Band 224, Heft 1-2

This tutorial introduction has been written for people who are not specialized in neutron scattering or in other scattering methods but who are interested and would like to get an impression and learn about the method of Quasielastic Neutron Scattering (QENS). The theoretical (scattering process) as well as the experimental basics (neutron sources, neutron scattering instruments, experimental periphery) are explained in a generally understandable way, with only the most essential formulas. QENS addresses the stochastic dynamics in condensed matter, and it is pointed out for which problems and for which systems in condensed matter research QENS is a powerful method. Thus sufficient information is provided to enable non-experts to think about their own QENS experiment and to understand related literature in this area of research.

We present new developments in neutron backscattering spectroscopy from two major neutron scattering centers. The first example is the reactor backscattering project IN16B, which is under construction at the Institut Laue-Langevin (ILL), Grenoble, France, the second is the spallation source near-backscattering spectrometer BASIS at the spallation neutron source SNS, Oak Ridge National Laboratory, USA, in operation since 2007. After explaining the energy resolution contributions to neutron backscattering and presenting the current status, we describe the layout, performance and the complementarity of the two most advanced backscattering projects. A few examples should illustrate the status and new experimental possibilities.

We review recent advances in neutron and helium spin-echo spectroscopy, applied to the diffusion of atoms and simple molecules adsorbed on surfaces. A comparative introduction to the techniques is given, including advances in theory and simulation techniques, followed by a summary of several recent sets of experimental results.

The quasielastic dynamics in liquid metals has been investigated by neutron scattering extensively during the past years. Precise measurements of the self-particle dynamics and the coherent quasielastic response provided conclusive evidence that the relaxation dynamics is governed by at least two processes. These two time scales in the correlation functions appear as two dynamical processes in their associated memory functions. One process shows a fast decay and is related to stochastic binary collisions. The second slow process stems from a non-linear coupling of slow collective modes. These slow density fluctuations arise mainly from the dynamics around the structure factor maximum. A rigorous treatment of these effects can be accomplished by mode coupling theory. A few selective experiments from monatomic metals are presented to demonstrate the influence of slow effects on the dynamics. Experiments and MD-simulations show a surprising good agreement with predictions of mode coupling theory on a quantitative level. The slow decay process appears to be related to structural arrest in the supercooled state and might indicate a link to the solidification process, starting deep in the liquid state. Accurate quasielastic neutron scattering experiments are still and will remain a fundamental pillar for elucidating the complex dynamics in the liquid state of metals.

We have made a series of Quasi-Elastic Neutron Scattering (QENS) studies of supercooled water confined in 3-D and 1-D geometries, specifically, interstitial water in aged cement paste (3-D) and water confined in MCM-41-S and Double Wall Nano Tube DWNT (1-D). In addition, we also include the cases of hydration water on protein surface and other biopolymer surfaces (pseudo 2-D). By analyzing the QENS spectra using Relaxing Cage Model (RCM), we are able to extract accurately the self-intermediate scattering function of hydrogen atoms F H (Q,t), at low-Q as a function of temperature T, showing an α-relaxation process at long time. We can then construct the Dynamic Response Function χ T (Q,t) = -dF H (Q,t)/dT. χ T (Q,t) as a function of t at constant Q shows a single peak at the characteristic α -relaxation time 〈 τ 〉, the amplitude of which grows as we approach the dynamic crossover temperature T L observed before in each of these geometries. However, the peak height of χ T (Q,t) decreases after passing the crossover temperature T L . We make an argument to relate the occurrence of the extremum of the peak height in χ T to the existence of the dynamic crossover temperature in each of these cases.

Here we report dynamics of propylene (smallest asymmetric hydrocarbon) adsorbed in two different host systems – Na-Y and Na-ZSM5 zeolites as studied by quasielastic neutron scattering (QENS) techniques and molecular dynamics (MD) simulation techniques. MD simulation studies suggested that the rotational motion of propylene is much faster than the translation. Therefore, two different spectrometers were used to determine the translational and rotational contributions. Analysis of the QENS data (ΔE~200 μeV) revealed that the translational motion of propylene in Na-ZSM5 zeolite is a factor of 3 slower than that in Na-Y zeolite. This is consistent with the smaller void volume available in Na-ZSM5 zeolite as also the fact that the adsorption of propylene is stronger in Na-ZSM5 as compared to that in Na-Y zeolite as indicated in the Fourier Transformed Infra Red (FTIR) studies. This result is further corroborated by the MD simulation studies. It was found that the translation motion occurs at three time scales in both the systems, only one of which corresponds to that observed through the QENS measurements. To probe the faster rotational motion experiments were carried out on a Triple axis spectrometer with wider energy window (ΔE~3meV). The variation of the elastic incoherent structure factor (EISF) suggests that the observed QE broadening corresponds to isotropic rotational motion of propylene in both the hosts. Rotational diffusion coefficients of propylene molecules are found to be of the same order in both zeolites. Trajectories obtained from MD simulation corresponding to the rotational motion revealed that although on a large time scale, the rotation appears isotropic, but at short time scales the channel structure of the Na-ZSM5 restrict the rotation of propylene molecules. This is in contrast with propylene adsorbed in Na-Y zeolite where rotation remains isotropic even at short time scale. A comparison with other hydrocarbon-zeolite systems as studied by us is also presented here.

