Refinement of the atomic structure of specimens cut out from different growth pyramids of a K(H0.052D0.948)2 PO4 single crystal

The structures of two crystalline specimens cut out from the pyramidal and prismatic growth sectors of a K(H_0.052D_0.948)₂PO₄ single crystal have been studied by diffuse neutron scattering and precision diffuse X-ray scattering. Diffuse scattering is concentrated in the vicinity of the Bragg reflections and is practically the same in specimens cut out from different growth sectors of a single crystal. X-ray diffraction analysis using the extinction parameters provided the establishment of a higher perfection of the specimen cut out from the prismatic growth sector. The precision X-ray studies revealed different configurations of hydrogen bonds in the specimens.     

Dynamical Properties of a Disordered Crystal: Adamantane—C10D16

Inelastic neutron scattering measurements on C₁₀D₁₆ show the existence of underdamped collective librational modes with temperature-independent frequency widths.

A preliminary dynamical model, with no adjustable parameters, shows that the observed widths are consistent with the existing quasi-static disorder.

Direct measurements of the orientational pair correlation function by means of elastic neutron scattering agree with the prediction of a simple Ising model.

Intensity measurements between room-temperature and the ordering temperature (T _c = 208.6K) indicate that the orientational correlations develop very slowly as T → T _c.     

Über das Wachstum und den Abbau von Gallium- und CBr4-Einkristallen aus eigener Schmelze

The morphology and the temperature dependence of the velocity of crystal growth and dissolution of gallium- and carbon tetrabromide single crystals are investigated using a capillary technique. These two model substances have a relative low entropy of melting, ΔS_m, which in the case of Ga is above, and in the case of CBr₄ is below the critical value of ΔS_m = 4.0. According to recent theoretical considerations this parameter determines the formation or the absence of an atomically rough crystal-melt interface. Particular interest is given to the dissolution kinetics of Ga and CBr₄. Crystallization and dissolution behaviour of Ga has been shown to be similar to substances having high ΔS_m-values, whereas the morphology, the crystallization and the dissolution of CBr₄ can be explained by assuming an atomically rough crystal-melt boundary.     

Structure of poorly crystalline MoS2 — A modeling study

Current interest in amorphous and poorly crystallized phases of the layered transition metal dichalcogenides arises primarily from their unique catalytic and electrochemical properties not present in the corresponding crystalline phases. Computer calcultions of the scattered X-ray intensity from model structures of poorly crystalline (px-) MoS₂ are compared here with experimental patterns from materials prepared at different temperatures. The results are used to identify the salient features of the observed diffraction patterns of px-MoS₂ in which the measured diffuse scattered intensity may be clearly attributed to the stacking and rotational disorder of the layers as well as the limited number of these layers. A procedure for the interpretation of the X-ray diffraction pattern of px-MoS₂ has thereby been developed in which the entire scattering pattern may be calculated without separation into Bragg peaks (of varying breadth and shape) and diffuse background.     

Observation of chiral ordering of moments in the magnetic mesostucture of the (Pd0.984Fe0.016)0.95Mn0.05 alloy by means of polarized neutrons

Investigation of the magnetic structure of the diluted (Pd_0.984Fe_0.016)_0.95Mn_0.05 alloy at meso-and nanoscale levels by means of two techniques of small-angle polarized-neutron scattering (within a direct beam and beyond it) has been performed in the temperature range 10<T<60 K. The dependences of the neutron beam depolarization, the polarization rotation angle, and the polarization-dependent magnetic scattering cross section on a weak (0<H<40 A/cm) external magnetic field have been studied. A simple model of neutron beam depolarization by a sample with uniaxial magnetic anisotropy was used to analyze the results obtained. The experiment on polarized-neutron scattering in the so-called direct geometry showed the existence of a polarization-dependent scattering cross section. This scattering has left-right asymmetry and depends on temperature. Comparison of these results with the depolarization data leads to a conclusion about the existence of static chiral fluctuations in large-scale inhomogeneities.     

Structural study of levitated liquid Y2O3 using neutron scattering

The local structure of liquid Y₂O₃ at 2800 K has been investigated by combining aerodynamic levitation and laser heating with the neutron scattering technique. Despite relatively low counting statistics, we have been able to determine the short-range order in the liquid. In particular, we found Y–O and Y–Y coordination numbers around 6 and 12, confirming our previous study by anomalous X-ray scattering. The unusually sharp main diffraction peak observed with X-ray measurements, and characteristic of a high degree of chemical ordering, is strongly reduced in the neutron structure factor and thus indicates that it is mostly due to Y–Y correlations.     

