-Hopping Global Optimization Employing Tiny Periodic Simulation Cells, with Application to Water-Ice.

A crystal structure prediction algorithm for use in periodic boundary conditions with empirical rigid models is presented, which employs (i) unrestricted cutoff radii for the real-space interactions, thus allowing the treatment of even very small unit cells, and (ii) a global-optimization algorithm based on the basin-hopping method of Wales et al. (D. J. Wales and J. P. K. Doye, J. Phys. Chem. A 1997, 101, 5111). The algorithm is then applied to the TIP4P model of water (W. L. Jorgensen et al., J. Chem. Phys. 1983, 79, 926.) in order to find the lowest enthalpy water-ice crystalline structures in the pressure region 0-8000 bar, in unit cells holding in the range of 1-16 molecules, and a database of the 10 lowest enthalpy structures found at pressures 0, 4000, and 8000 bar is presented. The algorithm finds many of the ice polymorphs and, in particular, finds that the lowest energy structure at zero pressure is almost exactly tied between an ice Ic (cubic ice) and ice Ih (hexagonal ice) structure, having near-identical energies.

Ordered Structures in Liquid Water as Studied by Raman Spectroscopy and the Phonon Confinement Model

Abstract Raman spectroscopy and the phonon confinement model (PCM) have been used to study the two ordered structures in liquid water, “structured hydrogen bonded water” and “nano-ice”, whose existence has recently been confirmed by a hyperspectral analysis of 140 temperature dependent Raman spectra (−23 to 45 °C). The PCM spectral simulation based on the ice Ih structure indicates that vibrations are limited within a short range of few molecules in “structured hydrogen bonded water”, while they extend to about a 2–4 nm range in “Nano-ice”.

First principles study of structural and optical properties of [formula omitted] isomers

In this work we undertake a comprehensive numerical study of the ground state structures and optical absorption spectra of isomers of B12 cluster. Geometry optimization was performed at the coupled-cluster-singles-doubles (CCSD) level of theory, employing cc-pVDZ extended basis sets. Once the geometry of a given isomer was optimized, its ground state energy was calculated more accurately at the coupled-cluster-singles-doubles along with perturbative treatment of triples (CCSD(T)) level of theory, employing larger cc-pVTZ basis sets. Thus, our computed values of binding energies of various isomers are expected to be quite accurate. Our geometry optimization reveals eleven distinct isomers, along with their point group, and electronic ground state symmetries. We also performed vibrational frequency analysis on the three lowest energy isomers, and found them to be stable. Therefore, we computed the linear optical absorption spectra of these isomers of B12, employing large-scale multi-reference singles-doubles configuration-interaction (MRSDCI) approach, and found a strong structure-property relationship. This implies that the spectral fingerprints of the geometries can be utilized for optical detection, and characterization, of various isomers of B12. We also explored the stability of the isomer with the structure of a perfect icosahedron, with Ih symmetry. In bulk boron icosahedron is the basic structural unit, but, our vibrational frequency analysis reveals that it is unstable in the isolated form. We speculate that this instability could be due to Jahn-Teller distortion because five-fold degenerate HOMO orbitals in Ih structure are unfilled.

Transition mechanism of sH to filled-ice Ih structure of methane hydrate under fixed pressure condition

The phase transition mechanism of methane hydrate from sH to filled-ice Ih structure was examined using a combination of time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device (CCD) camera observation under fixed pressure conditions. Prior to time-resolved Raman experiments, the typical C-H vibration modes and their pressure dependence of three methane hydrate structures, fluid methane and solid methane were measured using Raman spectroscopy to distinguish the phase transitions of methane hydrates from decomposition to solid methane and ice VI or VII. Experimental results by XRD, Raman spectroscopy and CCD camera observation revealed that the structural transition of sH to filled-ice Ih occurs through a collapse of the sH framework followed by the release of fluid methane that is then gradually incorporated into the filled-ice Ih to reconstruct its structure. These observations suggest that the phase transition of sH to filled-ice Ih takes place by a typical reconstructive mechanism.

