Toshiya Otomo

Large-angle X-ray scattering, small-angle neutron scattering, and NMR relaxation studies on mixing states of 1,4-dioxane-water, 1,3-dioxane-water, and tetrahydrofuran-water mixtures

Abstract Aqueous mixtures of cycloethers, 1,4-dioxane (pdio), 1,3-dioxane (mdio), and tetrahydrofuran (THF) have been investigated by using large-angle X-ray scattering (LAXS), small-angle neutron scattering (SANS), and NMR relaxation techniques. The results from the LAXS experiments have revealed that at a microscopic level the structures of the cycloether-water mixtures change with cycloether mole fraction xC (C = pdio, mdio and THF) in a similar way: in the range of xC 1,3-dioxane-water ⪢ 1,4-dioxane-water mixtures. The NMR relaxation data have revealed that the rotational motion of water molecules is most restricted at xC ≈ 0.3, suggesting that the hydrogen bonds among water molecules are most strengthened at this mole fraction. On the basis of the static structures from the microscopic to mesoscopic levels and the dynamic property of water molecules, the effects of the size of cycloether molecules and the position of oxygen atoms in the molecules on the mixing states of the mixtures are discussed.

Structure and dynamics of hexafluoroisopropanol-water mixtures by x-ray diffraction, small-angle neutron scattering, NMR spectroscopy, and mass spectrometry

The structure and dynamic properties of aqueous mixtures of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) have been investigated over the whole range of HFIP mole fraction (xHFIP) by large-angle x-ray scattering (LAXS), small-angle reutron scattering (SANS), 19F-, 13C-, and 17O-NMR chemical shifts, 17O-NMR relaxation, and mass spectrometry. The LAXS data have shown that structural transition of solvent clusters takes place at xHFIP∼0.1 from the tetrahedral-like hydrogen bonded network of water at xHFIP⩽∼0.1 to the structure of neat HFIP gradually formed with increasing HFIP concentration in the range of xHFIP⩾0.15. The Ornstein–Zernike plots of the SANS data have revealed a mesoscopic structural feature that the concentration fluctuations become largest at xHFIP∼0.06 with a correlation length of ∼9 A, i.e., maximum in clustering and microhetrogeneities. The 19F and 13C chemical shifts of both CF3 and CH groups of HFIP against xHFIP have shown an inflection point at xHFIP∼0.08, implying that the environment of ...

Intermediate range structure and low-energy dynamics of densified vitreous silica

Abstract The structure and low-energy dynamics of densified vitreous silica are studied with high accuracy by neutron scattering over a wide range of momenta and energy space. The structure factors (S(Q)) indicate that the change of the SiO4 tetrahedron (the short range order) by compaction is very small, but the intermediate range structure (IMRS) represented by the first sharp diffraction peak is strongly affected. This mainly attributed to a deformation of the sixfold ring structure. The low-energy dynamics the boson peak, BP also has a strong correlation with the change of IMRS. A reduction of the intensity of the boson peak by densification, however, cannot be explained by simple models such as phonon dumping or an eigenmode of IMRS. A shrinkage of the void space, surrounded by the sixfold ring structure, is related to the suppression of the low-energy dynamics. We consider that the origin of the boson peak can be attributed to a soft mode composed of an additional degree of freedom such as the void space.

Peculiar suppression of the specific heat and boson peak intensity of densified SiO2 glass

Abstract Low-energy dynamics of normal and densified vitreous silica was investigated by neutron scattering combined with the specific heat measurements. A large evolution of the Boson peak was clarified to be a suppression of the density of states at low energy, but not a peak shift as observed by Raman scattering. These results are inconsistent with a scenario based on a phonon scattering by a local density fluctuation, but consistent with soft potential model and a recent model by Nakayama.

Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4–xHx

Compositionally tunable vanadium oxyhydrides Sr2VO(4-x)H(x) (0 ≤ x ≤ 1.01) without considerable anion vacancy were synthesized by high-pressure solid-state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands.