Vibrational modes of hydrogen in (NH<Subscript>4</Subscript>)H<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> (Neutron scattering and simulation)

The vibrational modes of hydrogen in (NH₄)H₅(PO₄)₂ have been investigated by inelastic incoherent neutron scattering at temperatures above (220 K) and below (160 and 5 K) the phase-transition point T _c = 180 K. Computer simulation of vibrational modes has been performed for the low-temperature phase (NH₄)H₅(PO₄)₂ at 5 K. The calculated generalized incoherent dynamic factor and the partial incoherent dynamic factors for all hydrogens make it possible to identify the observed modes from acid hydrogens and hydrogens of ammonium ions.     

Paramagnons in the PrNi system with an induced magnetic moment

The recent studies of the PrNi system have shown that, at a temperature slightly higher than the ferromagnetic ordering temperature (T _C ～ 21 K), softening of some part of magnetic excitations occurs near the Γ point of the Brillouin zone. The magnetic modes observed have been qualitatively described by a model taking into account the crystal field and the exchang interaction within the mean-field random-phase approximation. To refine the model parameters and obtain a complete set of excitation modes, inelastic neutron scattering measurements on PrNi single crystals and polycrystals have been performed at T = 23 K. An acoustic branch related to the level with high excitation energy has been observed for the first time. The character of its dispersion suggests that the low-energy acoustic mode is responsible for the magnetic phase transition.     

Structural Investigations of Polyacetylene and Polyphenylene by Neutron Scattering

Wide angle neutron scattring patterns of deuterated cis- and trans-polyacetylene (PA) and deuterated polyphenylene (PPh) doped with AsF₅ have been measured. For undoped trans-PA a refined crystal structure is proposed to reproduce the neurtron scattering data. Doping of PA with AsF₅ is explained on the basis of an intercalation model. Deuterated PPh has been prepared by different polyerisation procedures, variing the crystallinity of the undoped material in a wide range. Strong extra peaks are observed during doping with AsF₅, wheres the crystallinity of the samples remains essentially unchanged. An intercalation model similar to AsF₅.-doped graphite is proposed.     

Cold Neutron Scattering Investigation of Molecular Orientational Motion in Smectic A Phase of 4-Cyano- 4′-N-Alkylbiphenyl Liquid Crystal

The measured cold neutron scattering spectra obtained in the S_A phase of 4-cyano-4-alkylbiphenyl sample are investigated with emphasis: a) on the molecular uniaxial, within two potential barriers angular confined, stochastic reorientational motion, b) on the long molecular axis orientational fluctuations between two potential barriers as well as uniaxial simple rotational diffusion, and c) on the correlations of orientation between adjacent molecules due to Faber, with no localized motion implied.

It is shown that, for low Q values, where Faber approach could be utilized, the spectra are well represented by his continuum theory of molecular disorder, although the first model above, in a modified form, can not be completely ruled out.     

Simulation of dense random-packed models for the amorphous TbxFe1−x alloy system

A recently developed computer algorithm for simulating dense random-packed (DRP) structures has been applied to the amorphous Tb_xFe_1?x alloy system, for which neutron scattering data are available with x=0.12, 0.33, 0.45 and 0.75. The pair correlation functions resulting from the models are presented and the corresponding interference functions calculated: these latter correctly predict the observed composition dependent peak shifts, as well as the overall features of the data.     

High frequency collective dynamics in liquid potassium

We investigated by inelastic X-ray scattering the dynamical properties of molten potassium in a wide range of momentum transfer, Q, from 1 nm⁻¹ up to the main peak of the structure factor Q ≈ 17 nm⁻¹. The observed increase of sound velocity in the low Q region (1 < Q < 3 nm⁻¹), has been described within a model characterized by two distinct relaxation processes for the collective dynamics. The obtained results are discussed and compared with those from previous neutron scattering experiments. In particular, we associate the speed-up of the sound velocity to the ‘instantaneous’ disorder of the liquid as opposed to the argument, supported by some neutron scattering studies, of a transition from a liquid to solid like response of the system.     

Influence of pressure on fast dynamics in polyisobutylene

Influence of pressure on fast dynamics and elastic properties in polyisobutylene is studied using Raman, Brillouin and neutron scattering spectroscopy. Analysis of the results shows that the boson peak frequency increases with pressure stronger than the longitudinal sound velocity measured by Brillouin scattering. Moreover, the boson peak intensity decreases under pressure stronger in Raman scattering than in neutron scattering suggesting a decrease in the light-to-vibrations coupling coefficient C(ν). The strong decrease of the microscopic peak intensity under pressure in Raman spectra supports this suggestion. We argue that variations in C(ν) might be related to amplitude of structural fluctuations. We speculate that change in disorder and/or overall density under pressure is the main cause for the observed variations.     