Rod structures of bound water: a possible role in self-organization of biological systems and nondissipative energy transmission

Cluster–rod structure were designed, which are comprised of tetrahedral atoms with a typical torsion angle of ~38° at interatomic bonds. These structures correspond to a muscle tissue and clathrin lattice by their metrics and topology and can be formed by bound water in these systems. It is shown that the considered rod structures, which are fragments of bound water structures, can also be involved in nondissipative energy transmission as elastic energy storing structures. The estimated length of the bound water rod structure required to absorb the energy of decomposition of an ATP molecule into ADP and a phosphate group is comparable with myosin head sizes and its step along an actin filament. A mechanism of cooperative transition of the rod structure to a fragment of the ice Ih structure was demonstrated. This transition is accompanied by nondissipative release of stored energy.

Benchmarking a first-principles thermal neutron scattering law for water ice with a diffusion experiment

The neutron scattering properties of water ice are of interest to the nuclear criticality safety community for the transport and storage of nuclear materials in cold environments. The common hexagonal phase ice Ih has locally ordered, but globally disordered, H2 O molecular orientations. A 96-molecule supercell is modeled using the VASP ab initio density functional theory code and PHONON lattice dynamics code to calculate the phonon vibrational spectra of H and O in ice Ih . These spectra are supplied to the LEAPR module of the NJOY2012 nuclear data processing code to generate thermal neutron scattering laws for H and O in ice Ih in the incoherent approximation. The predicted vibrational spectra are optimized to be representative of the globally averaged ice Ih structure by comparing theoretically calculated and experimentally measured total cross sections and inelastic neutron scattering spectra. The resulting scattering kernel is then supplied to the MC21 Monte Carlo transport code to calculate time eigenvalues for the fundamental mode decay in ice cylinders at various temperatures. Results are compared to experimental flux decay measurements for a pulsed-neutron die-away diffusion benchmark.

Phase Transition Dynamics of Three Types of Water within Poly(N,N-dimethylacrylamide) Hydrogels

Water in hydrogels has been classified into three types: bound, intermediate, and free water. To investigate the individual phase transition dynamics for each type of water, differential scanning calorimetic (DSC) curves and Raman spectra of poly(N,N-dimethylacrylamide) hydrogels were measured with heating from 130 to 310 K. Bound and intermediate water showed glassy initial states at 130 K, whereas free water became hexagonal ice (Ih) structure. Intermediate water in glassy state undergoes four phase transition steps: glass-to-liquid transition (at 160–190 K), crystallization from liquid state to cubic ice (Ic) (at 200–230 K), Ic–Ih transition (at 240–250 K), and melting (at 250–273 K). It is concluded that pre-melting of ice, which has been observed in various polymer hydrogels, results from the exothermic Ic–Ih transition of intermediate water.

Pre-resonance-stimulated Raman scattering for water bilayer structure on laser-induced plasma bubble surface.

Pre-resonance-stimulated Raman scattering (PSRS) from water molecules in the air/water interfacial regions was studied when the laser-induced plasma bubble was generated at the interfaces. A characteristically lower Raman shift of OH-stretching vibrational modes of water molecules at around 3000 cm(-1) (370 meV) was observed, in which the mechanisms were possibly attributed to the strong hydrogen bond in a well-ordered water bilayer structure that was formed on a laser-induced plasma bubble surface. Simultaneously, the PSRS of ice Ih at about 3100 cm(-1) was obtained, which also belonged to the strong hydrogen bond effect in ice Ih structure.

Temperature and Doping Effect on Thermal Conductivity of Copper–Gold Icosahedral Bimetallic Nanoclusters and Bulk Structures

Molecular dynamics simulations based on analytic potentials are performed to investigate the coefficient of thermal conductivity (CTC) of gold–copper (Au–Cu) nanoclusters with 55 atoms and icosahedral (Ih) structure at different compositions via a Green–Kubo formalism, and the results are compared with the corresponding quantities for bulk systems. The temperature dependence of CTC is considered for both AuCu nanoclusters and bulk systems in the 40 K < T < 273 K temperature range. For bulk systems, our results are in excellent agreement with the experiment and show that thermal conductivity decreases with temperature in the range of 40 K < T < 273 K, whereas it increases with temperature in the same range for Au–Cu alloys. The dependence of CTC for bulk AuCu on Cu mole fraction at 273 K is investigated, and a plateau is found as a function of copper doping. Heat transfer for pure copper and gold bulk systems occur mostly via a phonon mechanism, whereas for bulk copper–gold alloys a diffusion mechanism is ...

