Diffuse Neutron Scattering Research Articles

Estimation of Force Constants of Al from Diffuse Neutron Scattering Measurement

Neutron diffraction measurement of an aluminum powder sample at 290 K was carried out at the high resolution powder diffractometer installed at JRR-3. Broad oscillations of the diffuse scattering intensity were observed and explained by the correlation effects among the thermal displacements of atoms. The interatomic force constants were determined from the correlation effects using a newly introduced equation. The derived force constants and the crystal structure of Al were used to estimate the phonon dispersion relations, phonon density of states, and specific heat by computer simulation. The calculated phonon dispersion relations and specific heat of Al are similar to those obtained from inelastic neutron scattering and specific heat measurements, respectively.

Force Constants of Cu Crystals from Diffuse Neutron Scattering Measurement

Diffuse neutron scattering measurement on Cu crystals was performed at 10 K and 300 K. Oscillatory forms were observed in the diffuse scattering intensities. The observed diffuse scattering intensities are analyzed by including the correlation effects among thermal displacements of atoms in the theory. Using the values of correlation effects among neighboring atoms and the values of Debye-Waller temperature parameter, force constants among first, second and third nearest neighboring atoms have been evaluated. The result of correlation effects in Cu crystals are compared to that of ionic crystal and semiconductor. The relation between correlation effects and the inter-atomic distance is not depending much on the crystal binding types. Received: 12 October 2010; Revised: 22 October 2010; Accepted: 16 December 2010

Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn2O5 was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at � 45 K. A broad peak below � 6 K in the heat capacity measurements corresponds to the crossover transition of Tb 3+ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn2O5 with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by

Diffuse Neutron Scattering of KBr at Room Temperature and Its Application as Background Function in Rietveld Analysis

Diffuse neutron scattering from powder KBr was measured at room temperature. The oscillatory diffuse scattering is clearly observed. The diffuse scattering theory including correlation effects among thermal displacements of atoms is applied to background function in the Rietveld analysis. The observed scattering data are analyzed by including the correlation effects among thermal displacements of first, second and third nearest neighboring atoms. The inter-atomic distance and temperature dependence of the values of correlation effects is discussed.

Diffuse Neutron Scattering in the Mn0.4Cu0.6Alloy

Diffuse Neutron Scattering in the Mn_0.4Cu_0.6Alloy

The instrument DNS: polarization analysis for diffuse neutron scattering

The diffuse neutron scattering (DNS) instrument is a time-of-flight spectrometer that similar to the D7 in Grenoble has been equipped with neutron polarizers and analyzers covering a wide angular range. A particular feature of this instrument is its compact size and consequently, a focusing layout of the polarizers. The polarizing devices are benders that are made of a stack of magnetic supermirrors. Details of the instrumental design and its performance will be discussed.

Short-Range Order in Ni-Rich Ni–Ti Studied by Diffuse Neutron Scattering

Diffuse wide-angle neutron scattering has been measured for 58 Ni-5.8 at% Ti at room temperature after quenching from 777 K, and for 5 8N i-9.6 at% Ti at 1103 K. The fitted sets of short-range order and static displacement parameters are similar to those found in a previous investigation of short-range ordered Ni-8.9 at% Al. Applying a Bragg-Williams model, the coherent solvus of the γ phase is well reproduced. Neither the displacement parameters nor the phonon dispersior branches measured along (ξξξ) for the sample containing 9.6 at% Ti show any signs of instability of the short-range ordered face centered cubic structure.

Defects and lattice distortions in the superionic conductor Ba 0.84La 0.16F 2.16

Diffuse neutron scattering (DNS) experiments have been performed on a single crystal sample of Ba 0.84La 0.16F 2.l6 at temperatures from 600 to 1000°C. The results confirm that the defects form clusters and cluster aggregates, the concentration of which diminishes upon heating. Furthermore, the data yield information about local lattice deformations and suggested that the mobile fluorine ions move in the closest vicinity of clusters, in agreement with the enhanced ionic motion model. Quenching is shown to influence the defect structure and the ionic conductivity.

Focusing polarisation analysis at the diffuse neutron scattering spectrometer (DNS) in Jülich

A focusing neutron polariser system was developed for the instrument for diffuse neutron scattering (DNS) located in the neutron guide laboratory at the research centre Jülich. The focusing polarisers of the bender type are equipped with polarising FeCoV/Ti:N supermirrors. Results of Monte Carlo calculations of the transmission and focusing properties for different wavelength are given. Comparing the focusing polariser system with a conventional non-focusing set up, there is a considerable gain in intensity. For a wavelength of 3.3 Å the largest enhancements are obtained; at the positions of the sample and detectors these are given by the factors 1.8 and 2.7, respectively.

