Theory of Thermal Neutron Scattering

Marshall, W.; Lovesey, S. W.; Nelkin, Mark

doi:10.1119/1.1987042

Cited by 432 publications

(317 citation statements)

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“…• l'interaction entre les moments magnétiques des neutrons et le champ électrique crée par l'ensemble des électrons et des noyaux [6].…”

Section: Contributions De La Polarisation Nucléaireunclassified

La technique classique du rapport de flipping. Application aux aimants moléculaires et aux aimants photo-commutables

Gillon

2007

Journées de la Neutronique

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Résumé. Des exemples d'applications de la méthode classique du rapport de flipping à la détermination de la densité de spin par diffraction de neutrons polarisés (DNP) sont décrits dans ce chapitre. Dans le domaine des composés magnétiques à base moléculaire, de telles investigations fournissent des informations directes sur la nature et l'origine des interactions intra-et inter-moléculaires. Ainsi, la visualisation de la distribution de spin dans la maille cristalline et dans la molécule permet de mettre en évidence les chemins d'interaction entre les centres magnétiques (ions de transition, radicaux organiques) et d'étudier le rôle des effets de délocalisation de spin et de polarisation de spin dans les mécanismes d'interaction. De plus, la densité de spin expérimentale dans des clusters moléculaires permet d'élucider la nature de l'état fondamental magnétique qui peut résulter de la compétition entre des interactions antiferromagnétiques intramoléculaires. Le photomagnétisme dans les composés à base moléculaire suscite un interêt croissant, en raison des applications potentielles des matériaux magnétiques photo-commutables. Les composés à transition de spin contenant un ion Fe 2+ en coordination octaédrique présentent un état fondamental bas spin diamagnétique (S = 0) qui peut être commuté, sous irradiation lumineuse de longueur d'onde adéquate, vers un état métastable paramagnétique haut spin (S = 2) ayant une très longue durée de vie à basse température. La densité d'aimantation dans un état photo-excité magnétique a été déterminée pour la première fois pour le complexe à transition de spin [Fe II (ptz)6](BF4)2 (ptz = 1-propyltetrazole).

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“…• l'interaction entre les moments magnétiques des neutrons et le champ électrique crée par l'ensemble des électrons et des noyaux [6].…”

Section: Contributions De La Polarisation Nucléaireunclassified

La technique classique du rapport de flipping. Application aux aimants moléculaires et aux aimants photo-commutables

Gillon

2007

Journées de la Neutronique

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“…Now, in a harmonic crystal W is proportional to (sin 0/,~,) 2 (Marshall & Lovesey, 1971), but the effect of anharmonicity is to introduce higher-order terms (Willis, 1969) W= B(sin 0/I) 2 + C(sin 0/i) 4 +..., where C depends on the indices of the Bragg reflection. Hence if a is proportional to (sin 0/1) 2 we would obtain a value of B reduced from the true result.…”

Section: Ip Oc E-2w(1 + A) Oc E -(2w-a) (Ifa Is Small)mentioning

confidence: 99%

“…The standard expression for the one-phonon scattering cross section, d 2 a/dO dE' (Marshall & Lovesey, 1971 ;Squires, 1978), can be rewritten…”

Section: Theory Of the Correctionmentioning

confidence: 99%

“…If an incident neutron of wavevector k is scattered into a state with wavevector k', the conditions for one-phonon scattering are (Marshall & Lovesey, 1971) h 2 --(k 2 -k '2) = he9 = ehogj(q) (3) 2m and…”

Section: Theory Of the Correctionmentioning

confidence: 99%

“…Fig. 2 is a reciprocallattice diagram of the scattering process, and ~ and x are the scattering and reciprocal-lattice vectors respectively.If an incident neutron of wavevector k is scattered into a state with wavevector k', the conditions for one-phonon scattering are (Marshall & Lovesey, 1971) h 2 --(k 2 -k '2) = he9 = ehogj(q) (3) 2m andwhere x is a reciprocal-lattice vector and e = 1 for phonon creation and -1 for phonon destruction. The standard expression for the one-phonon scattering cross section, d 2 a/dO dE' (Marshall & Lovesey, 1971 ;Squires, 1978), can be rewritten…”

