Neutron powder diffraction structure solution from data

Early data analysis attempted to extract values of the individual structure factors from peak envelopes and then apply standard single crystal methods to obtain structural information. This approach was severely limited because the relatively broad peaks in a powder pattern resulted in substantial reflection overlap and the number of usable structure factors that could be obtained in this way was very small. Consequently, only very simple crystal structures could be examined by this method. For example, the neutron diffraction study of defects in CaF2-YF3 fluorite solid solutions used 20 reflection intensities to determine values for eight structural parameters. To overcome this limitation, H. M. Rietveld realized that a neutron powder diffraction pattern is a smooth curve that consists of Gaussian peaks on top of a smooth background... 

Because of the progress in structure solution from powders and in neutron diffraction, more than 300 crystal structures have already been fully determined, most of them during the past two decades. Here, crystal structures are only considered as fully characterized if all atomic positions have been determined and refined to an acceptable precision. The hydride fluoride database (HFD) provides a comprehensive, critical compilation of crystal structure data of metal hydrides. The power of the method described of combining X-ray and neutron powder diffraction may be illustrated by the example of MgeCoaDn, whose rather complex crystal structure, which contains 63 free positional parameters, was solved ab initio from high-resolution synchrotron and neutron powder diffraction data. Because of the main problems discussed in this section, the accuracy of metal hydride structural parameters is generally lower than for other inorganic compounds. 

In this chapter diffraction and in particular X-ray and neutron diffraction will be described in general, with an emphasis on powder diffraction techniques. The specific properties of X-ray and neutron diffraction and a description of sources and instruments for powder diffraction studies will be presented. Furthermore the use of powder diffraction data, from the simple use for phase identification to structure solution and refinements with the Rietveld methods, will be described. Two examples showing the potential for powder... 

Another t)rpe of phase transition in NdAlOa, namely the LT transformation from the rhombohedral to orthorhombic structure, has been predicted at 16 K from the extrapolation of transition temperatures in the Ndi, Smj A103 solid solution (Yoshikawa et al., 1998). This transition, however, was not confirmed using s)mchrotron powder diffraction data collected in situ at temperatures down to 12 K (Vasylechko et al., 2007b) and neutron diffraction measurements at 0.5 and 1 K (Palacios et al., 2003). Figure 5 displays the temperature evolution of the lattice parameters and cell volume of NdA103. Structural parameters of the rhombohedral phase of NdAlOs over a broad temperature range are given in Table 8. 

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