Comparative Investigation of Elastic Diffuse Neutron Scattering in C60Powder

Abstract C60 powder samples of varying impurity content have been investigated by neutron scattering techniques. For Q > 2.5 Å−1 (Q = transferred momentum) strong elastic diffuse scattering is found below Tc ~ 260 K providing evidence of static disorder in the low temperature phase. The elastic scattering increases on cooling from Tc to 150 K but remains virtually constant below this temperature. There is no marked dependence on the impurity level. For Q > 2.5 Å−1 elastic diffuse scattering obeying a sin(QR)/(QR) law is observed both above and below the phase transition (R= radius of C60 molecule). This scattering is nearly independent of temperature, however, depends strongly on the amount of impurities in the sample. In order to account for the unusually strong elastic diffuse scattering intensity model calculations have been performed which relate orientational disorder in C60 to the elastic scattering observed in powder samples.

Diffuse Neutron Scattering from Surfaces and Interfaces

ABSTRACTThe use of neutron scattering as a tool for exploring surfaces and interfaces has become more prevalent over the last several years, mainly due to the increasing popularity of reflectivity techniques, which study specular reflection from single surfaces or multilayers. Due to intensity limitations, the use of off-specular or grazing-incidence neutron scattering techniques has been much less prevalent. In this paper we shall discuss the origins of magnetic off-specular scattering (e.g., as observed in magnetic multilayers) and the origins of anomalies seen in the X-ray or neutron diffuse scattering from multilayers near Bragg reflections in terms of the Dynamical Theory of Scattering.

Diffuse Neutron Scattering from Cubic Lithium Sulphate at 908 K

The diffuse neutron scattering intensity for Li 2 SO 4 at 908 K has been calculated using a treatment which includes correlation between thermal displacements of the atoms and reorientation of the SO 4 2- ions. The calculation of the diffuse neutron diffraction scattering for Li 2 SO 4 powder is based on a structural model obtained from a recent analysis of single crystal neutron Bragg reflections for the high temperature cubic phase of Li 2 SO 4 . Relatively large correlations are required between the displacements of the atoms (λ s ( S ) s ( O ) and λ s n ( O ) s n ′ ( O ) for n ≠ n ′ ) to obtain good agreement between calculated and observed diffuse intensities. It is shown that evidence for a “paddle-wheel mechanism” is difficult to obtain from an analysis of the diffuse neutron scattering intensities.

Diffuse Neutron-Scattering Study of Local Atomic Arrangements in fcc Cu-Zn Alloys

Single crystals of 65 Cu-31 at.% Zn, nat Cu-31 at.% Zn, and 65 Cu-22 at.% Zn were investigated by diffuse neutron scattering. The contributions from short-range order and static atomic displacements were separated. Because of the different scattering lengths of 65 Cu and nat Cu species-dependent displacements could be extracted for the alloys with 31 at.% Zn. Effective pair interactions obtained by the inverse Monte-Carlo method show a dominant V 110 of œ520 meV and no distinct concentration dependence. All three sets of effective pair interactions give a low antiphase boundary energy of œ55 mJ/m 2 facilitating the formation of long-period superstructures

Diffuse Neutron Scattering and Short-Range Order in Alloys – Contrast Variation and Contrast Enhancement by Isotopic Replacement

ABSTRACTIn Al-Cu alloys containing Guinier-Preston zones, the scattering contributions due to short-range order and atomic displacements vary differently when the scattering contrast between Al and Cu is changed. The diffuse neutron scattering of Al-Cu single crystals containing 63Cu or 65Cu is compared and evaluated to test the current methods of separating the two scattering contributions. - The nature of a sugggtted ordered state in α-brass is also studied by neutron scattering, using Cu to enhance the scattering contrast in a Cu-30 at.% Zn alloy. A broad diffuse peak is found in powdered, appropriately aged samples, confirming that the first Warren-Cowley short-range order parameter is negative.

Diffuse and Small Angle Neutron Scattering Studies of Clustering and Decomposition in Electron-Irradiated Cu-Ni-Fe Alloys

ABSTRACTEarlier investigations of short range clustering (SRC) and long range decomposition (LRD) in Cu-Ni alloys upon electron irradiation [1,2] have been extended to alloys containing 0.5 and 2 at.% Fe. Moreover, quenching instead of furnace cooling was applied to achieve a well defined homophase equilibrium state with a small degree of SRC. Under subsequent irradiation at temperatures between 373 K and 623 K the decomposition kinetics of the alloys have been followed by diffuse and small angle neutron scattering experiments within a momentum transfer range of 0.001 < κ(Å−1) < 8. The addition of Fe is found to extend the miscibility gap to higher temperatures. The observed scattering curves are discussed in terms of the existing models for the initial stages of spinodal decomposition, irradiation enhanced diffusion and phase stability under irradiation.

Investigation of the localized magnetic moments on Fe and Mn in Disordered Fe-Mn-Al Alloys by Magnetic Diffuse Neutron Scattering

Localized magnetic moments of Fe and Mn are determined by magnetic diffuse neutron scattering and magnetization measurements. The observed decrease in μFe and μMn with increasing Mn content is discussed on the basis of the modified Zener model. Lokalisierte magnetische Momente von Fe und Mn werden durch magnetische diffuse Neutronen-streuung und Messung der Magnetisierung bestimmt. Die beobachtete Abnahme von μFe und μMn mit zunehmendem Mn-Gehalt wird auf der Basis des modifizierten Zener-Modells diskutiert.