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confidence: 99%

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The thermal diffuse correction in neutron time-of-flight diffraction

Cole¹,

Windsor²

1980

Acta Cryst Sect A

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A numerical method is developed for calculating the thermal diffuse scattering (TDS) correction from phonon inelastic scattering in a neutron diffraction pattern measured by the time-of-flight method. The correction is evaluated for a nickel powder spectrum at room temperature. It is shown to be more important than had been suggested previously. It gives rise to a reduction of the apparent Debye-Waller factor by about 10%. It may also lead to the appearance of the higher-order Debye-Waller factor previously ascribed to anharmonicity. IntroductionIn a diffraction experiment, the measured peak intensity of a Bragg reflection will in general include a contribution arising from the inelastic scattering of the incident beam by phonon excitations; this is referred to as the thermal diffuse scattering (TDS). Several theories have been developed to correct data from monochromatic X-ray or neutron diffraction experiments for the effects of TDS (e.g. Walker & Chipman, 1972;Cooper, 1971;Suortti, 1967). Recently, much attention has been given to the advantages of pulsed neutron sources for diffraction (Carpenter, 1977). In this type of diffraction experiment, scattering from neutrons with different wavelengths present in the neutron pulse can be analysed by timing over a flight path of a few metres to give the diffraction pattern at one or more fixed angles as a function of wavelength. Analogous experiments with a polychromatic X-ray beam, such as that from a synchrotron radiation source, can be performed with an energy-sensitive X-ray detector (Buras, Staun Olsen & Gerward, 1976). Its high epithermal flux compared with a reactor (Windsor, 1977) allows diffraction at good resolution * Present address: Engineering Department, Plessey Radar, Weybridge, Surrey, England.0567-7394/80/040697-08501.00 to be performed over a wide range of scattering vectors. However, if these sources are to be successfully used for accurate structural studies it is important for TDS corrections to be obtained for time-of-flight experiments. Windsor & Sinclair (1976) obtained the neutron diffraction spectrum of powdered nickel by the timeof-flight method and argued that, at the high scattering vectors used, thermal diffuse scattering would appear as a relatively smooth background beneath the Bragg peaks and would have only a small effect on the integrated intensities. This is because the multi-phonon scattering, which grows with increasing scattering vectors, is relatively featureless on a time-of-flight scale. In addition, the increasing overlap of the Bragg peaks implies overlap of the TDS around each peak. However, the values obtained for the Debye-Waller exponent B [defined in equation (22)] were smaller than in previous measurements and this was explained in terms of anharmonicity in the lattice potential.It is well known (Willis, 1969) that the effect of TDS in a conventional constant-wavelength experiment is to produce an apparent reduction in the value of B if no corrections are applied, and similar effects in a time-of-flight expe...

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Single‐Crystal Neutron Diffraction

Koetzle

2012

Characterization of Materials

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Single‐crystal neutron diffraction allows determination of the distribution of nuclear scattering density in crystalline structures. It is especially suited to the location of hydrogen and characterization of hydrogen bonding in structures ranging from ice to small proteins, to the determination of magnetic structures, and to experiments on crystals under extreme conditions of temperature, magnetic field, and pressure. The principal requirement is a good quality single crystal of volume 0.1–10 mm 3 , although ongoing instrumental developments, including the introduction of large area detectors and Laue methods, are continually decreasing the lower limit. There are currently over 30 dedicated single‐crystal neutron diffractometers at steady‐state reactor and neutron spallation sources worldwide, accessible via peer‐reviewed proposal mechanisms. The experimental and analysis techniques are well established and are very similar to those used for single‐crystal x‐ray diffraction.

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Theory of Thermal Neutron Scattering

Cited by 432 publications

References 2 publications

La technique classique du rapport de flipping. Application aux aimants moléculaires et aux aimants photo-commutables

La technique classique du rapport de flipping. Application aux aimants moléculaires et aux aimants photo-commutables

The thermal diffuse correction in neutron time-of-flight diffraction

Single‐Crystal Neutron Diffraction

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