Asen K. Kirov  Mois I. Aroyo  J. Manuel Perez‐Mato 《Journal of Applied Crystallography》2003,36(4):1085-1089

NEUTRON is a computer program for calculating the phonon extinction rules for inelastic neutron scattering experiments. Given the space group and the phonon symmetry specified by the wavevector, the program examines the inelastic neutron scattering activity of the corresponding phonons for all possible types of scattering vectors. The systematic selection rules are also useful in the interpretation of the results of thermal diffuse scattering. NEUTRON forms part of the Bilbao Crystallographic server ( http://www.cryst.ehu.es ) and can be used via the Internet from any computer with a Web browser.  相似文献   

    

Klaus Habicht  Thomas Keller  Robert Golub 《Journal of Applied Crystallography》2003,36(6):1307-1318

A resolution function for inelastic neutron spin‐echo spectroscopy on a three‐axis spectrometer is derived. Inelastic dispersive excitations where the tilted field technique applies are being considered. Using a Gaussian approximation of the transmission function of the three‐axis spectrometer and a second‐order expansion of the total Larmor phase, the instrumental resolution function of an idealized spin‐echo instrument is obtained. Furthermore, the resolution function is extended to include the effects of sample properties, such as mosaicity, spread in lattice spacings and the curvature of the four‐dimensional dispersion surface in a line‐width measurement.  相似文献   

    

A. N. Falco  F. M. A. Margaa  F. G. Carvalho 《Journal of Applied Crystallography》2003,36(5):1266-1269

The feasibility of constructing a variable‐geometry converging multichannel collimator consisting of interlaced planes of thin neutron‐absorbing strips is discussed. A process is reported for producing strips of a composite material incorporating gadolinium oxide as the neutron absorber with the required mechanical and neutron‐absorbing properties.  相似文献   

    

L. D. Cussen  A. R. Wildes 《Journal of Applied Crystallography》2003,36(5):1225-1229

Newly developed reflecting neutron collimators promise increased detector count rates in a field where most work is intensity‐limited. The effects of such elements on instrument resolution are complicated and appear to be even more subtle than previously imagined. Numerical tests using the McSTAS ray‐tracing program ( http://neutron.risoe.dk/mcstas ) are reported which support and extend the recent analysis of these effects. If reflecting collimators are used in all instrument positions they give unusual scan profiles. Using reflecting collimators only before the monochromator and immediately before the detector has no deleterious resolution effects and increases count rates by a factor of more than two on powder diffractometers.  相似文献   

    

Paul F. Henry  Mark T. Weller  Chick C. Wilson 《Journal of Applied Crystallography》2003,36(6):1361-1367

The distribution of divalent iron and nickel over the two metal sites with differing coordination geometry in Fe₂Ni(PO₄)₂, sarcopside, has been investigated using time‐of‐flight powder neutron diffraction of nickel isotopically substituted materials. Data from four separate samples were collected using HRPD at ISIS, containing ^natNi, ⁵⁸Ni, ⁶⁰Ni and ⁶²Ni, under identical conditions. The occupancy of iron on the M(1) site was found to be 0.290 (1) from a combined‐data‐set Rietveld refinement of the three isotopically substituted samples, compared with 0.26 (4) and 0.26 (15) respectively from this and a previous time‐of‐flight powder neutron diffraction study using natural‐abundance nickel, and 0.366 (6) and 0.376 (3) using anomalous X‐ray scattering techniques. A critical comparison of isotope substitution neutron diffraction and anomalous X‐ray scattering methods for distinguishing nickel and iron from powder data is presented.  相似文献   

    

Jinkui Zhao 《Journal of Applied Crystallography》2003,36(5):1177-1181

Following previous works on data interpretation in the case of a one‐dimensionally encoded spin‐echo small‐angle neutron scattering instrument when the scattering data are collected along a single direction, further studies for the more general case of data collection in two dimensions are presented. A mathematical relationship between the correlation function measured in two dimensions and the pair‐distance distribution function of the scattering sample is established. The relationship between the correlation function measured in one and two dimensions is also examined.  相似文献   

    

I. G. Wood  Lidunka Vo adlo  K. S. Knight  David P. Dobson  W. G. Marshall  G. David Price  John Brodholt 《Journal of Applied Crystallography》2004,37(1):82-90

The cementite phase of Fe₃C has been studied by high‐resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with T_c≃ 480 K, greatly affects the thermal expansion coefficient of the material. The average volumetric coefficient of thermal expansion above T_c was found to be 4.1 (1) × 10⁻⁵ K⁻¹; below T_c it is considerably lower (< 1.8 × 10⁻⁵ K⁻¹) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a third‐order Grüneisen approximation to the zero‐pressure equation of state, combined with a magnetostrictive correction based on mean‐field theory.  相似文献   

    

Xiangyun Qiu  Emil S. Boin  Pavol Juhas  Thomas Proffen  Simon J. L. Billinge 《Journal of Applied Crystallography》2004,37(1):110-116

An atomic pair distribution function (PDF) neutron powder diffraction round‐robin experiment was performed on six diffractometers at three spallation sources. Instrument‐specific effects on the real‐space PDF were investigated, such as finite measurement range, the instrument resolution and the asymmetric shape of diffraction peaks. Two illustrative samples, a perfectly long‐range‐ordered element, Pb, and a locally strained alloy ZnSe_0.5Te_0.5, were measured at low temperatures. Various aspects of the PDF were explored, either qualitatively by direct comparison or quantitatively via structural modelling. Future implementation of modelling codes incorporating some of these instrumental effects are also discussed.  相似文献   

    

M. Theo Rekveldt  Wicher Kraan  Thomas Keller 《Journal of Applied Crystallography》2002,35(1):28-33

A new Larmor precession device, consisting of two separate precession modules, is introduced. It operates as a high‐resolution neutron diffractometer, without the need for a highly collimated beam, necessary in conventional high‐resolution neutron diffraction. The same instrument operates in different modes as an effective small‐angle scattering device or accurately determines the line profile, the lattice plane orientation or the angular spread in crystal orientations. A dramatic intensity gain can be obtained in all applications discussed, compared with the conventional analogues, owing to the possibility to use a relaxed wavelength and angular resolution. Experiments in the high‐resolution diffraction mode on Si and Al are discussed.  相似文献   

    

X.‐L. Wang  Y. D. Wang  J. W. Richardson 《Journal of Applied Crystallography》2002,35(5):533-537

A significant error may occur in the measured lattice parameters when the sample is displaced from the diffractometer center. For time‐of‐flight neutron diffractometers, this error gives rise to different lattice parameters, not only for detectors at different 2θ but also for detectors at the same 2θ but on opposite sides of the incident beam. Tests made on GPPD at IPNS, Argonne National Laboratory, show that this error largely arises from a change in the diffraction constant, 1/Lsinθ, where L is the total flight path. Modeling of the experimental data indicates that in order to achieve a precision of 10⁻⁴, a typical requirement for strain measurements, for a wide angular range of detectors, the sample should be positioned to within ∼0.1 mm of the diffractometer center. Equations are given to relate the errors measured at different diffraction angles. For small sample displacement, the error may be corrected by introducing the displacements along and normal to the incident beam as refinable parameters in least‐squares‐based analysis programs.  相似文献