Refinement using neutron diffraction 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. 

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. 

X-ray powder data obtained from the cubic form of 02PtF6 were consistent with the presence of 02+ and PtFfi ions.863 The structure was refined using neutron diffraction powder data. The 02+ cation 340 resembles nitrosonium ion.864 The IR, Raman, and ESR spectra of 02+ salts have been studied in detail.865-868... 

When we solved this crystal structure using neutron diffraction data, we found a model Table 6.28) where the origin of coordinates was shifted with respect to that constructed fi om x-ray diffraction data Table 6.21 and Table 7.10). Here, we will first use the coordinates of atoms determined from x-ray data (this fully refined crystal structure is found in the data file Ch7Ex03a.inp) and then perform a refinement of the original model as established fi-om a neutron diffraction experiment (data file Ch7Ex03b.inp). 

Table 7.13. Structural parameters of CeRhGes, fully refined by Rietveld teehnique using neutron diffraction data. The space group is I4mm. The unit cell dimensions are a = 4.39180(4), c = 10.0238(1) A. Some of the anisotropic displacement parameters are fixed by...

B. C. Chakoumakas, C. J. Rawn, A. J. Rondinone, S. L. Marshall, L. A. Stern, S. Circone, S. H. Kirby, C. Y. Jones, B. H. Toby, and Y. Ishii, The Use of Rigid Body Constraints in Rietveld Refinements of Neutron Diffraction Data of Clathrate Hydrates, in Proc. Fourth Int. Conf. Gas Hydrates, Yokohama 655 (2002). 

As mentioned above, Garrett et al. [1] first demonstrated the equivalence of the bond valence sum mismatch pathways in a-Agl with pathways determined experimentally from an anharmonic atomic displacement refinement of neutron diffraction data as a justification for predicting transport pathways for their then new fast Ag" ion conductor Agig I12P 2O7 using the same approach. In our earliest work on pathway models for similar Ag" ion conducting oxyhalide systems [3, 18], we used a parameter set by Radaev et al. [19],... 

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. 

Currently, techniques for analyzing powder diffraction that involve the analytical dissection of measured Bragg reflection shapes " " can be used to derive a data set similar to that for a single crystal. As a result, crystal structures may be determined, and in some cases, refined anisotropically using powder diffraction data for X rays or neutrons. Powder diffraction methods can be used at a wide variety of temperatures and pressures and are well suited for studies of phase transformations. 

Just as in the case of full pattern decomposition, we will use two freely available software codes (LHPM-Rietica and GSAS ) to carry out Rietveld refinements using either or both x-ray and neutron diffraction data. Many... 

Figure 7.15. The two models of the crystal structure of CeRhGcs refined using neutron powder diffraction data collected at T = 200 K. The thermal displacement ellipsoids are shown at 99% probability. See Figure 6.20 and relevant discussion explaining the choice of the origin of coordinates in the two drawings.

Figure 7.23. The observed and calculated powder diffraction patterns of NiMn02(0H) after the completion of combined Rietveld refinement using neutron (a) and x-ray (b) powder diffraetion data.

C. Rietveld performs crystal structure refinement and quantitative phase analysis using powder diffraction data and the Rietveld method. Combining refinement programs and advanced modeling tools allows a faster route to determining the structures of both inorganic and molecular crystals. An effective way to know the atomic structure is by means of diffraction techniques using neutrons from nuclear reactors and particle accelerators or X-rays from... 

The above-mentioned optimal methods were also used in combination with experimental chlorine NMR data to refine the hydrogen-bonded proton positions in some amino acid hydrochlorides for which neutron diffraction data are unavailable. The combined experimental-theoretical procedure provided proton positions which resulted in back-calculated values of the chlorine quadrupolar coupling constant typically within 15% of experiment. The values of O were found to be less sensitive to the proton positions. 

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