Neutron diffuse diffraction

Consider now real materials with model micropores, that is to say with regular dimensions and whose pore walls consist of well-defined crystalline adsorption sites (including possible cationic sites). Such solids can be found within the realm of zeolites and associated materials such as the aluminophosphates. One can imagine the probability that a fluid adsorbed within such micropores may be influenced by the well-defined porosity and thus become itself "ordered. Such phenomena have already been highlighted with the aid of powerful but heavy techniques such as neutron diffraction and quasi-elastic incoherent neutron diffusion. The structural characterisation of several of the following systems was carried out with the aid of such techniques in collaboration with the mixed CNRS-CEA Leon Brillouin Laboratory at Saclay (France). 

It is shown that both the Bragg and the diffuse scattering parts of neutron powder diffraction data on ice Ih can be interpreted simultaneously by constructing large models of the structure that are consistent with the measured total scattering functions within errors. The RMCPOW algorithm proved to be readily applicable for the purpose. 

Figure 2.1 A neutron powder diffraction pattern collected from the superionic 8 phase of Bi2O3 at 1033 K, illustrating the rapid decrease in the intensities ofthe Bragg peaks with increasing Q and the presence of diffuse scattering observed as broad undulations (S. Hull et al., unpublished results).

Neutron powder diffraction spectrum of (hexagonal) D2O ( H20) ice (phase Ih) at 260 K, close to the melting point. Note the large amount of diffuse scattering under the Bragg peaks, indicating effects of structural disorder (Pusztai and McGreevy 1997, unpublished data)... 

Some 4503, 5 perovskite-structured materials, where A and B represent larger and smaller cations, are ionic conductors, while some other 503 s perovskite-type compounds are mixed conductors. Heavy elements such as La and Ba occupy the A site, but because the mobile O anion is a light element, conventional X-ray powder diffractometry is not sensitive to positional and occupational disordering of oxide ions. To investigate the diffusion path of mobile oxide ions, and structural disorder and crystal structure in perovskite-structured ionic and mixed conductors [5, 6, 8, 10-14], we applied a high-temperature neutron powder diffraction method. Our reasons for choosing this method were as follows [24] ... 

The X-N technique is sensitive to systematic errors in either data set. As discussed in chapter 4, thermal parameters from X-ray and neutron diffraction frequently differ by more than can be accounted for by inadequacies in the X-ray scattering model. In particular, in room-temperature studies of molecular crystals, differences in thermal diffuse scattering can lead to artificial discrepancies between the X-ray and neutron temperature parameters. Since the neutron parameters tend to be systematically lower, lack of correction for the effect leads to sharper atoms being subtracted, and therefore to larger holes at the atoms, but increases in peak height elsewhere in the X-N deformation maps (Scheringer et al. 1978). 

X-ray and diffuse neutron scattering and diffraction studies of PMN have been interpreted in terms of the spherical layer model of Vakhrushev et al. [25,26]. The Pb atom is not situated at the (000) position as it should be for an ideal perovskite lattice, but is distributed over a sphere of radius R around this position. 

Direct Synthesis reaction of, 6 395 fluoride, 21 235, 237, 239, 249 homopolyatomic cations, 17 82 ion, stereochemistry, 2 40-41, 44-45 isocyanates, preparation, 9 158 properties, 9 157 isothiocyanates, properties, 9 177 mixed valence compounds of, 10 375-381 crystal structure of, 10 376 diffuse reflectance spectrum of, 10 380 structure of Pb," ion, 10 381 nuclear magnetic shielding, 22 224 organometallic compounds, 2 82, 88, 89 oxide, neutron diffraction studies on, 8 231-233... 

The opposite signs for the neutron scattering power of hydrogen and deuterium (—0.38 and +0.65) offers the possibility for investigating (slow) self-diffusion between different water sites and/or localization of water molecules with different mobility if diffraction experiments are carried out for a sample where D20 is exchanged in steps vs. H20. 

The most important use of neutron diffraction in the general field encompassed by this book is in the study of alternating layers of deuterated and undeuterated films. At the time of writing, three papers have appeared on this topic. They are by Buhaenko et al. [56], Grundy et al. [57] and Stroeve et at. [58]. In the latter study, alternate layers of deuterated and undeuterated fatty acids were deposited and studied by neutron diffraction. Subsequently this ordered structure was destroyed by thermal diffusion and the gradual loss of order was monitored. The ordered structure is only destroyed at temperatures well above ambient. Applications of neutron diffraction to the study of lipid films at the air/water interface will be discussed in Chapter 8. 

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