Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been applied to study the sodium mobility on nanosecond time scales in the perovskite fluoride cryolite, Na₃AlF₆, at high temperatures. Up to T = 1153 K the diffusion of Na ions is well described by a diffusion process of jumps between six and eight-fold coordinated sites. Above this temperature, where a step-like increase in the electrical conductivity occurs, the jump length increases, which indicates additional jumps over larger distances. The electrical conductivity derived from the self-diffusion coefficient via the Nernst–Einstein relation and the corresponding activation energy are in excellent agreement with the previous conductivity measurements. We conclude that the jump diffusion of sodium ions is the dominant mechanism for the electrical conductivity in cryolite at high temperatures up to T = 1153 K.     

Investigation of diffusion processes in liquid sodium and sodium-hydrogen melt by quasielastic neutron scattering

The quasielastic neutron scattering experiments on liquid sodium (at T = 378, 573, and 693 K) and sodiumhydrogen melt (T = 693 K, hydrogen concentration ～ 0.4 at %) were performed with the DIN-2PI time-of-flight spectrometer. The characteristics of the diffusion mobility for particles comprising the liquids studied are extracted from the experimental results and analyzed with the help of the phenomenological and “relaxing cage” models. The self-diffusion coefficient in liquid sodium obtained for all temperatures is in the agreement with the values known from literature. The diffusion mobility in pure sodium and in sodium-hydrogen melt were found to be coinciding; it leads to the conclusion that in our experimental conditions hydrogen exists and diffuses in melt in the form of hydride NaH.     

Quasielastic neutron scattering and correlated self-diffusion in crystals

The effect of correlations between successive jumps of atoms during self-diffusion on the quasielastic neutron-scattering line wodth is investigated. Temporal and spatial aspects of these correlations are described in terms of the so-called “encounter model”, which had originally been developed for the interpretation of nuclear-magnetic resonance studies on self-diffusion. Owing to the rather high temperatures (i.e. large diffusivities) to which quasielastic neutron-scattering experiments on self-diffusion in crystals are restricted, the effect of both mono- and divacancies is considered. It is found that the line width associated with a correlated mechanism is smaller and in single crystals shows a different orientation dependence than expected for a random-walk diffusion mechanism. Numerical results are presented for (i) the monovacancy and two divacancy mechanisms of self-diffusion in a b.c.c. lattice, a situation likely to be encountered in sodium metal, and (ii) mono- and divacancies in the f.c.c. lattice. In the random-walk limit earlier results of Gissler and Rother and Chudley and Elliot are reproduced. All of the results derived also apply to the diffusion-induced broadening of Mössbauer lines. The article therefore presents a generalization of a recent theory in which only the effect of monovacancies on the Mössbauer line broadening has been considered.     

Self-diffusion in liquid lithium from coherent quasielastic neutron scattering

Using the method of quasielastic coherent neutron scattering by liquid lithium, the temperature dependence of self-diffusion coefficient is investigated and compared with analogous data extracted by the incoherent scattering method.     

Investigating one-dimensional diffusion by quasielastic neutron scattering: a theoretical approach

Scattering functions and full widths at half maximum for quasielastic neutron scattering (QENS) are calculated for diffusion in systems of one-dimensional channels. The self-correlation function for diffusion in isotropically oriented channels is given and it is found that this function diverges at the origin. The calculations are carried out for both normal and single-file diffusion and the influence of the ballistic phase is investigated. It is found that the ballistic phase influences the scattering functions very strongly for large diffusion coefficients. QENS data from the literature are analyzed with respect to this influence. The influence of three different resolution functions (triangular, Gaussian, and Lorentzian) is considered.     

The anomalous Debye-Waller factor of hydrogen in niobium as determined by quasielastic neutron scattering

The integrated intensity of quasielastic neutron scattering by protons in polycrystalline NbH_0.16 and in a single crystal of NbH_0.045 was investigated as a function of the scattering vector Q. Strong deviations from a harmonic Debye-Waller factor behavior were observed at elevated temperatures. The results show a temperature dependent delocalization of the proton extending as far as the neighboring sites of the interstitial lattice. Experiments on the single crystal indicate a directional dependent mean-square amplitude of the proton even at room temperature.     

Elastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals

Neutron scattering studies in (KBr)_1–x(KCN)_x mixed crystals are presented utilizing powder diffraction, single crystal diffraction and time-of-flight techniques. Forx>0.6 (KBr)_1–x(KCN)_x crystals exhibit ferroelastic and ferroelectric low-temperature phases. Crystals withx<0.6 undergo transitions into an orientational glass state. Here we present a detailed phase diagram including new results for x=0.85 and x=0.65. For the latter system a stable rhombohedral low-temperature phase has been detected where the orientational disorder of the plastic phase is only partly removed and quadrupolar relaxations between three body diagonals are still possible. From the powder diffraction experiments we determined further the concentration dependence of the static Debye-Waller factors which can be explained by an interplay of the rotation-translation and the rotation-random strain coupling. With single crystal diffraction techniques we studied the diffuse scattered intensities which are directly related to the order parameter of the glass state. The temperature dependence of the quasielastic intensities near the critical concentration shows a strong increase forT<110K indicative for a freezing-in of shear fluctuations which is a characteristic feature of a non-ergodic instability. This phenomenon appears for ordering (x=0.65) and for non-ordering, glassy compounds. A further anomaly in (KBr)_0.43(KCN)_0.57 at 75 K is interpreted in terms of a residual elastic ordering process. With high-resolution time-of-flight techniques we analysed the dynamic structure factor for x=0.57. We demonstrate that the central peak consists of a static and a dynamic component. The results are compared with mode coupling theories which describe the glass transition in supercooled liquids.     