Mobility of ions and water in clays is at the heart of their remarkable properties of water retention and ion-exchange. It has been addressed here using two microscopic techniques: neutron scattering and molecular dynamics simulations. Neutron scattering gives access exclusively to water dynamics in clays, due to the exceptional sensitivity of neutrons to H atoms. The data interpretation can be challenging, especially for natural clays such as montmorillonite, with inhomogeneous swelling characteristics. A great improvement is achieved with the use of synthetic materials, as demonstrated here on the case of synthetic (fluoro)hectorite. The standard analytical models for long-range diffusive motion, isotropic translation and its derivative, powder averaged two dimensional translation, have been used to interpret the neutron scattering data. They both agree on the order of magnitude for the diffusion coefficient of water in monohydrated and bihydrated clays, 10 −10 m 2 s −1 and 10 −9 m 2 s −1 respectively. While the two-dimensional nature of water diffusion in clays is seen clearly from molecular dynamics simulations, its signature in neutron scattering data is obscured by the powder-averaging of the signal. A novel method, based on a multi-resolution analysis of scattering functions from powder samples, allows never-the-less a clear determination of the dimensionality of water motion in the system. Extracting information on local water motion is difficult on the basis of neutron scattering data only. Various models for localised motion, rotation on a sphere or jump diffusion, have been proposed and used to interpret the observed neutron data, however their applicability is questionable in light of information from molecular dynamics simulations. Aside from aiding the interpretation of neutron scattering data, MD simulations are most valuable in providing information on the behaviour of ions in clays. MD estimates the interlayer ion coefficients as of the some order of magnitude as water, even if the details of ionic motion are strikingly different between the two ions considered here, Na + and Cs + . Further, MD has also allowed to address the topic of ion exchange between clay interlayers and bulk aqueous solution. The microscopic picture of water and ion motion in clays, emerging from neutron scattering and MD simulations, should be treated as a building block of the overall modelling of macroscopic transport in clays, the ultimate property of interest for many clay applications.

In a world where the effects of climate change on weather patterns is accepted as real and serious, the problem of decreasing the production of carbon dioxide is perceived as increasingly important. The cement industry produces 5–7% of the world’s carbon dioxide emission and its survival will depend on improvements in the production of concrete which will be both more durable and require less carbon dioxide per unit of manufacture than the currently produced concrete. The durability of concrete is related to its ability to limit fluid transmission and knowledge of how to reduce the rate at which water will be transmitted through cement paste is critical to improving durability. However, because of the complex chemical and physical nature of cement pastes, understanding water mobility is a great challenge. Many techniques are not applicable simply because they are not sensitive to the range of size from angstroms to microns and the extent of water interaction with the cement where water can either be chemically bound at hydroxyls or physically free in large pores. In this review paper, we present the most up to date results on the physical chemistry of the water/ cement paste interactions studied by quasi-elastic neutron scattering. These results bring new insight to the mobility of water in the gel pores, the small pores (radius less than 50Å) that control the rate of water transmission in the cement pastes from which high quality concrete will be made.

Quasielastic incoherent neutron scattering is a well suited and established experimental method to study protein and water dynamics in the picosecond to nanosecond time- and Ångstrom length-scale. Using deuterium labelling either protein or water motions can be selected and brought into focus. Protein and cell water dynamics were separately studied in red blood cells. A consistent picture of cytoplasmic water and protein dynamics in whole cells is emerging from recent experimental results.

