L.A. de Graaf

Non-analytic dispersion relations in liquid argon

Abstract Neutron scattering experiments on liquid argon reveal an anomalous approach to the hydrodynamic limit for the frequency of sound and for the ratio of the sound and heat mode damping factors. In both cases the behavior agrees with non-analytic dispersion relations predicted by the mode coupling theory.

Extended hydrodynamic modes in a dense hard sphere fluid

Abstract A computer simulation experiment of a dense hard sphere fluid of 256 particles shows that the intermediate scattering function and the longitudinal velocity correlation function can be described by three extended hydrodynamic modes, the properties of which agree well with those predicted by the revised Enskog theory.

Collective modes at very short wavelengths in liquid lithium

Abstract Inelastic neutron scattering data on liquid 7 Li at 470 K are presented. The results from two measurements are combined: the first with neutrons of 20 meV and a resolution at the detectors of 1.3 meV, the second with 525 meV neutrons and 18 meV resolution. This resolution enables one to investigate short-wavelength sound modes over a wavevector range 10 κ −1 . The data are analyzed in terms of extended hydrodynamic modes and the sound dispersion ω s ( κ ) over this entire κ-range are determined. The data indicate that very short wavelength sound modes do exist up to at least κ = 33 nm −1 . The position of the top of the longitudinal current correlation function deviates significantly from ω s ( κ ). The present data confirm recent inelastic X-ray scattering experiments on liquid lithium as well as a neutron scattering experiment on liquid cesium.

Rotational motions of NH+4 groups in NH4ZnF3 studied by quasielastic neutron scattering

The rotational behavior of the ammonium groups in the high‐temperature cubic phase of the perovskite NH4ZnF3 has been investigated, using incoherent quasielastic neutron scattering. The measurements were carried out for various wave vector transfers between 0.68 and 5.52 A−1 and for temperatures between the transition temperature of 115.1 and 300 K. The data are compared with some reorientational jump models. It is assumed that in the cubic phase the NH4 group is a slightly distorted tetrahedron, apparently due to strong hydrogen–fluorine interactions. The degree of distortion could, however, not be determined. On the time scale of the experiment (?3 ps) the reorientational behavior of the NH4 group can be described by 90° jumps about its axis of smallest moment of inertia. Residence times between 0.4 and 7 ps were found. Their temperature dependence follows an Arrhenius behavior, yielding Ea/kB=580±15 K, which agrees with calculations for the quantum mechanical energy level scheme.

Observation of fast sound in liquid Li4Tl and liquid Li4Pb by inelastic neutron scattering

We performed inelastic neutron scattering experiments on liquid Li4Pb and Li4Tl. We obtained the first experimental evidence for the existence of fast sound in a molten binary alloy of elements of strongly different weight. This phenomenon was observed for the first time ten years ago in a computer molecular dynamics (CMD) simulation. The results are compared with the results of the CMD simulation and with experimental results for the sound dispersion of pure lithium.

Rotational correlation functions of plastic-crystalline neopentane from neutron scattering experiments

Abstract The rotational motions of neopentane molecules in the orientationally disordered phase have been studied by incoherent neutron scattering. The measurements were performed with a rotating-crystal spectrometer (incoming wavelength 4.1 A), covering a wave-vector transfer range of 0.23–2.18 A −1 . Its resolution leads to an observation time up to about 5 ps. From the time-of-flight spectra intermediate scattering functions and frequency distributions were obtained. The frequency distributions were fitted with a damped Debye-model for the translational motions. From the intermediate scattering functions the rotational part was deduced and, using the partial-wave expansion, rotational correlation functions could be determined up to third order. These first-and second-order correlation functions are compared with infrared and Raman data and they are found to be in good agreement. From the behaviour of the elastic intensity of the spectra it can be concluded that on the time-scale covered the distribution of the protons of the molecule does not randomize. A comparison of elastic intensities and rotational correlation functions with the predictions of several, symmetry adapted, jump reorientation models is made. The best fit is obtained for a model with D2d symmetry where 90° reorientations about the fourfold axis occur more frequently than 180° jumps about the two perpendicular twofold axes.

Microscopic dynamics and structure of liquid 7Li

We report an inelastic neutron scattering (INS) experiment on liquid 7Li at 470 K, 526 K and 574 K using thermal neutrons. Fully corrected structure factors S( kappa , omega ) were obtained. We have assessed the quality of the data by means of the first frequency moment of S( kappa , omega ) and by the detailed balance condition. The coherent dynamic structure factor Sc( kappa , omega ) and the incoherent dynamic structure factor Ss( kappa , omega ) were separated by fitting models. We compare the data for Ss( kappa , omega ) at small kappa with several models using no adjustable parameters. The Lovesey model for Ss( kappa , omega ) gives very good agreement with the experimental data at all three temperatures. Ss( kappa , omega ) is not well described by the predictions from simple diffusion, mode-coupling theory or the model of Nelkin-Ghatak. The Lovesey model also gives the best agreement with the data when we fit one parameter.

Neutron diffraction study of liquid LixSi1-x alloys where x=0.80, 0.65 and 0.57

We report a neutron diffraction experiment on liquid 7LixSi1-x alloys (x=0.80, 0.65 and 0.57) just above the liquidus and on 7Li57Si43 at about 200 K above the liquidus. We obtained absolutely normalized and fully corrected structure factors and radial distribution functions. Which provide indications for the occurrence of chemical ordering in all three alloys. We have analysed the data by fitting models to the structure factor and to the radial distribution function, and by using the reverse Monte Carlo (RMC) method. We find strong indications for the presence of covalently bonded Si in liquid Li65Si35 and Li57Si43. The Si-Si coordination number ZSiSi varies from about 1.5 in Li65Si35 to about 2 in Li57Si43. For cSi>0.2, Si forms poly-anions ranging from dumbbells to large three-dimensional structures. ZSiSi in Li57Si43 is not strongly temperature dependent and consequently the poly-anions are fairly stable. Covalently bonded Si is absent in liquid Li80Si20. The RMC modelling indicates that individual Si atoms are well separated. This suggests that the covalent character of Si in the Si-rich alloys (Li65Si35 and Li57Si43) changes to a more ionic character in Li80Si20.

Microscopic dynamics in a dense helium argon gas mixture

Abstract By inelastic neutron scattering we determine the dynamic structure factor S ( k , ω) of a dense He-Ar gas mixture. S ( k , ω) is the sum of two Gaussians in frequency ω with halfwidths ω Ar H ( k ) ⪡ ω He H ( k ) the decay rates of fluctuations in the density of Ar and He atoms. The results agree with the Enskog theory.

The structure of liquid LiSi alloys

The reverse Monte Carlo method is used to analyse neutron diffraction data on liquid Li 65 Si 35 . A well defined Si-Si partial pair correlation function is obtained with strong indications for covalent Si-Si bonds. It is also clear that most of the Si 4 stars and Si 5 rings occuring in solid Li 12 Si 7 have disappeared in the liquid state. Si clusters are present in various forms, ranging from single Si atoms to large clusters