W.S. Howells

Quasielastic neutron scattering of two-dimensional water in a vermiculite clay

A well-characterized Na–vermiculite clay, containing zero, one, or two molecular layers of water between the clay platelets, has been studied by quasielastic neutron scattering (QENS). Experiments were carried out at a temperature of 300 K in two different scattering geometries; the clay platelets being at 45 and 135° angles to the incident beam in order to make the elastic Q-vector perpendicular and parallel, respectively, to the clay platelets for a scattering angle of 90° (Q≈1.33 A−1). The resulting QENS spectra show that almost no hydrogen motion occurs perpendicular to the clay platelets on the experimental time scale (about 2–40 ps). The two-H2O layer vermiculite exhibits a planar rotational motion of water molecules, forming hydration shells around the Na ions, and a basically two-dimensional translational jump-diffusion motion. The translational motion was modeled using the Gaussian jump-length distribution model, resulting in a mean jump length of 1.1 A and an average residence time of 2.3 ps. Us...

Dynamics of water in a molecular sieve by quasielastic neutron scattering

We have investigated the dynamics of water confined in a molecular sieve, with a cylindrical pore diameter of 10 A, by means of quasielastic neutron scattering (QENS). Both the incoherent and coherent intermediate scattering functions I(Q,t) were determined by time-of-flight QENS and the neutron spin-echo technique, respectively. The results show that I(Q,t) is considerably more stretched in time with a slightly larger average relaxation time in the case of coherent scattering. From the Q dependence of I(Q,t) it is clear that the observed dynamics is almost of an ordinary translational nature. A comparison with previous dielectric measurements suggests a possible merging of the alpha and beta relaxations of the confined water at T=185 K, although the alpha relaxation cannot be directly observed at lower temperatures due to the severe confinement. The present results are discussed in relation to previous results for water confined in a Na-vermiculite clay, where the average relaxation time from spin-echo measurements was found to be slower than in the present system (particularly at low temperatures).

Collective excitations in liquid methanol: A comparison of molecular, lattice‐dynamics, and neutron‐scattering results

The collective dynamics of liquid methanol‐d4 is studied by means of molecular‐dynamics simulation. The model potential is validated by means of lattice energy calculations and shows a very good agreement with the experimentally obtained crystal structure. Center‐of‐mass density and momentum fluctuations are investigated in the (Q,ω) region which is also accessible to inelastic neutron‐scattering (INS) techniques. A simple viscoelastic model previously used for the analysis of INS data is tested against the dynamic structure factor computed from the simulation. A direct comparison with the INS results themselves is also made and qualitative agreement is found. Also, a tentative assignment of the peaks appearing in the current–current correlations is made on the basis of lattice‐dynamics calculations for the polycrystalline low‐temperature α phase.

An atomic-scale study of the role of titanium in TiO2:SiO2 sol-gel materials

Abstract Neutron diffraction data is used to reveal details of the effect of adding titania to a silica-based sol-gel, including a first direct observation of the TiO correlation. At low Ti contents the data are consistent with the 4-fold oxygen coordination derived from NMR. Even at these concentrations the silica network is shown to have been affected significantly, with bond-length distributions being narrowed, and with a high level of network-OH indicating a qualitative change in the mesoscopic structure associated with interfacial surfaces. Data on a phase separated sample supports the conclusion that the network-OH is associated with the phase separation process.

A spectroscopic study of the structure of amorphous hydrogenated carbon

A range of amorphous hydrogenated carbon (a-C:H) samples have been studied using inelastic neutron spectroscopy (INS) and Fourier transform infrared (FTIR) spectroscopy. Using these complementary techniques, the bonding environments of both carbon and hydrogen can be probed in some detail, with the INS data providing not only qualitative but also quantitative information. By comparing the data from each of the samples we have been able to examine the effects of different deposition conditions, i.e. precursor gas, deposition energy and deposition method, on the atomic-scale structure of a-C:H.

The effect of temperature on the structure of amorphous hydrogenated carbon

The results of a neutron diffraction study on the structure of amorphous hydrogenated carbon a‐C:H are presented up to a maximum temperature of 1000 °C. The data show clearly the effect on atomic correlations of elevated temperatures, with the initial room‐temperature amorphous network (a mixture of single bonds and olefinic double bonds) becoming progressively aromatic, then graphitic as hydrogen is evolved. Complementary x‐ray diffraction and infrared spectroscopy data are also presented, the infrared data enabling a more detailed discussion of the temperature‐dependent hydrogen environment, and the x‐ray data are used to highlight the change in the carbon network. Comparisons have been made with previous work on similar systems and a brief summary of these results is given.

The structural properties of liquid and quenched sulphur II

Neutron diffraction experiments over the momentum transfer range 0<Q(AA)-1<40 have been used to investigate the structural properties of liquid sulphur from 140 degrees C up to 300 degrees C, i.e. from conditions near the melting point of sulphur up to temperatures well above its lambda -transition (Tlambda =159 degrees C), and to study how the structure of sulphur rapidly quenched from different liquid temperatures changes in relation to the corresponding liquid structure. The data have been Fourier transformed to give an accurate pair correlation function g(r) from which absolute numbers and distances of atoms in given molecular configurations may be obtained. The results indicate which molecular bonds are changed in the different solid and liquid states and shed new light on the lambda -transition from a molecular point of view. In particular we show that the number of nearest neighbours probably increases with temperature above the lambda transition. The data on g(r) are also interpreted with recent reverse Monte Carlo calculations.

A neutron diffraction study of amorphous boron using a pulsed source

The structure of amorphous boron has been investigated with pulsed neutron diffraction techniques using the ISIS facilities at the Rutherford Appleton Laboratory. The experimental static structure factor S(Q) and derived radial distribution function support a model based on building blocks of B12 icosahedra resembling those found in β-rhombohedral boron, but with disorder occuring in the linking between these subunits.

Bayesian analysis of quasielastic neutron scattering data

We consider the analysis of quasielastic neutron scattering data from a Bayesian point-of-view. This enables us to use probability theory to assess how many quasielastic components there is most evidence for in the data, as well as providing an optimal estimate of their parameters. We review the theory briefly, describe an efficient algorithm for its implementation and illustrate its use with both simulated and real data.

Polymer dynamics in 3PEG–LiClO4–TiO2 nanocomposite polymer electrolytes

The microscopic polymer dynamics in nanocomposite polymer electrolytes has been investigated using quasielastic neutron scattering experiments. Four samples based on the fully amorphous copolymer trihydroxy poly(ethylene oxide-co-propylene oxide) (3PEG) were investigated: 3PEG, 3PEG 1.5 mol/kg LiClO4, 3PEG 1.5 mol/kg LiClO4 10 wt % TiO2, and 3PEG 1.5 mol/kg LiClO4 20 wt % TiO2. In addition to a slow relaxation giving rise to elastic scattering, at least two dynamical processes were observed: a fast, local chain motion and a slower diffusive segmental motion. No changes of the quasielastic peak widths could be observed between the filled or unfilled composites; however, the elastic scattering of the sample was found to increase upon addition of the filler to the polymer electrolyte. The results indicate the presence of a roughly 5-nm-thick immobilized polymer layer (∼5 vol %) around filler particles, while the dynamics of the bulk polymer (∼95 vol %) is not influenced by the filler. The results imply that ...