Powder neutron diffraction experiments

In the conventional view of the structure of zeolite A each Si atom is surrounded by four A1 atoms and vice-versa, but recent m.a.s.n.m.r. data56 was interpreted to indicate that the structure was actually one in which each Si atom was surrounded by one Si and three A1 atoms and vice-versa. In support of this work a powder neutron diffraction experiment found a rhombohedral distortion in a sample of Na zeolite A.57... 

Neutrons in thermal equilibrium at 298 K can be used for diffraction in a similar way to X-rays, since they also have wavelengths comparable to interatomic spacings. In contrast to X-ray diffraction, the powder neutron diffraction experiment is much more common than single crystal neutron diffraction, since the beam intensity tends to be 1000 times less than for X-ray diffraction, so that single crystals of a sufficient size to collect good data are difficult to grow. 

The Bragg equation is utilized in two different ways in two sorts of powder neutron diffraction experiment ... 

The structural determination of Ba CugO for 6.8 < x < 7.0, was completed in several laboratories by Rietveld analysis of powder neutron d iffraction data (10-15). The neutron diffraction experiments confirmed the space group Pmmm and the main structural features found by x-rays by Siegriest et al. (7), but revealed that some of the oxygen assignments made in the x-ray studies were not entirely correct. The refined structural parameters obtained in four of these... 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

In powder diffraction, x-ray photons or neutrons (in x-ray and neutron diffraction experiments, respectively) are registered by the detector as random events. The measured intensity is directly proportional to the number of counts and therefore, the accuracy of intensity measurements is governed by statistics. Even though below we will refer to x-ray diffraction and photons, all conclusions remain identical when neutron diffraction and neutron count is considered. 

The crystal structure of Li4Re6Sn is depicted in Fig. 8. In addition to the X-ray single crystal structure investigations, the positions of the lithium atoms were determined by neutron diffraction experiments on samples of Li4Re6Sn powder. A... 

The analysis of the polarised neutron experiment on K2NaCrFe (Fig. 9) has been completed< >. The aspherical form factor component/4(0 was shown to be particularly sensitive to the covalent spin distribution, and the shape of fi Q) supported the inference for eg spin polarization drawn from the comparison of powder neutron diffraction data and resonance data (Section VI. B). A preliminary report of the form factor determination for both tetra-hedrally and octahedrally coordinated Fe + in YaFesOis by polarized neutrons has been given ). The form factors are not the same and that for tetrahedral Fe + is contracted relative to that for octahedral Fe +, which follows closely the calculated free-ion curve. The ligand forward peak (Section VI. B) has been directly observed ) in a measurement of the critical scattering of neutrons by ferromagnetic K2CUF4. 

Die focus of much of the preceding discussion has been issues associated with diffraction data measurement and processing. Di action techniques continue to develop at a rapid pace. Applications of powder neutron diffraction and, particularly, exploitation of the properties of synchrotron X-radiation in single crystal and high resolution powder diffraction experiments were highlighted here. Several of the techniques mentioned such as microcrystal diffraction, time-resolved Laue diffi action and differential anomalous diffraction are still at early stages of development. 

From the neutron diffraction study the site occupancies of the ytterbium, platinum, and bismuth atoms are clear (Robinson et al. 1994). Since these heavy atoms differ only by few electrons, it is difficult to assign the correct sites only on the basis of X-ray powder diffraction data. Another important feature concerns the wet chemical analysis. Since all crystals of YbPtBi were grown from a bismuth flux, elemental bismuth occurs as an impurity phase in all samples. An amount of 8.1wt.% was detected in a neutron diffraction experiment. In view of this impurity, the absolute values of all physical property measurements should be discussed with caution. 

Recent examinations of the LT behaviour of YAIO3 crystals using neutron and s)mchrotron diffraction techniques revealed pronounced anomalies in the expansion along the fc-direction (Figure 21B). It should be noted that for synchrotron powder diffraction experiment, an Mn-doped crystal was chosen, whereas for neutron diffraction experiment, a sample containing 1 at.% Nd was used. At LT, the anomaly occurs in both Mn- and Nd-doped crystals. 

For a number of years the question of magnetic ordering in dhcp Pr has been puzzling. An antiferromagnetic structure below 25 K was found by Cable et al. (1964) in neutron diffraction experiments on a powder sample, with a spon-... 

Susceptibility measurements on powder samples (Troc and Lam 1974) and on monocrystalline samples (Busch and Vogt 1978) show very distinctly the transition at 66 K (see fig. 52). Up to 1000 K the Curie-Weiss law is strictly obeyed (Busch et al. 1979a). The anisotropy of the magnetic susceptibilities is very well marked. The transition at 66 K was further confirmed by neutron diffraction experiments (Lander et al. 1972b, Marples et al. 1975). There were various attempts to find distortions in UAs, Knott et al. (1980) and Lander and Mueller (1974) could not detect measurable distortions. Only recently a tetragonal distortion with c/a = 1.0002 was identified for the type lA structure (McWhan et al. 1990), an indication of strong orbital contributions. 

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