Quasi-elastic Neutron Scattering Signal Research Articles

Rotation of complex ions with ninefold hydrogen coordination studied by quasielastic neutron scattering and first-principles molecular dynamics calculations

Quasielastic neutron scattering (QENS) and neutron powder diffraction of the complex transition metal hydrides ${\mathrm{Li}}_{5}{\mathrm{MoH}}_{11}$ and ${\mathrm{Li}}_{6}{\mathrm{NbH}}_{11}$ were measured in a temperature range of 10--300 K to study their structures and dynamics, especially the dynamics of the hydrogen atoms. These hydrides contain unusual ninefold H-coordinated complex ions (${\mathrm{MoH}}_{9}^{3\ensuremath{-}}$ or ${\mathrm{NbH}}_{9}^{4\ensuremath{-}}$) and hydride ions $({\mathrm{H}}^{\ensuremath{-}})$. A QENS signal appeared >150 K due to the relaxation of H atoms. The intermediate scattering functions derived from the QENS spectra are well fitted by a stretched exponential function called the Kohlrausch-Williams-Watts functions with a small stretching exponent \ensuremath{\beta} \ensuremath{\approx} 0.3--0.4, suggesting a wide relaxation time distribution. The $Q$ dependence of the elastic incoherent structure factor is reproduced by the rotational diffusion of $M{\mathrm{H}}_{9}$ ($M=\mathrm{Mo}$ or Nb) anions. The results are well supported by a van Hove analysis for the motion of H atoms obtained using first-principles molecular dynamics calculations. We conclude that the wide relaxation time distribution of the $M{\mathrm{H}}_{9}$ rotation is due to the positional disorder of the surrounding Li ions and a unique rotation with $M{\mathrm{H}}_{9}$ anion deformation (pseudorotation).

Water Mobility in Reverse Micelles Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulation

AbstractReverse micelles (RMs) are aggregates in which nanoscale droplets of a polar liquid, usually water, are surrounded by a surfactant layer in a nonpolar continuous phase. They are widely used as media for reactions in which the extent of confinement or the presence of a surfactant interface play a central role. We have used molecular dynamics (MD) computer simulation and quasielastic neutron scattering (QENS) and to investigate the mobility of water molecules in reverse micelles. The contribution of water to the QENS signal is enhanced by deuterating the surfactant and the nonpolar phase. Our studies of water mobility have focused on the effects of water pool size, determined by the water/surfactant mole ratios w0, as well as on the properties of the water-surfactant interface. Specifically, we have examined the effects of varying w0 and of substituting other alkali ions for the usual Na+ counterion of the anionic surfactant AOT (bis (2-ethylhexyl) sulfosuccinate)). We find good agreement between the QENS signal and its prediction from MD simulation. This allows us to obtain additional insight into water mobility by analyzing the MD self-intermediate scattering function (ISF) of water hydrogens in terms of contributions from molecular rotation and translation and from molecules in different interfacial layers. MD data indicate that the translational ISF decays nonexponentially due to lower water mobility close to the interface and to confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay. However, rotational mobility of O-H bond vectors in the interfacial region remains fairly high due to the lower density of water-water hydrogen bonds in the vicinity of the interface.

Investigation of the State of Water in Hydrating Layered Sodium Disilicate in Crystalline and Amorphous Forms by Quasi-Elastic Neutron Scattering

The structure and state of water within hydrating crystalline and amorphous sodium disilicate were monitored for the first time using quasi-elastic neutron scattering (QENS) and X-ray diffraction (XRD). The QENS kinetic data collected for the first 12 h of the reaction were fitted to a model consisting of a Lorentzian and a Gaussian function each convoluted with the energy resolution of the instrument. This allowed the QENS signal from water to be associated with two states that included bound and free water, as confirmed by thermogravimetric analyses and 2H NMR. In situ QENS data were collected for a set of sodium disilicate and silica mixtures at a series of discrete temperatures between 20 and 40 °C. The bound water was successfully modeled with first-order reaction kinetics to quantify the hydration of the layered silicate structure within kanemite. First-order reaction kinetics was also used to model the reaction using amorphous starting material, which may also suggest the presence of hydrated layered structure in the reaction product. The presence of the hydrated layered structure in amorphous material was confirmed by XRD analyses. The consequences of this for the alkali−silica reaction (ASR) and its swelling mechanism are discussed. On the basis of these results, a standard test for ASR reactivity of aggregates could be developed using natrosilite as a reagent, and the kinetic parameters as a scale of reactivity.

State of Water in Hydrating Tricalcium Silicate and Portland Cement Pastes as Measured by Quasi‐Elastic Neutron Scattering

Quasi‐elastic neutron scattering (QENS) was used to monitor the state of water in portland cement and tricalcium silicate pastes during the first 2 days of hydration at three different temperatures. By applying a double‐Lorentzian rather than a single‐Lorentzian fitting function, the QENS signal from water at a given hydration time was divided into three separate populations arising from liquid water, chemically bound water, and constrained water. The constrained water population consisted of water adsorbed on surfaces and contained in very small (<10 nm) pores, and could be associated primarily with the calcium‐silicate‐hydrate (C‐S‐H) phase. The rate of increase in the chemically bound water population closely followed the exothermic heat output, while the constrained water population increased more rapidly during the first several hours of hydration and then leveled off.

Coherent quasielastic neutron scattering: A theorem about total neutron scattering functions for rotational jump diffusion of molecules on a lattice

We derive a theorem about the coherent quasielastic neutron-scattering signal from a lattice of N molecules that are undergoing rotational jump diffusion. If no correlations between the molecular jumps exist then the coherent quasielastic scattering signal is just N times that of a single molecule. The elastic contribution is just the diffraction diagram of the system. This implies, among others, that between the Bragg peaks there is no coherent elastic contribution, i.e., between the Bragg peaks the coherent signal is purely quasielastic. These results for coherent scattering are in marked contrast to those in the case of incoherent scattering where one also can find elastic scattering outside the Bragg peaks.