Protein fibril formation has been often associated to manifestation of serious and devastating amyloyd diseases, including Alzheimer and BSA. The main mechanism for the formation of amyloid fibrils is the accumulation of protein aggregates in body´s organs.In this paper, we try to compare the dynamical behaviour of two amyloidogenic proteins, the Insulin and the Myoglobin. Insulin has been chosen for its pharmacological extensive use in diabete´s therapy, while Myoglobin is used as control, since its dynamics is now largely known. The investigation has been performed through incoherent elastic neutron scattering over a wide temperature range. Our results suggest an enhanced stiffness of Insulin with respect to Myoglobin.

In recent biomedical studies different nanocarrier systems have been proposed for ´smart´ drug delivery. The engineering of these systems requires as a prerequisite a detailed knowledge of their structure and dynamics at the molecular level. Quasielastic neutron scattering is an ideal tool for dynamic studies of these complex systems since it provides information on molecular motions in a time window which is important to relate the local dynamics to the macroscopic functional properties of the drug vectors. Some selected examples referring to different nanostructures will be analysed in the following.

The elastic constants of bicontinuous microemulsions, consisting of water, oil (decane) and a surfactant, can be modified by the addition of diblock-copolymers (boosting effect). A similar decoration of the surfactant membrane is possible in the lamellar phase. At high diblock copolymer densities, in the brush regime, a change in the dependence of the bending rigidity on the molecular size and concentration of the diblock copolymer is predicted theoretically. In particular a very strong dependence of the bending rigidity on the radius of gyration of the diblock copolymer ( κ ∝ R 5 ) is predicted. We have studied the structure and dynamics of lamellar microemulsions by small angle neutron scattering (SANS) and neutron spin echo spectroscopy (NSE) in the brush regime, with high diblock copolymer density. Decyl polyglycol ether (C 10 E 4 ) has been used as surfactant, the amphiphilic diblock-copolymers were polyethylenepropylene/polyethyleneoxide (PEP 5 –PEO 5 ). The influence of the diblock copolymer concentration when entering the brush regime on the bending rigidity κ and the correlation length ξ will be discussed.

Monomer dynamics in sodium dodecyl sulfate (SDS) micellar solution at room temperature as studied by quasi elastic neutron scattering (QENS) technique is reported. The elastic incoherent structure factor (EISF) obtained from the analysis of QENS data suggests that the SDS chains perform uniaxial rotational diffusion along the long molecular axis. The rotation diffusion coefficient was found to decrease with the increase in the concentration of the surfactants. It is observed that on addition of electrolyte (0.8 M NaCl) the motion is arrested. Addition of salt is known to induce the growth in SDS micelles, here it is found to affect the dynamics of monomers also. This is due to the combined effect of closer packing of the monomers within the micelle and the tendency of amphiphilic molecules to stabilize due to the presence of electrolytes.

The high hydrogen content in complex hydrides such as M[AlH 4 ] x and M[BH 4 ] x (M = Li, Na, K, Mg, Ca) stimulated many research activities to utilize them as hydrogen storage materials. An understanding of the dynamical properties on the molecular level is important to understand and to improve the sorption kinetics. Hydrogen dynamics in complex hydrides comprise long range translational diffusion as well as localized motions like vibrations, librations or rotations. All the different motions are characterized by their specific length- and timescales. Within this review we give an introduction to the physical properties of lightweight complex hydrides and illustrate the huge variety of dynamical phenomena on selected examples.

The defect pyrochlores have become the focus of attention for the development and optimization of novel solid-state proton conductors as alternatives to costly polymer-based materials. A promising candidate is (H 3 O)SbTeO 6 , a super-proton conductor characterized by an outstanding conductivity of 10 –1 S cm –1 at ambient conditions. The crystal structure of this compound has been known for some time yet, to date, the physical mechanism underpinning proton conduction has remained elusive. Structurally, (H 3 O)SbTeO 6 consists of corner-shared (Sb/Te)O 6 octahedra forming a cubic framework of three-dimensional interconnected cages partially occupied with hydronium H 3 O + ions. These ions are connected to the inorganic framework by weak hydrogen bonds. In this work, we present the first characterization of this compound by means of quasielastic and inelastic neutron spectroscopy over a wide temperature range (T = 1.5–500 K), as well as by heat capacity measurements. Our results show that the onset of low-frequency stochastic motions of the hydronium moiety takes place around room temperature and persists up to the highest temperatures investigated. Unlike other inorganic proton conductors, these motions are effectively decoupled from vibrational motions at higher frequencies, as evidenced by the temperature dependence of the vibrational density of states. These findings have important implications for the exploitation of the defect pyrochlores as proton-conducting media for fuel-cell and gas-sensing applications.