Neutron refinement data

The complexity of the solvable structure strongly depends on the spectral resolution of the diffraction method in use. Structures with about 60 atoms in the asymmetric unit were solved from powder data combining synchrotron X-ray diffraction with refinement from neutron diffraction data from the same material (Morris et al. 1994 Admans 2000). About half of that complexity can be achieved with good laboratory X-ray diffractometers (Masciocchi et al. 1996 Kariuki et al. 1999). Neutron diffraction data can better be used for structure refinement than for structure determination, for the same reason. 

Since scattering factors for neutron diffiaction are independent of the scattering angle (in contrast to the case of XRD and ED), neutron diffraction data are more frequently used for refinement than the others. 

Structure determination from X-ray and neutron diffraction data is a standard procedure. Starting with a rough model, the accurate structure is determined using a least-squares structure refinement, which is based on kinematic diffraction and in which the differences between calculated and experimental intensities are minimized. X-ray and neutron diffraction are not applicable to all crystals. To determine crystal structures of thin layers on a substrate or small precipitates in a matrix (see figure 1) only electron diffraction (ED) can lead you to the crystal structure. 

R. M. Wilson, J.C. Elliott, S.E.P. Dowker, R.l. Smith, Rietveld structure refinement of precipitated carbonate apatite using neutron diffraction data. Biomaterials 25 (2004) 2205-2213. 

As has become clear in previous sections, atomic thermal parameters refined from X-ray or neutron diffraction data contain information on the thermodynamics of a crystal, because they depend on the atom dynamics. However, as diffracted intensities (in kinematic approximation) provide magnitudes of structure factors, but not their phases, so atomic displacement parameters provide the mean amplitudes of atomic motion but not the phase of atomic displacement (i.e., the relative motion of atoms). This means that vibrational frequencies are not directly available from a model where Uij parameters are refined. However, Biirgi demonstrated [111] that such information is in fact available from sets of (7,yS refined on the same molecular crystals at different temperatures. 

Table 2 Refined structural parameters of Ba(Pb0 76Bi 25)Os (Powder neutron diffraction data) (Ref. 12.)...

Soper, A.K., Tests of the empirical potential structure refinement method and a new method of application to neutron diffraction data on water, Mol. Phys., 99, 1503-1516,2001. 

All of these hexafluorides are dimorphic, with a high-temperature, cubic form and an orthorhombic form, stable below the transition temperature (92). The cubic form corresponds to a body-centered arrangement of the spherical units, with very high thermal disorder of the molecules in the lattice, leading to a better approximation to a sphere. Recently, the structures of the cubic forms of molybdenum (93) and tungsten (94) hexafluorides have been studied using neutron powder data, with the profile-refinement method and Kubic Harmonic analysis. In both compounds the fluorine density is nonuniformly distributed in a spherical shell of radius equal to the M—F distance. Thus, rotation is not completely free, and there is some preferential orientation of fluorine atoms along the axial directions. The M—F distances are the same as in the gas phase and in the orthorhombic form. 

Refined structures of the five homologous phases of the lanthanide higher oxides from experimental neutron diffraction data... 

Results of Rietveld refinement of neutron powder data for Na-A zeolite at various temperatures above and below the cubic-rhombohedral transition. The space group used in each refinement is as shown. In the rhombohedral refinements, the positional parameters were constrained to satisfy Fm3c symmetry as discussed in text, f is the fractional occupancy and N the number of reflections included in this refinement. Also shown in columns I and II are the X-ray single crystal results of Pluth and Smith(t), but with their anisotropic temperature factors converted to the average isotropic values. Neutron... 

Table 3.2. Extinction and refinement data for some neutron diffraction precision structure analyses ...

Hanson JC, Schoenbom BP (1981) Real space refinement of neutron diffraction data from sperm whale carbonmonoxymyoglobin. J Mol Biol 153 117-146... 

Wiener, M. C. and White, S. H. (1992). Stmcture of a fluid dioleoylphosphatidylcholine bilayer determined by joint refinement of x-ray and neutron diffraction data. HI. Complete stmcture. Biophys. J. 61 434. 

Implementation of neutron diffraction data allowed much better refinement of the position of the hydrogens, and thus of the putative H-bonds themselves. Of particular interest... 

Deformation density calculations [66] based on X-ray and neutron diffraction data indicated a substantial electron deficiency in the sulphur valence shell. Kappa refinement based on the point charge model relates a positive charge close to 1.8 to the sulphur, and about -1.0 on each of the sulphate oxygen atoms [66]. The comparison of these results to those obtained from MO cluster model calculations, provides an indication of the goodness of the applied atomic basis sets. Such a comparison is presented in Table 4. 

Recently, the crystal structures of Ce Ojj and Ce,O20 have been confirmed by refinement of syngle cry.stal neutron diffraction data. ... 

Recently, the crystal structure of CeO, (CejOj,) has been determined by refinement of single crystal neutron diffraction data A value of the unit cell parameter of 11.111(2) A has been found (space group la3), in agreement with the value of 11.126 A reported by Sevan. ... 

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