Refinement of the crystal structure of the high-temperature phase G 0 in (NH4)2WO2F4 (powder, X-ray, and neutron scattering)

The (NH₄)₂WO₂F₄ compound undergoes a series of phase transitions: G ₀ → 201, K → G ₁ → 160, and K → G ₂, with a significant change in entropy (ΔS ₁ ～ Rln10 at the G ₀ → G ₁ transition), which indicates significant orientational disordering in the G ₀ phase and the order-disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G ₀ phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WO₂F₄]^2? ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH₄]⁺ ions calls for the combined method of elastic and inelastic neutron scattering. Inelastic neutron scattering is used to determine the state of hindered rotation for ammonium ions in the G ₀ phase. Powder neutron diffraction shows that the orientational disorder of NH₄ ions can adequately be described within the free-rotation approximation.     

Neutron studies of the structure and dynamics of Rb1 − x (NH4)xI mixed crystals

The structural and dynamical properties of the x-T phase diagram of the system of Rb_{1 ? x} (NH₄)_xI mixed crystals is of great interest, because such solid solutions are almost free of internal stresses due to almost equal ionic radii of ammonium and rubidium. The x-T phase diagram of Rb_{1 ? x} (NH₄)_xI is studied on samples with ammonium concentration ranging from 0.01 to 0.73 over the temperature range from 15 to 300 K by the methods of powder neutron diffraction and inelastic incoherent neutron scattering. The results obtained by powder neutron diffraction show that the α-β phase transition at low temperature is rather extended and occurs at the ammonium concentrations x = 0.50 and 0.66. The region of orientational state glass is determined by inelastic incoherent neutron scattering at the concentrations x = 0.29 and 0.40 at the temperature T = 20 K.     

Complexes of poly-N-vinylpyrrolidone with sulfonated tetraphenylporphins

The interaction of sulfonated tetraphenylporphins H₂ TPPS₄(HCl)₂ and in CuTPPS₄ with poly-N-vinylpyrrolidone (PVP) is studied using small-angle neutron scattering, dynamic light scattering, visible spectrophotometry, and viscometry. It is shown that porphyrin molecules form complexes with PVP, thus charging the polymeric chain.     

Small-angle neutron scattering data on C60 clusters in weakly polar solutions of fullerenes

Solutions of fullerence C₆₀ in carbon disulfide CS₂ have been investigated by small-angle neutron scattering. Combination of solubility, contrast, and incoherent scattering make it possible to measure and analyze the relatively small scattering cross section of this system. Along with single fullerene molecules, a small amount of large fullerene clusters (more than 100 Å in size) is found in these solutions. The formation of these clusters depends on the procedure of solution preparation. The size distribution functions of clusters are compared with the results of the phenomenological cluster model of fullerene solubility.     

The short-range structure of alkali disilicate glasses by pulsed neutron total scattering

Time-of-flight total neutron scattering experiments using pulsed neutrons with short wavelength produced by an electron linear accelerator have been carried out to measure the structure factors of SiO₂ and M₂O · 2 SiO₂ (M = Li, Na and Li_0.63Na_0.37) glasses over a wide range of the scattering vector up to Q (=4π sin θ/λ) ～480 nm^?1. The Placzek correction has been considered up to the second order in the energy transfer. The Fourier transform of the structure factors yield the pair distribution functions highly resolved in real space, from which the stretching of the SiO and OO atomic distances and the number of bridging and non-bridging oxygen atoms in the alkali disilicate glass have been obtained.     

A neutron small angle scattering study of SiO2Na2 O glasses

The first results obtained by small-angle neutron scattering (SANS) study of sub-liquidus immiscibility of glasses are presented. Measurements were performed on the neutron small-angle scattering spectrometer of the Institut Laue-Langevin (Grenoble, France). The glass studied was 0.88 (SiO₂), 0.12 (Na₂O) from SiO₂Na₂O system which presents a well-known miscibility gap already explored by small-angle X-ray scattering (SAXS) method. Absolute values of the neutron scattering cross section as a function of scattering vector were obtained for this glass quenched and heat treated at 560°C for various lengths of time. It is shown that the ANS method can be used to follow phase separation kinetics and the comparison with SAXS results can in principle be used to separate the effects of density and concentration fluctuations in this system.     

Structure characterization of CVD amorphous Si3N4 by pulsed neutron total scattering

The structure factor S(Q) of chemically vapor-deposited (CVD) amorphous Si₃N₄ has been measured by pulsed neutron diffraction over the range of the scattering vector Q from 1–330 nm^?1. The oscillatory behavior in the S(Q) persists up to Q = 300 nm^?1 and there is appreciable small angle scattering intensity. The SiN bond length is l_SiN = 0.1729 nm, and its coordination numbers n_SiN and n_NSi are 3.70 and 2.78 respectively. The bond angles around a Si and a N atom are found to be 109.8 and 121°. Analysis of the small angle scattering intensity shows the existence of voids with an average diameter of about 1 nm and a volume fraction of about 4%, which may stabilize the amorphous structure of Si₃N₄ having rigid covalent bonds due to relaxing the strain energy accumulated in the matrix.