Intramolecular diffusive motion in alkane monolayers studied by high-resolution quasielastic neutron scattering and molecular dynamics simulations

Molecular dynamics simulations of a tetracosane (n-C24H50) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of approximately 0.1-4 ns. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.     

New interpretation of local dynamics of poly(dimethyl siloxane) observed by quasielastic neutron scattering

Synthetic and natural polymers have complex dynamic behavior with distinct motions taking place on a wide range of time and length scales. For poly(dimethyl siloxane) we show that, at temperatures above the melting point, the reorientation of the CH3 groups provides a non-negligible contribution to the incoherent dynamic structure factor. Analysis of the quasielastic neutron scattering data is carried out using a model function that includes fast rotational motion of the CH3 groups and local conformational transitions between isomeric states. By using this model, detailed comparison between experimental data and theoretical predictions at distances where deviations from the traditional Rouse model are expected becomes possible.     

Origin of dynamic heterogeneities in miscible polymer blends: A quasielastic neutron scattering study

In order to investigate the origin of the often invoked nanoheterogeneities in miscible polymer blends, we have performed quasielastic neutron scattering experiments on the component dynamics within the miscible polymer blend polyisoprene/polyvinyl ether including the pure components as a reference. We find that the apparent local heterogeneities observed by spectroscopic techniques originate from the chain specific crossover properties between entropy driven and local chain dynamics and are, thus, a purely dynamical phenomenon.     

Incoherent quasielastic neutron scattering from H2O and aqueous ZnCl2 solutions

A study of the diffusive motions of the protons in pure water and ZnCl₂ aqueous solutions has been performed, using incoherent quasielastic neutron scattering. It is shown that it is essential to take into account the rotational motion of the water molecules. The translational linewidth is conveniently fitted over the whole Q-range, using the Random Jump Diffusion model for which the jump length turns out to be roughly the same for pure H₂O and the saturated solution, fairly close to the distance between protons in the water molecule.     

Studying the structural features of oxide nanoclusters of cerium and titanium in a silicate glass by means of the small-angle neutron scattering

The features of the formation of Ce-Ti-O complex oxide nanoclusters in a silicate glass are studied by means of the small-angle neutron scattering technique. It is found that bounded regions of density fluctuation of the glass material are formed in the initial glass matrix without the addition of titanium and cerium oxides. These regions could serve as nucleation centers for oxide clusters of cerium and titanium upon their introduction into the matrix. The calculated average size of these inhomogeneities does not exceed 30 ± 1 nm, and their surface volume equals 0.72 ± 3 nm³. A structural mechanism for Ce-Ti-O oxide formation in a silicate glass, in which the nanoclusters are formed within a bounded region of glass material inhomogeneities at low concentrations of the initial cerium oxide (CeO₂), is proposed. At high cerium oxide concentrations, oxide nanocluster growth occurs predominantly on the surface of these inhomogeneities. This leads to a sharp change in the nanocluster sizes and their fractal dimension.     

The diffusive motion of silver ions in α-AgI: Results from quasielastic neutron scattering

The diffusive motion of silver ions in σ-AgI at 250°C has been studied by quasielastic cold neutron scattering. Spectra were taken in the range of wavevector transfer 0.5 < Q < 2.2Å^?1 for elastic scattering. The quasielastic line shapes contain a narrow and a broad component. They are compared to model calculations allowing for the superposition of two kinds of motion on two different time scales, a local random motion and a translational motion of the jump-diffusion type. The model closely fits the data. The local random motion takes place on a time scale of the order of 10^?12 s, with amplitudes of the order of 1 Å. It is probably caused by rapid fluctuations of the local potentials due to the diffusive motion of the other cations. The translational motion results in a mean displacement of the silver ion over a distance of the order of a lattice constant (5 Å) with a correlation time of the order of 10^?11s. This correlation time is composed of a residence time and a time-of-flight, which are both of comparable magnitude.     

Scaling violation in quasielastic electron scattering

The scaling law in quasielastic electron scattering is broken due to exchange forces. The violation in the longitudinal response function is attributed to the central part of exchange forces, while in the transverse response the tensor part also leads to the breaking. The effects of one-boson exchange potentials are discussed.     

Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies

The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.     

Axial mass in quasielastic antineutrino-nucleon scattering accompanied by strange-hyperon production

Reactions of quasielastic Λ-, Σ⁻-, and Σ⁰-hyperon production in antineutrino-nucleon interactions are studied. An axial-mass (M _A ) value that agrees with a fit to all accelerator data on exclusive and inclusive νN and νN reactions was extracted from a global statistical analysis of experimental data on differential and total cross sections for ΔY = 0 and 1 quasielastic reactions of neutrino and antineutrino scattering on various nuclear targets.