Crystal structures powder neutron data

A complete understanding of the structure of the material under study or application is a sine qua non condition for the successful research or use of the material. In the case of powders, the best way to decipher the structure of new materials is the Rietveld method. This methodology was initially developed by Hugo M. Rietveld in 1969 [23] as a procedure for refining crystal structures using neutron powder diffraction data. To implement the method in practice, certain information about the estimated crystal structure of the phase or phases of interest in the diffraction profile under test is necessary. 

Solving Crystal Structures from Powder Neutron Data... 

Table 28 The Crystal Structure Data for Two Static Copper(II) Structures at a Range of Temperatures for a-[Cu(HC02)2] Neutron Diffraction Using Powder Profile Analysis569,570...

During the last five years, a powerful new method of getting crystal structural information from powder diffraction patterns has become widely used. Known variously as the Rietveld method, profile refinement1, or, more descriptively, whole-pattern-fitting structure refinement, the method was first introduced by Rietveld (X, 2) for use with neutron powder diffraction patterns. It has now been successfully used with neutron data to determine crystal structural details of more than 200 different materials in polycrystalline powder form. Later modified to work with x-ray powder patterns (3, X) the method has now been used for the refinement of more than 30 crystal structures, in 15 space groups, from x-ray powder data. Neutron applications have been reviewed by Cheetham and Taylor (5) and those for x-ray by Young (6). 

Recently, Wu [37] reported the crystal structure of the ternary imide Li2Ca(NH)2 which was determined using neutron powder diffraction data on a deuterated sample. In his paper, the reaction, Li2Ca(NH)2 + H2 LiNH2 + LiH + CaNH, was indicated. However, with respect to the reaction mechanism of the Li-Mg-N-H system [38], the reaction process in this report was thought to be stopped halfway. 

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. 

Abstract. - High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerene Cm in the temperature range 5 K to 320 K. Solid Cm adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the high-temperature face-centred-cubic phase (T > 260 K), the Cm molecules are completely orientation-ally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single (111) direction. In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists. 

Figure 2.61. Patterson functions calculated in the uOw plane using Eq. 2.136 and employing experimental x-ray (left) and neutron (right) powder diffraction data shown in Figure 2.58. The strongest peak in any Patterson function is always observed at (0,0, 0) beeause the origins of all vectors coincide with the origin of coordinates. Since in this particular example the real crystal structure contains an atom in (0, 0,0, see Figure 2.59), some of the peaks on the Patterson map correspond to the actual locations of atoms (i.e. = x- 0).The contour of...

The very existence of the powder diffraction pattern, which is an experimentally measurable function of the crystal structure and other parameters of the specimen convoluted with various instrumental functions, has been made possible by the commensurability of properties of x-rays and neutrons with properties and structure of solids. As in any experiment, the quality of structural information, which may be obtained via different pathways (two possibilities are illustrated in Figure 2.62 as two series of required steps), is directly proportional to the quality of experimental data. The latter is usually achieved in a thoroughly planned and well executed experiment as will be detailed in Chapter 3. Similarly, each of the data processing steps, which were described in this chapter and are summarized in Figure 2.62, requires knowledge, experience and careful execution, and we will describe them in practical terms in Chapters 4 through 7. 

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