Mass-fractal clustering and power-law decay of cluster size in 1-propanol aqueous solution

Mesoscale structure of 1-propanol aqueous solutions with propanol mole fraction xp ranging from 0.1 to 0.33 has been studied by means of small angle neutron scattering (SANS) and large-scale reverse Monte Carlo (RMC) technique. Analysis of the SANS intensities in terms of a fractal model shows that the fractal dimension df of mesoscale structure of the solution is about 1.8-1.9 for water-rich solution and about 1.5 for propanol-rich solution. Percolation analysis on the RMC results reveals that the water molecules and the propanol molecules cluster, respectively, as a mass fractal, the dimension dM of which is about 2.3-2.5 for both clusters for water-rich solution. Furthermore, the distribution of the cluster size is expressed by a simple power law with an exponent tau of about 1.35-1.5 for the propanol clusters and 1.05-1.2 for the water clusters. These results imply that the current solution is characterized by polydisperse mass fractals. In fact, a theoretical relation for polydisperse system of mass fractals, df = dM(2-tau), holds well in the current solution. The characteristic change in df from 1.8-1.9 to 1.5 described above is attributed to the crossover between the water-rich regime and the propanol-rich regime. Most of the water molecules and the propanol molecules are located on the interface between clusters, and the water molecules form thin layers of about 10 A thick irrespective of 1-propanol content studied.

Inelasticity Effect on Neutron Scattering Intensities of the Null-H2O

Time-of-flight (TOF) neutron scattering measurements have been carried out for liquid null-H2O, in which the average coherent scattering length of hydrogen atoms is zero. In order to determine the inelasticity effect depending on both the scattering angle and the neutron flight path ratio, γ [ = l_{s}/(l_{0} + l_{s}), l 0 and l s denote the moderator-sample and sample-detector distances, respectively], neutron scattering measurements have been performed using three neutron spectrometers, HIT-II, RAT, and SWAN, installed at KENS, Tsukuba, Japan. The self-scattering intensity for the null-H2O was derived by subtracting the known O–O partial structure factor from the observed scattering cross-section. It has been revealed that the magnitude of the inelasticity distortion involved in the self-scattering term is still significant even at a smaller scattering angle than that expected from the first-order inelasticity correction formulas proposed in the literature. The inelasticity distortion in the self-scatter...

Neutron production from lead targets for 12-GeV protons

Abstract We carried out an experiment on neutron yields from typical cylindrical lead targets for 12-GeV protons by means of the Mn-bath moderation method. Our experimental results are, roughly speaking, consistent with those by Russian groups done for lower energy protons by different methods. The number of neutrons absorbed in the Mn-bath was determined to be 195 neutrons/proton, and the total neutron yield, including leakage neutrons, which were evaluated by calculations, was estimated to be 201 neutrons/proton for a 20cm (diameter) × 60cm (length) lead target. The total neutron yield as a function of the proton energy Ep, fitted to our data and the results of others, is expressed by the formula 2.0 + 29.2Ep 0.78, where Ep is expressed in GeV.

Neutron Scattering Studies on Spin-Peierls Material CuGeO3

Crystal structure and phonon dynamics up to 150 meV of the spin-Peierls material CuGeO 3 have been investigated by neutron scattering experiments. We found a large lattice anomaly in the a-b plane, vertical to the one-dimensional antiferromagnetic spin chain, associated with additional diffraction peaks in the b * -axis at temperatures much higher than the spin-Peierls transition temperature. The phonon modes of the chain oxygens at 100 meV show consistent evolution with diffraction. Therefore, we conclude that the structural evolution of the chain oxygens in the a-b plane, which mediates the spin-spin interaction along the c -axis, drives the spin-Peierls transition.

Dispersive excitation in different forms of SiO2

Abstract The dynamical structure factors of vitreous silica (v-SiO2), polycrystalline α-cristobalite and polycrystalline α-quartz are measured by inelastic neutron scattering. Clear dispersive behavior extending up to 55 meV is observed for all samples, irrespective of structural order. Especially, the similarity of dynamical behavior between v-SiO2 and polycrystalline α-cristobalite is found to be extremely remarkable in the range of E⩾7 meV. We consider that the essential difference between a glassy system and a crystalline one should be in the low-energy region below the Boson peak energy. The quantum excitation between a two-level system based on buckling motion may contribute to the anomalous low-energy excitation in glasses.