Time-resolved X-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate.

The mechanisms by which methane hydrate transforms from an sI to sH structure and from an sH to filled-ice Ih structure were examined using time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device camera observation under fixed pressure conditions. The XRD data obtained for the sI-sH transition at 0.8 GPa revealed an inverse correlation between sI and sH, suggesting that the sI structure is replaced by sH. Meanwhile, the Raman analysis demonstrated that although the 12-hedra of sI are retained, the 14-hedra are replaced sequentially by additional 12-hedra, modified 12-hedra, and 20-hedra cages of sH. With the sH to filled-ice Ih transition at 1.8 GPa, both the XRD and Raman data showed that this occurs through a sudden collapse of the sH structure and subsequent release of solid and fluid methane that is gradually incorporated into the filled-ice Ih to complete its structure. This therefore represents a typical reconstructive transition mechanism.

Magic-number gold nanoclusters with diameters from 1 to 3.5 nm: relative stability and catalytic activity for CO oxidation.

Relative stability of geometric magic-number gold nanoclusters with high point-group symmetry ((Ih), D(5h), O(h)) and size up to 3.5 nm, as well as structures obtained by global optimization using an empirical potential, is investigated using density functional theory (DFT) calculations. Among high-symmetry nanoclusters, our calculations suggest that from Au(147) to Au(923), the stability follows the order Ih > D(5h) > Oh. However, at the largest size of Au(923), the computed cohesive energy differences among high-symmetry I(h), D(5h) and O(h) isomers are less than 4 meV/atom (at PBE level of theory), suggesting the larger high-symmetry clusters are similar in stability. This conclusion supports a recent experimental demonstration of controlling morphologies of high-symmetry Au(923) clusters ( Plant, S. R.; Cao, L.; Palmer, R. E. J. Am. Chem. Soc. 2014, 136, 7559). Moreover, at and beyond the size of Au(549), the face-centered cubic-(FCC)-based structure appears to be slightly more stable than the Ih structure with comparable size, consistent with experimental observations. Also, for the Au clusters with the size below or near Au(561), reconstructed icosahedral and decahedral clusters with lower symmetry are slightly more stable than the corresponding high-symmetry isomers. Catalytic activities of both high-symmetry and reconstructed I(h)-Au(147) and both Ih-Au(309) clusters are examined. CO adsorption on Au(309) exhibits less sensitivity on the edge and vertex sites compared to Au(147), whereas the CO/O2 coadsorption is still energetically favorable on both gold nanoclusters. Computed activation barriers for CO oxidation are typically around 0.2 eV, suggesting that the gold nanoclusters of ∼ 2 nm in size are highly effective catalysts for CO oxidation.

Phase changes induced by guest orientational ordering of filled ice Ih methane hydrate under high pressure and low temperature

Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under pressure and temperature conditions of 2.0 to 77.0 GPa and 30 to 300 K, respectively, using diamond anvil cells and a helium-refrigeration cryostat. Distinct changes in the axial ratios of the host framework were revealed by In-situ X-ray diffractometry. Splitting in the CH vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules, was observed by Raman spectroscopy. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the axial ratio changes. The results indicated that orientational ordering of the guest methane molecules from orientational disordered-state occurred at high pressures and low temperatures, and that this guest ordering led to the axial ratio changes in the host framework. Existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray data. In addition, above the pressure of the guest-ordered phase, another high pressure phase was developed at a low-temperature region. The deuterated-water host samples were also examined and isotopic effects on the guest ordering and phase changes were observed.